U.S. patent number 3,686,471 [Application Number 05/092,803] was granted by the patent office on 1972-08-22 for system for recording and/or reproducing four channel signals on a record disc.
This patent grant is currently assigned to Victor Company of Japan, Ltd.. Invention is credited to Nobuaki Takahashi.
United States Patent |
3,686,471 |
Takahashi |
August 22, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
SYSTEM FOR RECORDING AND/OR REPRODUCING FOUR CHANNEL SIGNALS ON A
RECORD DISC
Abstract
This system records four channel signals on or reproduces them
from a record disc. The four channel signals are combined into sum
signals and difference signals, respectively, each composed of two
signals. Each difference signal is converted into an angle
modulated wave by modulating a carrier from a single oscillator and
is multiplexed with each sum signal. The four channel signals
picked up from the record disc are reproduced and sounded
respectively from four speakers placed at predetermined
positions.
Inventors: |
Takahashi; Nobuaki (Yamato,
JA) |
Assignee: |
Victor Company of Japan, Ltd.
(Kanagawa-ku, JA)
|
Family
ID: |
27457646 |
Appl.
No.: |
05/092,803 |
Filed: |
November 25, 1970 |
Foreign Application Priority Data
|
|
|
|
|
Nov 28, 1969 [JA] |
|
|
44/95587 |
Mar 15, 1970 [JA] |
|
|
45/21826 |
Mar 24, 1970 [JA] |
|
|
45/24579 |
Jul 5, 1970 [JA] |
|
|
45/58313 |
|
Current U.S.
Class: |
369/90 |
Current CPC
Class: |
H04S
3/006 (20130101) |
Current International
Class: |
H04S
3/00 (20060101); G11b 003/00 (); G11b 003/74 ();
H04b 005/00 () |
Field of
Search: |
;179/1.4ST,1.4M,1.4C,1.1TD,1G,15BT,1.1S,16P,16A,15BM,15BC |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Konick; Bernard
Assistant Examiner: Cardillo, Jr.; Raymond F.
Claims
What I claim is:
1. A system for recording four channel signals on a record disc,
said system comprising a signal source means for supplying four
separate channel signals, matrix circuit means for composing a
first sum signal and a first difference signal responsive to the
first and second channel signals and a second sum signal and a
second difference signal responsive to the third and fourth channel
signals among the four separate channel signals, single oscillator
means for oscillating to provide a signal having a predetermined
frequency, frequency divider means operated responsive to the
oscillator signal of predetermined frequency for dividing the
frequency thereof, means responsive to the output of the frequency
divider means for generating a saw-tooth waveform signal, first
serrasoid modulator means for angle-modulating said saw-tooth
waveform signal with the first difference signal, second serrasoid
modulator means for angle-modulating said saw-tooth waveform signal
with the second difference signal, first frequency multiplier means
for multiplying the frequency of the output signal of the first
serrasoid modulator means, second frequency multiplier means for
multiplying the frequency of the output signal of the second
serrasoid modulator means, first mixer means responsive to the
output signal of the first frequency multiplier means and the
output signal of the single oscillator means for producing a signal
having frequencies equal to the difference between the frequencies
of the output signal of the first frequency multiplier means and
the predetermined frequency of the single oscillator means, second
mixer means responsive to the output signal of the second frequency
multiplier means and the output signal of the single oscillator
means for producing a signal having frequencies equal to the
difference between the frequencies of the output signal of the
second frequency multiplier means and the predetermined frequency
of the single oscillator means, means whereby the carrier
frequencies of the output signals of said first and second mixer
means have frequencies in a recordable frequency range of the
record disc, the maximum phase angle deviation of the carrier wave
of each of the output signals of said first and second mixer means
being between 0.3 and 3 radians and the lower frequency limit of
said carrier wave being higher than the upper frequency limit of
the first and second sum signals, third mixer means for mixing and
multiplexing the first sum signal with the output signal of the
first mixer means, fourth mixer means for mixing and multiplexing
the second sum signal with the output signal of the second mixer
means, and means for simultaneously recording the output signal of
the third mixer means on one wall of a single groove of the record
disc and the output signal of the fourth mixer means on the other
wall of the groove.
2. A system for recording four channel signals in a single groove
cut on a record disc, said system comprising signal source means
for supplying four separate channel signals, matrix circuit means
for composing a first sum signal and a first difference signal
responsive to the first and second channel signals and also a
second sum signal and a second difference signal responsive to the
third and fourth channel signals of the four separate channel
signals, single oscillator means for generating a carrier wave
having a predetermined frequency, first modulator means for
angle-modulating said carrier wave with the first difference
signal, second modulator means for angle-modulating said carrier
wave with the second difference signal, each of said modulator
means comprising means whereby the maximum phase angle deviation of
said carrier wave is between 0.3 and 3 radians for all frequencies
which are included in the four channel signals, the lower frequency
limit of the carrier wave in the output of each of said modulator
means being higher than the upper frequency limit of the first and
second sum signals, first mixer means for mixing and multiplexing
the first sum signal with the output signal of the first modulator
means, second mixer means for mixing and multiplexing the second
sum signal with the output signal of the second modulator means,
and means for simultaneously recording the output signal of said
first mixer means on one wall of a single groove of the record disc
and recording the output signal of said second mixer means on the
other wall of the groove.
3. The recording system as defined in claim 2 wherein said signal
source means comprises four microphones which are placed at
predetermined distances and positions relative to a sound source
for converting the sounds from said sound sources into distance and
position dependent electric signals, means responsive to each
electric signal generated by each of said microphones for recording
said signals as the four channel signals on a magnetic tape while
said tape is running at a first running speed, means for
reproducing the four channel signals from said magnetic tape while
said tape is running at a second running speed which is slower than
said first running speed, and means for supplying the reproduced
four channel signals to said matrix means.
4. A system for reproducing the four channel signals which are
recorded on the record disc according to claim 2, said system
comprising pickup means for generating electrical signals
responsive to the signals recorded on the one wall of the groove
and electrical signals responsive to the signals recorded on the
other wall of the groove, first separator means for separating the
first sum signal and the angle modulated signal which is modulated
with the first difference signal, second separator means for
separating the second sum signal and the angle modulated signal
which is modulated with the second difference signal, first
demodulator means for demodulating the angle modulated signal
separated by the first separator means and thus reproducing the
first difference signal, second demodulator means for demodulating
the angle modulated signal separated by the second separator means
and thus reproducing the second difference signal, second matrix
circuit means responsive to the separated first sum signal and the
reproduced first difference signal for separately reproducing the
original first and second channel signals, third matrix circuit
means responsive to the separated second sum signal and the
reproduced second difference signal for separately reproducing the
original third and fourth channel signals, and means comprising
four speakers for respectively generating sound responsive to the
reproduced four channel signals.
5. The reproducing system as defined in claim 4 wherein said four
speakers are arranged with respect to the listener so that the
first speaker is placed at the front and on one side, the second
speaker at the rear and on the one side, the third speaker at the
front and on the other side, and the fourth speaker at the rear and
on the other side.
6. The reproducing system as defined in claim 4 wherein said four
speakers are arranged with respect to the listener so that the
first speaker is placed at the front and on one side, the second
speaker at the rear and on the other side, the third speaker at the
front and on the other side, and the fourth speaker at the rear and
on the one side.
7. A system for recording four channel signals on a record disc,
said system comprising signal source means for supplying four
separate channel signals, matrix circuit means for composing a
first sum signal and a first difference signal responsive to the
first and second channel signals and a second sum signal and a
second difference signal responsive to the third and fourth channel
signals among the four separate channel signals, first equalizer
means for receiving the first difference signal, second equalizer
means for receiving the second difference signal, said first and
second equalizer means having a characteristic frequency response
which increases in proportion to an increase of frequency at a
slope of 6 dB/octave, single oscillator means for generating a
carrier wave having a predetermined frequency, first modulator
means for frequency modulating said carrier wave responsive to the
output signal of the first equalizer means, second modulator means
for frequency modulating said carrier wave responsive to the output
signal of the second equalizer means, the maximum phase angle
deviation of the modulated carrier wave being between 0.3 and 3
radians in each output of said first and second modulator means and
the lower frequency limit of said modulated carrier wave being
higher than the upper frequency limit of said first and second sum
signals, first mixer means for mixing and multiplexing the first
sum signal with the output signal of the first modulator means,
second mixer means for mixing and multiplexing the second sum
signal with the output signal of the second modulator means, and
means for simultaneously recording the output signal of said first
mixer means on one wall of a single groove of the record disc and
the output signal of said second mixer means on the other wall of
the groove.
8. The recording system as defined in claim 7 wherein said first
and second equalizer means respectively have a rising slope
characteristic of 6 dB/octave in a frequency range which is higher
than a predetermined frequency and a flat characteristic in a
frequency range which is lower than said predetermined
frequency.
9. The recording system as defined in claim 8 wherein said
predetermined frequency in the first and second equalizer means is
selected at a certain frequency within a range of 100 to 2,000
Hz.
10. A system for reproducing the four channel signals which are
recorded on the record disc according to the system of claim 7,
said system comprising pickup means for playing back as electrical
signals the signals recorded on the one wall of the groove of the
record disc and also the signals recorded on the other wall of the
groove, first separator means for separating the first sum signal
and the frequency modulated signal modulated with the first
difference signal, second separator means for separating the second
sum signal and the frequency modulated signal modulated with the
second difference signal, first demodulator means for demodulating
the frequency modulated signal separated by the first separator
means and thus reproducing the first difference signal, second
demodulator means for demodulating the frequency modulated signal
separated by the second separator means and thus reproducing the
second difference signal, first equalizer means for receiving the
demodulated first difference signal, second equalizer means
receiving the demodulated second difference signal, said first and
second equalizer means having a characteristic frequency response
which decreases in proportion to an increase of frequency at a
slope of 6 dB/octave, second matrix circuit means responsive to the
separated first sum signal and the output signal of the first
equalizer means for separately reproducing the original first and
second channel signals, third matrix circuit means responsive to
the separated second sum signal and the output signal of the second
equalizer means for separately reproducing the original third and
fourth channel signals, and means comprising four speakers for
respectively reproducing as sound the reproduced four channel
signals.
11. The reproducing system as defined in claim 10 wherein said
first and second equalizer means have a falling slope frequency
characteristic of 6 dB/octave in a frequency range which is higher
than a predetermined frequency and a flat characteristic in a
frequency range which is lower than said predetermined
frequency.
12. The reproducing system as defined in claim 11 wherein said
predetermined frequency is a selected certain frequency within a
range of 100 to 2,000 Hz.
Description
This invention relates to a system for recording four channel
signals on and/or reproducing them from a record disc and, more
particularly, to a system for recording four channel signals on a
single groove of a record disc and/or reproducing these signals
from this single groove.
In a conventional stereophonic record disc, two channel signals are
recorded. In this stereo record, signals of the left and right
channels are respectively recorded on two walls of a groove in the
disc. This groove is called a 45-45 stereo system groove because
both walls thereof are inclined at an angle of 45.degree. with
respect to a perpendicular line which passes through the deepest
point of the groove. In this conventional two channels stereophonic
system, there are two sound sources forming one sound plane, which
enables this system to have a sound source reproductivity twice as
large as that of a monaural system in which there is only one sound
source and no sound plane whatsoever.
However, there has recently been an increasing demand for
reproducing and a real atmosphere of the live performance in a
reproduced sound field. It will be apparent that the degree of
reality is enhanced if the number of signal sources can be
increased. For this purpose, a proposed magnetic tape reproducing
apparatus has sounds reproduced from a magnetic tape having four
channel signals recorded on four tracks. However, it has heretofore
been considered impossible to record four channel signals on a
single groove of a record disc without providing a plurality of
grooves for four channels.
This invention has made it possible to record four channel signals
on a single groove of a record disc.
Even if a recording of four channel signals in a single groove of a
record disc is made possible, a requirement for compatibility with
the conventional two channel stereo record disc must be considered.
More specifically, a stereophonic reproduction of a two channel
record disc must be possible on a four channel record disc
reproducing apparatus. Also, a stereophonic reproduction of a four
channel record disc must be possible on a two channel record disc
reproducing apparatus. Accordingly, a four channel record and a
reproducing apparatus therefor which satisfy the aforementioned
needs are required.
Further, in an amplitude modulation system, the signal-to-noise
ratio cannot be improved in consideration of energy of side bands.
In a frequency modulation system, the signal-to-noise ratio can be
improved. If, however, there is a crosstalk between output signals
from modulators having separate oscillators, a disturbing noise
which did not exist originally will be produced. Therefore, a four
channel record disc which can be of a practical use is not feasible
unless the problem of disturbing noises caused by a crosstalk is
satisfactorily settled.
Furthermore, in cutting and recording on an original lacquer disc,
it is necessary to compress frequency bands of signals to be
recorded due to limitation in operable frequency range of a cutting
machine. This recording with the compressed bands must be effected
without producing noises.
This invention has eliminated the barriers which heretofore have
obstructed a realization of a four channel record and has satisfied
every requirement as hereinabove described. It is, therefore, a
general object of the present invention to provide a novel and
useful system for recording and/or reproducing four channel signals
on a record disc.
Another object of the invention is to provide a system for
recording and reproducing four channel signals in a single groove
of a record disc.
Yet another object of the invention is to provide a system for
recording four channel signals on a record disc in such a way that
no disturbing noise is produced by crosstalk which may occur
between each channel signal.
A further object of the invention is to provide a system for
reproducing four channel signals from a record disc, which system
is compatible with the system for playing back a two channel stereo
record.
A still further object of the invention is to provide an angle
modulation type system for recording four channel signals on a
record disc, in which four channel signals can be recorded without
producing noise, even in the where frequency bands of signals to be
recorded are compressed in recording.
Other objects and features of the invention will become apparent
from the description made hereinbelow with reference to the
accompanying drawings, in which:
FIG. 1 is a schematic diagram showing a state in which sounds from
sound sources are recorded as four channel signals on a magnetic
tape;
FIG. 2 is a block diagram showing one embodiment of a recording
system according to the invention for cutting and recording four
channel signals on a record disc;
FIGS. 3A and 3B are graphs as diagrams showing respectively
frequency characteristics of the recorded and reproduced
signals;
FIG. 4 is a vertical section of a groove of a record disc;
FIG. 5 is a block diagram showing one embodiment of the reproducing
system according to the invention;
FIG. 6 is a diagram illustrating a reproduced sound field;
FIG. 7 is a vector diagram showing the relationship between vectors
of a groove of a record disc and an actuating axis of a pickup
stylus;
FIGS. 8 and 9 are respectively vector diagrams for illustrating a
crosstalk;
FIGS. 10A and 10B are diagrams showing signal waveforms;
FIGS. 11A and 11B are diagrams showing, respectively, the
relationships between an angle .theta. and an angle .alpha., and
the angle .theta. and a vector l shown in FIG. 9;
FIG. 12 is a diagram showing relationship between frequency of the
modulating wave and the phase deviation angle;
FIGS. 13 and 14 are diagrams showing respectively examples of the
frequency response characteristics of equalizers used in the
recording system shown in FIG. 2;
FIGS. 15 and 16 are diagrams respectively showing examples of the
frequency response characteristics of equalizers used in the
reproducing system shown in FIG. 5;
FIG. 17 is a block diagram of an easily conceivable embodiment of a
frequency modulating means in the recording system; and
FIG 18 is a block diagram of a preferable embodiment of the
frequency modulating means in the recording system.
First, one embodiment of a recording system according to this
invention will be described with reference to FIGS. 1 and 2.
In FIG. 1, microphones 11 and 13 for the respective first and third
channels are placed on the left and right, close to the sound
sources 10. More remote from the sound sources 10, are microphones
12 and 14 on the left and right, for the respective second and
fourth channels. The sounds from the sound sources 10 which are
picked up by the microphones 11, 12, 13 and 14 are recorded as the
first, second, third and fourth channel signals on four tracks of a
magnetic tape provided in a cutting tape recorder 15. The frequency
band of each channel signal is about 30Hz to 15Hz.
The magnetic tape on which recording has been effected is run in
the tape recorder 15 at a speed of 1/S of the recording speed
thereby reproducing the recorded four channel signals. In the
present embodiment, the running speed of the tape during
reproduction is about 1/2.7 of the recording speed. Accordingly,
frequency bands of the first to fourth channel signals reproduced
from the tape recorder 15 are compressed to 1/S (1/2.7 in the
present embodiment) of the frequency bands of the recorded
signals.
The first and second channel signals Ch1 and Ch2 respectively,
having a frequency band of 15/S Hz, played back from the tape
recorder 15 are supplied to a matrix circuit 16 and the third and
fourth channel signals Ch3 and Ch4 to a matrix circuit 17. In the
matrix circuit 16, the first and second channel signals Ch1 and Ch2
are converted into a sum signal (Ch1+Ch2) and a difference signal
(Ch1-Ch2). The sum signal (Ch1+Ch2) is supplied through an
equalizer 18 to a mixer 25. The difference signal (Ch1-Ch2) is
supplied through an equalizer 19 to a frequency modulator 22. At
the frequency modulator 22, the difference signal (Ch1-Ch2)
frequency modulates a carrier frequency generated by a single local
oscillator 24. The frequency modulated difference signal F(Ch1-Ch2)
is supplied to the mixer 25. This modulated difference signal is a
signal which is frequency modulated with respect to input of the
frequency modulator 22 and is phase modulated with respect to input
of the equalizer 19. Accordingly, this modulated signal will
hereafter be referred to as an angle modulated difference
signal.
The sum signal (Ch1+Ch2) supplied to the mixer 25 has a band which
is shown by a broken line a in FIG. 3A. The angle modulated
difference signal (Ch1-Ch2) supplied to the mixer 25 has a band
shown by a broken line b in FIG. 3A. The frequency band b is higher
than the upper frequency limit of the band a. Accordingly, the sum
signal having the band a and the modulated difference signal having
the band b are multiplexed together at the mixer 25 without
overlapping each other.
Similarly, the third and fourth channel signals Ch3 and Ch4
respectively, having a frequency band of 15/S Hz, are converted
into a sum signal (Ch3+Ch4) and a difference signal (Ch3-Ch4) in
the matrix circuit 17. The sum signal (Ch3+Ch4) is supplied through
an equalizer 20 to a mixer 26. The difference signal (Ch3-Ch4) is
supplied through an equalizer 21 to a frequency modulator 23. At
the frequency modulator 23, the difference signal (Ch3-Ch4)
frequency modulates a carrier frequency generated by the local
oscillator 24, which carrier is the same as the one supplied to the
modulator 22. This frequency modulated difference signal F(Ch3-Ch4)
is supplied to the mixer 26. This modulated difference signal is
referred to as an angle modulated difference signal. The sum signal
(Ch3+Ch4) supplied to the mixer 26 has a band shown by a broken
line c in FIG. 3B. The angle modulated difference signal F(Ch3-Ch4)
supplied to the mixer 26 has a band shown by a broken line d in
FIG. 3B. The frequency of band d is higher than the upper frequency
limit of the band c. Accordingly, the sum signal having the band c
and the modulated difference signal having the band d are
multiplexed together at the mixer 26 without overlapping each
other. The equalizers 18 and 20 may be, for example, of a normal
type having RIAA characteristic.
The multiplexed signals [(Ch1+Ch2) + F(Ch1-Ch2)] and [(Ch3+Ch4) +
F(Ch3-Ch4)] from the mixers 25 and 26 are respectively amplified at
cutting amplifiers 27 and 28. The amplified multiplexed signals are
applied to a 45-45 system cutter 29. A lacquer disc 30, which is
used for original disc recording, is rotated at a speed of 1/S of a
normal rotating speed of a record disc (for example 331/3 rpm.).
The signals applied to the cutter 29 are recorded by a cutting
stylus 31 in a single spiral groove on the lacquer disc 30.
As in a conventional two channel stereo record disc, the cutting
stylus 31 cuts, as shown in FIG. 4, a so-called 45-45 system groove
32 on the lacquer disc 30. The groove 32 has walls 33 and 34
respectively inclining at an angle of 45.degree. with respect to a
perpendicular passing through the deepest point of the groove 32.
The multiplexed signal of the first and second channels [(Ch1+Ch2)
+ F(Ch1-Ch2)] is recorded on the left wall 33 (corresponding to a
wall where a left channel signal of the conventional two channel
stereo record is recorded) of the groove 32. This signal will
hereafter be referred to as a L channel multiplexed signal.
Similarly, the multiplexed signal of the third and fourth channels
[(Ch3+Ch4) + F(Ch3-Ch4)] is recorded on the right wall 34
(corresponding to a wall where a right channel signal of the
conventional two channel stereo record is recorded) of the groove
32. This signal will hereafter be referred to as a R channel
multiplexed signal. Accordingly, the signals of four channels are
simultaneously recorded in the single groove of the lacquer disc
30.
It is to be noted that the angle modulated difference signals
F(Ch1-Ch2) and F(Ch3-Ch4) should be maintained at a level of 1/10,
i.e., -20 dB, of the direct wave sum signals (Ch1+Ch2) and
(Ch3+Ch4).
A maximum operable frequency of the cutter 29 generally is about 17
Hz. According to this invention, each channel signal is compressed
in its band as described hereinabove, whereby the maximum frequency
of the angle modulated difference signals F(Ch1-Ch2) and F(Ch3-Ch4)
is limited to approximately 15 Hz. The cutter 29 is capable of
recording these angle modulated signals.
As an alternative method to effect compression of the frequency
bands employing the tape recorder 15, the first to fourth channel
signals collected from the sound sources 10 are supplied directly
to the matrix circuits 16 and 17 and processed similar to that
described hereinabove. The output signals from the mixers 25 and 26
are recorded on a magnetic tape by means of the tape recorder. In
this case, the magnetic tape is run at a speed, of 1/S of the
recording speed and the reproduced multiplexed signals which are
compressed in its frequency bands are recorded on the lacquer disc
30.
If, however, the difference signal is frequency modulated for the
purpose of multiplexing with the sum signal before it is compressed
in its band, the maximum frequency of the frequency modulated
difference signal will reach as high as 45Hz. There are generally
acicular crystals of magnetic material distributed at random on a
magnetic surface of a magnetic tape. As a result, if, for example,
an accurate sine wave is recorded on the magnetic tape, the
waveform of the signal reproduced from the tape is different in
each cycle, and a signal distortion is generated. Consequently, if
this signal is frequency demodulated, a white noise is produced. It
is, therefore, undesirable to effect recording and reproducing of
the frequency modulated signal on the magnetic tape for band
compression. It follows from this that the embodiment which has
been described with reference to FIGS. 1 and 2 is a preferable
embodiment.
FIG. 5 is a block diagram showing one embodiment of a reproducing
system, according to the invention. A record disc 50 is formed by a
conventional method, using the lacquer disc 30 into which the
groove has been cut and the four channel signals have been recorded
as hereinabove described. The record disc 50 has a groove of the
same shape as that of the lacquer disc 30. The record disc 50 is
placed on a turntable of a record player and rotated at a regular
rotating speed (for example 33 1/3 rpm) which is S times as fast as
the speed of the lacquer disc 30.
A stylus 52 of a pickup cartridge 51 contacts the groove of the
record disc 50 to reproduce the recorded signals. In the present
embodiment, the reproducing frequency characteristic of the pickup
cartridge 51 is 20Hz to 45Hz, and the stylus pressure of the
cartridge is 1.5 grams. Since the record 50 is rotated at a regular
rotating speed, the frequency characteristics of the signals is
reproduced respectively from both walls of the groove of the record
disc 50, as shown by solid lines in FIGS. 3A and 3B.
More specifically, the multiplexed signal reproduced from the left
wall of the groove consists of the direct wave sum signal (Ch1+Ch2)
shown by the solid line A in FIG. 3A and the angle modulated
difference signal F(Ch1-Ch2) shown by the solid line B in FIG. 3A.
Similarly, the multiplexed signal reproduced from the right wall of
the groove consists of the direct wave sum signal (Ch3+Ch4) shown
by the solid line C in FIG. 3B and the angle modulated difference
signal F(Ch3-Ch4) shown by the solid line D in FIG. 3B. Upper
frequency limits of the direct wave sum signals (Ch1+Ch2) and
(Ch3+Ch4) are respectively approximately 15Hz. Center carrier
frequencies of the angle modulated difference signals F(Ch1-Ch2)
and F(Ch3-Ch4) are respectively 30Hz. The upper frequency limits of
their frequency deviations are respectively 45Hz. The lower
frequency limits thereof are respectively 20Hz. The lower limits of
the frequency deviations are respectively cut by 5Hz, thereby to
limit their upper limits to 45Hz.
The multiplexed signal [(Ch1+Ch2) + F(Ch1-Ch2)] reproduced from the
pickup cartridge stylus 52 is supplied to a low-pass filter 53 and
a band-pass filter 54. The sum signal (Ch1+Ch2) filtered through
the low-pass filter 53 is supplied to a matrix circuit 58 through
an equalizer 57. The angle modulated difference signal F(Ch1-Ch2)
output from the band-pass filter 54 is demodulated by a demodulator
59. The demodulated signal (Ch1-Ch2) is supplied to the matrix
circuit 58 through an equalizer 60. Similarly, the reproduced
multiplexed signal [(Ch3+Ch4) + F(Ch3-Ch4)] is supplied to a
low-pass filter 55 and a band-pass filter 56. The sum signal
(Ch3+Ch4) filtered through the low-pass filter 55 is supplied to a
matrix circuit 62 through an equalizer 61. The angle modulated
difference signal F(Ch3-Ch4) output from the band-pass filter 56 is
demodulated by a demodulator 63. This demodulated signal (Ch3-Ch4)
is supplied to the matrix circuit 62 through an equalizer 64.
The matrix circuit 58 effects the following matrix operation;
1/2[(Ch1+Ch2) + (Ch1-Ch2)] = Ch1
1/2[(Ch1+Ch2) - (Ch1-Ch2)] = Ch2
Accordingly, the first and second channel signals Ch1 and Ch2 are
taken out of the matrix circuit 58. Similarly, the third and fourth
channel signals Ch3 and Ch4 are separately taken out of the matrix
circuit 62. The first to fourth channel signals Ch1 to Ch4 are
respectively amplified at amplifiers 65, 66, 67 and 68, and then
taken out of output terminals 69a, 70a, 71a and 72a as signals
having frequency bands of 30Hz to 15Hz.
The first to fourth channel signals output from the terminals 69a
to 72a are respectively supplied to speaker input terminals 69b,
70b, 71b and 72b, shown in FIG. 6. Speakers 73, 74, 75 and 76, for
the first to fourth channel signals, reproduce the sound of the
first to fourth channel signals. The speakers 73 and 74 for the
first and second channels are respectively placed at the left front
and the left rear of a listener 77 speakers 75 and 76 for the third
and fourth channels are respectively placed at the right front and
right rear of the listener 77.
In this case, six sound planes are formed, i.e., the planes between
the speakers 73 and 75, 73 and 74, 74 and 76, 75 and 76, 73 and 76,
and 75 and 74 as shown respectively by broken lines 78, 79, 80, 81,
82 and 83. Accordingly, the sound source reproductivity of the
system according to the invention is six times as large as the
conventional two channel stereo reproducing system in which only
one sound plane is formed.
If the record disc 50, constructed as hereinabove described, is
played back in a conventional two channel stereo record reproducing
apparatus, the angle modulated signal will be cut off. Only the
direct wave signal will be reproduced. Accordingly, the sum signal
of the first and second channels will be reproduced from a left
speaker and the sum signal of the third and fourth channels from a
right speaker. Conversely, if a conventional two channel stereo
record disc is played back in the four channel record reproducing
apparatus according to this invention, a left channel signal will
be reproduced from the speakers 73 and 74 and a right channel
signal from the speakers 75 and 76. Accordingly, the four channel
record disc and the reproducing apparatus therefor according to
this invention are completely compatible with the conventional two
channel stereo record disc and the reproducing apparatus
therefor.
Further, in the embodiment hereinabove described, the four channel
signals are obtained by microphones placed as shown in FIG. 1. The
signals are reproduced by speakers placed as shown in FIG. 6. In
the system according to this invention, however, arrangements of
the microphones and speakers are not limited to those shown in the
aforementioned figures.
For example, among the microphones 11 to 14 shown in FIG. 1, the
microphones 12 and 14 may be interchanged with each other. In this
case, the speakers 74 and 76 among the speakers 73 to 76 shown in
FIG. 6 must of course be interchanged with each other. In addition,
multiplexed signals recorded and reproduced in this case are, if
placed as illustrated with reference to FIG. 6, a multiplexed
signal of a sum and difference signals of channels corresponding to
the left front and right rear positions of the listener 77. A
multiplexed signal of a sum and difference signals of channels
corresponding to the right front and left rear positions.
Furthermore, arrangements of the microphones 11 to 14 and the
speakers 73 to 76 can be conveniently altered.
In multiplexing, the first and second (or third and fourth) channel
signals Ch1 and Ch2 (Ch3 and Ch4) are recorded on one wall of the
record groove. It is easily conceivable that only the second (or
fourth) channel signal Ch2 (Ch4) is frequency modulated and
multiplexed with the first (third) channel signal Ch1 (Ch3). If,
however, a four channel record disc is played back in the
conventional two channel stereo reproducing apparatus, the second
(fourth) channel signal is not reproduced at all. Accordingly, when
each channel signal has a separate sound information, the
reproduced sound will be extremely unnatural and incomplete.
Therefore, there is no compatibility in this easily conceivable
system. In the system according to this invention, a complete
compatibility is assured by effecting a matrix operation.
Nextly, the second reason for effecting the aforementioned matrix
operation in the system according to this invention will be
explained. When a signal formed by multiplexing an angle modulated
signal with a direct wave signal is transmitted, there is a
signal-to-noise ratio deterioration. Distortion of the demodulated
carrier will generally occur, which in turn will cause disturbance
and noise from the direct wave signal in the demodulated
signal.
The radius of tip of a playing back stylus for contacting the
record disc is practicably 5.mu. at the minimum, so as not to
injure the groove of the record disc. On the other hand, a half
wave length of a carrier having a superaudible frequency of 30Hz
is, for example, 3.5.mu. measured at the innermost groove having a
diameter of 120mm of a 331/3 rpm record disc. Consequently, the
amplitude of the carrier should be below a certain value in order
to be reproduced by the stylus having a radius of 5.mu.. In other
words, the amount of energy of the carrier for cutting on the
record disc cannot exceed a certain value. The energy of the
carrier is dependent upon the frequency and amplitude of the
carrier. Further, a record disc has inherent factors for generating
noise due to uneven plating in its production process, material of
record disc etc. Accordingly, the signal-to-noise ratio of the
reproduced carrier becomes a limited value.
Further, if there is a strong high frequency component in the
direct wave signal, its influence upon the carrier cannot be
ignored. The carrier is disturbed to a considerable degree by the
direct wave signal.
In the system according to this invention, noise is diminished by
applying the matrix operation to the channel signals. The reason
therefor will be set forth hereinbelow. Noises contained in the
signals recorded on and reproduced from the left and right walls of
the groove are respectively designated as L.sub.N and R.sub.N.
If, as described only the signals Ch3 and Ch4 are angle modulated
and recorded and reproduced after being multiplexed with the
signals Ch1 and Ch2, respectively, each reproduced channel signals
Ch1' to Ch4' are represented as follows;
Ch1' = Ch1
Ch2' = Ch2+L.sub.N
Ch3' = Ch3
Ch4' = Ch4+R.sub.N
On the other hand, in the system according to this invention, the
difference signals (Ch1-Ch2) and (Ch3-Ch4) are angle modulated and
multiplexed with the sum signals (Ch1+Ch2) and (Ch3+Ch4).
Accordingly, reproduced signals are represented by (Ch1+Ch2),
(Ch1-Ch2+L.sub.N), (Ch3+Ch4) and (Ch3-Ch4+R.sub.N). Therefore, each
reproduced signal Ch1' to Ch4' is represented as follows;
Ch1' = 1/2[(Ch1+Ch2) + (Ch1-Ch2+L.sub.N)] = Ch1+ L.sub.N /2
Ch2' = 1/2[(Ch1+Ch2) - (Ch1-Ch2+L.sub.N)] = Ch2- L.sub.N /2
Ch3' 1/2[(Ch3+Ch4) + (Ch3-Ch4+R.sub.N)] = Ch3+ R.sub.N /2
Ch4' 1/2[(Ch3+Ch4) - (Ch3-Ch4+R.sub.N)] = Ch4- R.sub.N /2
Accordingly, in the system according to this invention, noise is
likely to occur in the second and fourth channels, where it is
decreased by one-half, as compared to a system in which no matrix
operation is effected. Moreover, noise components are in opposite
phase between the first and second channels whereby an auditory
sensitivity to the noise becomes very low.
If the noise signal L.sub.N is amplified K times and applied to a
speaker having impedance of R.OMEGA., electric power P.sub.1
applied to the speaker in the system in which no matrix operation
is effected will be as follows:
P.sub.1 = (K.sup.. L.sub.N).sup.2 /R = (K.sup.2 /R ).sup.. L.sub.N
2
In the system according to this invention, electric power P.sub.2
applied to two speakers will be as follows:
Accordingly, sound output of the noise component will be halved by
effecting matrix operation as in the system according to this
invention.
Nextly, in case two angle modulated waves are transmitted by
separate transmission systems, a beat will generally be produced
between the carriers of the transmitted frequency modulated
signals, if there is crosstalk between the two transmission
systems. Beats will also be produced between the carrier and the
side band and between the side bands. Accordingly, if there is
crosstalk between the L and R channel multiplexed signals on both
walls of the groove in recording and reproducing on the
aforementioned four channel record groove, disturbing noises will
be generated by the beats. The system according to this invention,
is so constructed that the four channel record will be put into
practical use. The detail of this construction will be described
hereinbelow.
In FIG. 7, vectors EF and FG show movements of left and right
channels of the cutter. Vector EH and HJ show sensitivity axes of
left and right channels of the pickup stylus. If there is
discrepancy between the cutter and the pickup stylus by an angle
.delta., as shown in the figure, a crosstalk in the amount shown by
a crosstalk ratio C.sub.t, of the following equation, will be
generated;
C.sub.t .apprxeq. 20 log [(1 /sin .delta.)] (dB)
where .delta.<< 1 radian.
It will be understood that if the angle .delta. is for example 0.03
radian, the crosstalk ratio C.sub.t is about 30dB.
Further, in actual pickup operation, crosstalks will also occur due
to a partial deformation of the record groove or the construction
of vibration system of the pickup. Accordingly, it is difficult to
confine crosstalk ratio to 20 dB or above as a whole in recording
on and reproducing from the record groove of the 45-45 system.
Nextly, the reason why a disturbing noise is produced in a
reproduced signal due to crosstalk will be described. This
disturbing noise brings a bad effect to the reproduced sound, which
is different kind from a bad effect produced by crosstalk occurring
during recording and reproducing on a 45-45 system, two channel
stereo record, with respect to the orientations of the left and
right sound sources. As a cause for mutual interference between L
and R channel signals, the beat between the carriers of both
channels is the greatest one. In addition to this, there are also
beats which occur between the carrier of one channel and the side
band of the other channel and, though small in degree, between the
side bands of both channels.
For the sake of brevity, it is assumed that one, of separate but
mutually crosstalking separate L and R channel signals, is a
frequency modulated signal, and the other is a signal having a
constant frequency which is the same as the carrier of the
frequency modulated wave.
In case the crosstalk occurs between the separate channels, side
band component of the frequency modulated wave included in the
reproduced signal of each channel is affected by the crosstalk. As
a result, a disturbing noise is produced in a reproduced signal
obtained by demodulating the frequency modulated wave of each
channel signal. This relation will now be described with reference
to a vector diagram.
The frequency modulated wave of one channel is shown by vector Y,
and the wave having the constant frequency of the other channel is
shown by vector X. FIG. 8 is a vector diagram showing the
composition of the vectors X and Y caused by the crosstalk. In the
diagram, the arrow in a spiral form shows that the phase angle of
the carrier of the frequency modulated wave is rotated by the
modulating wave. The waveform of the wave represented by the vector
Y is shown in FIG. 10A, and the waveform of the composite vector Z
of the vectors X and Y is shown in FIG. 10B.
In FIG. 8, vector Z represents the composite vector of the vector X
and the vector Y. The tip of vector Z, which results from the
crosstalk between the left and right channels, moves around a
circle Ys as the vector Y is rotated. The deviation angle .alpha.
between the vector X and the vector Z changes as the vector Y is
rotated.
Accordingly, the variation of phase deviation in the composite wave
represented by the vector Z is different from the variation of
phase deviation in an single frequency modulated wave due to
presence of the deviation angle .alpha.. Consequently, harmonics of
higher orders of the modulating wave is contained in the reproduced
signal which is obtained after demodulation. These harmonics
generate the disturbing noises. The larger the phase deviation
angle .theta. of the crosstalking frequency modulated wave (the
rotating angle of the vector Y), the larger the amount of
disturbing noise thus generated in the reproduced signal. The above
description has been made on the assumption that the vector X and
the vector Y have the same carrier frequency. If these vectors have
different frequencies, interference noises will be generated
between the carriers and between the carrier and the side band. The
level of this interference noise is much higher than the
aforementioned disturbing noise. Accordingly, it is an essential
condition that the separate channel signals have the same carrier
frequency.
Further, in a frequency modulated wave, modulation index mf is
shown by the relation mf=(.DELTA.f/fm). The maximum frequency
deviation .DELTA.f is a constant value which is independent of the
modulating wave frequency fm, if the level of the modulating wave
is constant. The modulation index mf is varied inversely with the
modulating wave frequency fm. Accordingly, the phase deviation
angle of the frequency modulated wave is in inverse proportion to
the frequency of the modulating wave. The inverse proportion is
expressed by the straight line of -6 dB/oct as shown in FIG. 12. In
case the level of the modulating frequency is constant, the
deviation angle will become 1/2 if the modulating frequency is
doubled.
Consequently, in case the frequency modulated wave in each separate
channel has been frequency modulated by a modulating wave having a
low frequency component, the phase deviation angle .theta. of the
frequency modulated wave becomes large. A great deal of disturbing
noise is generated in the reproduced signal due to crosstalk
between the two channel digits.
It will be understood from the above description that the
disturbing noise may be reduced if the phase deviation angle of the
frequency modulated wave determined by the modulating wave is made
small. This relation will further be considered with reference to
FIG. 9.
In FIG. 9, the length of vector X is designated as m, the length of
vector Y as n, the length of vector Z as l the phase deviation
angle of vector Y as .theta. and the deviation angle of vector Z as
.alpha.. Then there is a relation
From Equation (1), Equations
l = .+-..sqroot.(m + ncos.theta.).sup.2 + n.sup.2 sin.sup.2 .theta.
(3)
are obtained.
The FIG. 11A shows a relation between the phase deviation angle
.theta. of the vector Y and the angle .alpha. of the vector Z
obtained by the above Equations (2) and (3). FIG. 11B shows the
relation between the phase deviation angle .theta. of the vector Y
and the length l of the vector Z. In FIG. 11A, if
.vertline..theta..vertline.<.pi./2, then .theta..varies..alpha.,
i.e., the phase deviation angle .theta. will substantially be
proportional to the deviation angle .alpha..
As described hereinabove, within a range where the phase deviation
angle .theta. and the deviation angle .alpha. are proportional, a
crosstalk may occur between separate channel signals. This
crosstalk will not produce a higher harmonic component of the
modulating wave in the reproduced signal of the other channel.
Accordingly, it will not produce a disturbing noise. The above
description has been made on the assumption that the direction of
the vector Y and the vector X is the same during the time when they
are not modulated.
In the system according to this invention, it is intended that the
same phase deviation angle .theta. is given to the angle modulated
waves of separate channels. Crosstalk which may inevitably occur
between separate channel signals will not produce a disturbing
noise in the reproduced sound. The phase deviation angle .theta.
satisfies the aforementioned relation .theta..varies..alpha. in a
standard level of the modulating wave in a frequency band ranging
over several hundreds to several thousands Hz, which is relatively
high in auditory sensitivity. This purpose of the invention is
achieved by supplying carriers having the same frequency and phase
to the frequency modulators 22 and 23 by means of the same local
oscillator 24.
In FIG. 2, the equalizers 19 and 21 have a frequency response
characteristic, as shown in FIG. 13 or FIG. 14.
In the characteristic shown in FIG. 13, the output level rises at a
slope of 6 dB per octave, as the frequency increases. Accordingly,
the frequency modulated waves which are obtained from the frequency
modulators 22 and 24 through the equalizers 19 and 21 are
substantially phase modulated waves. The phase deviation angle of
the phase modulated wave shows a constant value, which is
independent from variations of the frequency if the level of the
modulating wave is constant.
Thus, the phase deviation angle .theta. of the angle modulated wave
(which is obtained at the outputs of the frequency modulators 22
and 24) is set at a predetermined radian (for example,
approximately 0.3 to 3 rad.), when the level of the modulating wave
applied to the frequency modulators 22 and 24 is a standard level.
Then, the crosstalk which may occur will not finally produce a
disturbing noise in the reproduced signal.
In the characteristic shown in FIG. 14 as another example of
characteristic of the equalizers 19 and 21, the characteristic is
flat within a low frequency range. Sounds which are usually
recorded on a record disc scarecely contain a sound of very low
frequency component. Even if there is such very low frequency
component, its energy is very small. Further, the phase deviation
angle of the angle modulated wave modulated by a modulating wave
having a frequency corresponding to the ultra low sound frequency
range, may be large. The result is a generation of the disturbing
noise in the reproduced sound. The frequency component of the
disturbing noise is within a range where an auditory detecting
ability is relatively low. Therefore, the signal-to-noise ratio in
a low frequency range is improved by making the characteristic flat
below a suitable frequency, which is shown by point T in FIG.
14.
As a frequency at the point T, a suitably frequency within the
range of 100 to 2,000 Hz is selected. An experiment shows that a
good result is obtained when the frequency around 800 Hz is
selected.
In the reproducing system shown in FIG. 5, the equalizers 60 and 64
have a frequency response characteristic shown in FIG. 15 or FIG.
16. If the equalizers 19 and 20 have the characteristic shown in
FIG. 13, the equalizers 60 and 64 have the characteristic as shown
in FIG. 15 which is inverse of the characteristic shown in FIG. 13.
If the equalizers 19 and 21 have the characteristic shown in FIG.
14, the equalizers 60 and 64 have the characteristic as shown in
FIG. 16 which is the inverse of the characteristic shown in FIG.
14.
As described hereinabove, in the system according to this
invention, the phase deviation angle of the angle modulated wave
(obtained by angle modulation by a modulating wave at a reference
level) is set at a predetermined radian. This deviation angle is
set irrespective of the modulating wave frequency, particularly in
a high and intermediate frequency ranges where auditory sensitivity
is high. Accordingly, no disturbing noise is produced in the
reproduced signal when the composite signal containing a crosstalk
component is demodulated.
In case the level of the input modulating wave exceeds the standard
level, a harmonic component of higher order is developed in the
reproduced signal obtained by demodulating the composite signal of
the angle modulated wave and the crosstalk component. The harmonic
component, of higher order in the reproduced signal obtained by
demodulation, is reduced to an accoustically undetectable level by
the equalizers 60 and 64, which have characteristics for
attenuating signals in high frequency range. Accordingly, even in
case the level of input modulating wave exceeds the standard level,
no disturbing noise is produced in the reproduced signal.
The relative speed of a pickup stylus, with respect to a record
disc, is several hundred mm/sec. The size of dust particles which
may fall in the groove of the record disc is several tens .mu..
Hence, most frequency components of noise signals generated by such
dust particles are distributed within a high frequency range.
Accordingly, the signal-to-noise ratio is improved by reducing the
frequency characteristic of the modulated wave in a high frequency
range during reproduction of the signal.
The system according to this invention is further considered from
the aspect of transmission energy. A 100 percent-modulation in an
amplitude modulation system corresponds to the case in which the
phase deviation angle of the angle modulated wave is set at 1
radian in the system according to this invention. Accordingly, the
signal-to-noise ratio is improved by about 20 dB in a high
frequency range, as and about 40 dB in a low frequency range
compared with the amplitude modulation system.
In the aforementioned embodiment of the recording system, the
combination of the frequency modulators 22 and 24 and the
equalizers 19 and 21 is employed as a modulating means. However,
the modulating means is not limited to this, but a modulator may be
employed having a characteristic providing an angle modulated wave
having a constant phase deviation angle relative to an input
modulating wave. The deviation is at a certain level within a range
above several hundred Hz.
Nextly, a specific embodiment of a frequency modulating means
consisting of the frequency modulators 22 and 23 and the local
oscillator 24 which are adopted in the recording system shown in
FIG. 2 will be described.
FIG. 17 shows an example of a frequency modulating means which is
readily conceivable from the conventional prior art. Difference
signals (Ch1-Ch2) and (Ch3-Ch4) are respectively supplied from
input terminals 100 and 101 to mixers 108 and 109 through reactance
tubes 102 and 103, oscillating circuits 104 and 105, and buffer
amplifiers 106 and 107. At the mixers 108 and 109, the difference
signals are mixed with a signal from an oscillator 110. Outputs
from the mixers 108 and 109 are taken out of output terminals 113
and 114 through filters 111 and 112.
The easily conceivable frequency modulating means of the
aforementioned construction has the separate oscillating circuits
104 and 105 in the two channels. It is difficult to synchronize
these two oscillating circuits 104 and 105. If, for example, there
is no input at the terminal 100 and there is an input at the
terminal 101, the frequency of the oscillating circuit 105 deviates
in accordance with the input at the terminal 101. At the same time,
the frequency of the oscillating circuit 104 is also deviated by
the oscillating circuit 105 notwithstanding the absence of input at
the terminal 100, which produces an undesirable result. It is,
therefore, very difficult to control and synchronize this system so
as to obtain carriers having the same frequency and phase from the
terminals 113 and 114.
In the system according to this invention, it is necessary, to have
a modulator which is capable of obtaining signals of two channels
having the same carrier frequency and phase, in order to prevent
distortion and disturbing noise due to crosstalk in each channel
signal. Accordingly, the aforementioned modulating means which is
easily conceivable from the prior art is of no practical use in the
system according to this invention.
Then, in the system according to this invention, the frequency
modulating means shown in FIG. 18 is employed as a preferable
embodiment. A master oscillator 120 includes a crystal oscillator
with a stable output. Output from the oscillator 120 is supplied,
on one hand, to a frequency divider 121 at which the frequency is
divided into a frequency of q/p times. A saw tooth generator 122
generates a saw tooth wave responsive to the output of the
frequency divider 121. The output of the saw tooth generator 122 is
supplied as a carrier to frequency modulators 127 and 128. The
frequency modulators 127 and 128 respectively consist of an
equalizer having a characteristic response that increases in a low
frequency range and a phase modulator. The difference signals
(Ch1-Ch2) and (Ch3-Ch4) are applied to input terminals 123 and 124,
which are respectively supplied to the modulators 127 and 128 after
being amplified by amplifiers 125 and 126.
The modulators 127 and 128 are serrasoid modulators for effecting
modulation without changing the phase of the carrier. It is to be
noted here that, with this modulation, a carrier and a sideband
which compose a frequency modulated wave are separately considered.
The sideband is changed by a modulating wave and the carrier is
changed only in its level. When the modulation index is large, the
phase of the carrier changes by 180.degree.. Here, the level of the
carrier has changed to negative and the phase of the carrier
remains unchanged.
The output frequency modulated signals from the modulators 127 and
128, are multiplied in the frequency thereof by frequency
multipliers 129 and 130. This increases the modulation degree. The
resulting signal is supplied to mixing detectors 131 and 132. At
the same time, the output frequency from the master oscillator 120
is supplied to the mixing detectors 131 and 132. Obtained from the
mixing detectors 131 and 132 is an output signal which has been
beat down to a frequency which is the difference between the output
frequency from the oscillator 120 and the frequency of output
signals from the frequency multipliers 129 and 130. This output
signal is taken out of output terminals 135 and 136 through filters
133 and 134.
According to the frequency modulator means of the aforementioned
construction, the signals which actuate the modulators 127 and 128
and the signals which are supplied to the mixing detectors 131 and
132 are all maintained at the same frequency and in the same phase.
Accordingly, it is possible to transmit the angle modulated wave
without producing disturbing noises due to crosstalk, even in
transmission systems in which crosstalk occurs between the L and R
channels.
The modulation angle obtained from the serrasoid modulators 127 and
128 is on the order of .+-.1.5 radian. It is theoretically
impossible to obtain a modulation angle of more than .+-.3 radian.
Accordingly, a greater modulation angle is obtained by increasing
the modulation degree by means of the frequency multipliers 129 and
130.
If it is desired to obtain carriers of the output frequency
modulated signals from the terminals 135 and 136 at a low
frequency, the signals must be beat down after being multiplied by
the frequency multipliers 129 and 130. This will be understood from
the fact that the frequency of the saw tooth wave at the serrasoid
modulators must be selected at a frequency at least five times as
high as a maximum modulating frequency in order to avoid the beats
which will otherwise occur between the saw-tooth wave and the
modulating wave if the saw tooth is selected at a low
frequency.
If an oscillator has an unstable output frequency and its output
has already been modulated by noises such as hum, instead of the
stable master oscillator 120 which supply the signal to the mixing
detectors 131 and 132, the signal-to-noise ratio will extremely be
deteriorated. In this case, the signal-to -noise ratio will be
deteriorated in proportion to the multiplication of the frequency
multiplier. If, for example, the frequency is multiplied 81-times,
the signal-to-noise ratio will be deteriorated by a factor of
81.
In the system according to this invention, the output of the master
oscillator 120 is multiplied after being frequency divided and
frequency modulated. This multiplied output is beat down with the
output from the master oscillator 120. As a result, the variation
in the output of the master oscillator 120 is mostly cancelled by
being beat down. Accordingly, even if the output of the master
oscillator 120 is modulated in AC, there is not much variation in
the beat down output and hence no deterioration in the
signal-to-noise ratio.
This invention is not limited to the embodiments hereinabove
described but many modifications and variations may be made without
departing from the spirit and scope of the invention.
* * * * *