U.S. patent number 3,789,137 [Application Number 05/241,944] was granted by the patent office on 1974-01-29 for time compression of audio signals.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to George F. Newell.
United States Patent |
3,789,137 |
Newell |
January 29, 1974 |
TIME COMPRESSION OF AUDIO SIGNALS
Abstract
The invention pertains to a technique for time-compressing audio
signals without disturbing the relative position of the audio
samples through the use of a time buffer store and a subsequent FM
recording of the time-compressed audio signal in a format similar
to video signals and the subsequent demodulation of the FM signals
and time expansion of the signals to produce the original audio
signals. The use of FM modulation and demodulation of a type
similar to that used with video information for recording and
transmitting time-compressed audio information permits common
processing of audio and video information and the utilization of
standard video transmission and video tape and video disc recorders
to accommodate audio as well as video information.
Inventors: |
Newell; George F. (Pittsburgh,
PA) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
22912830 |
Appl.
No.: |
05/241,944 |
Filed: |
April 7, 1972 |
Current U.S.
Class: |
386/201; 386/328;
G9B/20.001; 386/E5.042; 348/E7.03; 348/E7.091; 386/E9.045; 704/205;
704/211; 348/484; 360/8 |
Current CPC
Class: |
H04N
7/002 (20130101); H04N 9/802 (20130101); H04N
7/087 (20130101); H04N 5/781 (20130101); G11B
20/00007 (20130101) |
Current International
Class: |
G11B
20/00 (20060101); H04N 7/00 (20060101); H04N
9/802 (20060101); H04N 7/087 (20060101); H04N
5/781 (20060101); H04n 005/76 () |
Field of
Search: |
;178/5.6,5.8,5.4CD,6.6A,DIG.3,6.6DD ;179/15.55T,1.2S |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Springborn; Harvey E.
Attorney, Agent or Firm: F. H. Henson et al.
Claims
I claim:
1. Apparatus for achieving time/bandwidth exchange to convert audio
signals of audio bandwidth into time-compressed audio signals of
video bandwidth for recording and transmission on video apparatus
comprising, first means for time compressing audio input signals of
an audio bandwidth into time-compressed audio signals of video
bandwidth without significant loss of information content, second
means operatively connected to said first means and adapted to
respond to said time-compressed audio signals by dividing said
time-compressed audio signals into spaced apart segments wherein
each segment is a duration substantially equivalent to the active
portion of a television line, and third means operatively connected
to said second means for inserting television synchronization
information into the spaces between said segments to produce audio
information in a waveform substantially identical to a video
composite waveform.
2. Apparatus as claimed in claim 1 including pedestal voltage means
for superimposing said segments of time-compressed audio signals on
a pedestal voltage and amplitude control means for adjusting the
amplitude of said time-compressed audio signals within limits
compatible with video recording and transmission apparatus.
3. Apparatus as claimed in claim 1 further including video
recording means operatively connected to said third means for
recording said audio information and fourth means operatively
connected to said video recording means for time expanding said
time-compressed audio signals into audio signals of audio bandwidth
for reproduction on audio apparatus.
4. Apparatus as claimed in claim 1 further including video
recording means adapted to respond to video information in a video
composite waveform and to said audio information to record said
audio information and said video information on said video
recording means.
5. Apparatus as claimed in claim 4 further including time
multiplexing means operatively connected to said video recording
means to time multiplex record said audio information and said
video information.
6. Apparatus as claimed in claim 4 further including means
operatively connected to said video recording means for separating
said audio information and said video information during playback,
further including means for displaying said video information on
video apparatus and means for time expanding said time-compressed
audio signals to produce audio signals of audio bandwidth for
reproduction on audio apparatus.
7. A method for achieving time/bandwidth exchange to convert audio
signals of audio bandwidth into time-compressed audio signals of
video bandwidth for recording and transmission on video apparatus
including the steps of, time-compressing audio input signals of
audio bandwidth into time-compressed audio signals of video
bandwidth, dividing said time-compressed audio signals into spaced
apart segments wherein each segment is of a duration substantially
equivalent to the active portion of a television line, and
inserting television synchronization information into spaces
between said segments to produce audio information in a waveform
substantially identical to the video composite waveform.
8. A method as claimed in claim 7 including the step of
transmitting the audio information on video transmission
apparatus.
9. A method as claimed in claim 7 further including the step of
recording said audio information on video recording means.
10. A method as claimed in claim 9 further including the step of
recording video information in combination with said audio
information on a video recording means.
11. A method as claimed in claim 10 further including the step of
transmitting said audio information and video information on video
transmitting apparatus.
12. A method as claimed in claim 11 further including a step
following the transmission of said audio and video information
consisting of recording said audio and video information on a video
recording means.
13. A method as claimed in claim 11 wherein said audio information
and video information comprise a plurality of separate programs,
and further including the step of identifying the audio information
and video information comprising a program.
14. A method as claimed in claim 12 further including the steps of
displaying the video information of video apparatus, time expanding
said audio information into audio signals of an audio bandwidth and
reproducing said audio signals of an audio bandwidth on an audio
apparatus.
Description
BACKGROUND OF THE INVENTION
The time compression of audio waveforms permits the transmission of
audio information via a transmission channel exhibiting an
appropriately wide bandwidth in a period of time significantly
short in comparison to the duration of the original audio signal.
The most desirable format for time-compressed audio information is
that corresponding to standard video format which would permit the
transmission of audio and video signals in a time multiplex mode
and permit the use of conventional TV channels and equipment for
processing the time-compressed audio signals. The capability of
time multiplexing audio and video signals facilitates the
transmission of audio accompanied by color video stills in a time
significantly shorter than the time required for presentation of
the original audio information.
A previous invention disclosed in U. S. Pat. application Ser. No.
76,572 by W. W. Ramage, now abandoned, entitled "Signal Processing
and Reproducing Method And Apparatus" filed Sept. 29, 1970 and
assigned to the assignee of the present invention described
apparatus for time compression of an audio waveform in order for
record and replay by modified video apparatus. The Ramage
disclosure sacrifices recording density of time-compressed audio
information in that disclosure describes a technique for recording
discrete samples on an individual basis. According to the Ramage
technique the samples which are adjacent in the original audio
signals are no longer adjacent in the time-compressed waveform and
the packing density must be sufficiently low such that the
intersample interference produced by the dispersion inherent in
practical circuits and networks does not cause unacceptable
distortion when the samples are finally rearranged in their correct
relative positions. Reference is made also to U.S. Pat. No.
3,564,127 entitled "System of Band Compression for Video Signals"
issued Feb. 16, 1971 to G. F. Newell and G. C. Sziklai and assigned
to the assignee of the present invention.
SUMMARY OF THE INVENTION
The invention described herein permits time-compressed audio and
video waveforms to be combined in time-multiplex form, recorded,
replayed and transmitted over a conventional television network
without modification to the network equipment. The transmitted
waveforms are received and reconverted to the original audio in
conjunction with the video waveforms. The disclosed invention is
based on the time compression of the audio signal without
disturbing the relative position of the audio samples comprising
the signal. This produces a time-compressed waveform which is an
accelerated replica of the original waveform except for
predetermined spaces occurring at regular intervals to provide a
format similar to that of video signals. The time-compressed audio
thus produced is capable of being recorded by the same FM method
used for conventional video and thus permits maximum recording
density and therefore maximum compression ratio. Furthermore, since
adjacent samples of the original audio are adjacent in the
time-compressed waveform, the effects of dispersion and bandwidth
limitations are no more serious than if the audio waveform were not
compressed and the bandwidth and frequency limitation were scaled
in proportion to the bandwidths of the time-compressed and the
original signal bandwidths.
While this disclosure relates to advantages and techniques for time
compression of audio, it is apparent that desirable advantages can
be achieved in the combination of the time-compressed audio signals
with conventional video signals. The reason for implementing the
time compression of audio in a manner to convert the audio waveform
into a video-like waveform with synchronizing signals added at
conventional TV sweep intervals is the fact that conventional TV
apparatus generally requires such synchronizing waveforms for
correct operation. As an example, a video tape recorder of the type
used in TV broadcasting search out the horizontal sweep
synchronizing pulses and vertical sweep synchronizing pulses and
uses them for the servo-control of the head-to-tape velocities and
for the control of voltage-controlled delay circuits that correct
for variations in the tape-to-head velocity. Conventional video
control circuits and transmitter circuits include stabilizing
amplifiers which use the synchronizing pulses for such purposes as
clamping the voltage excursions of video waveforms. In order for an
audio waveform to pass through such apparatus, the synchronizing
pulses must be added and the audio information confined to those
periods of the composite waveform that are conventionally occupied
by the active picture information in a video waveform.
The compression of the audio waveform and the modification
necessary to effect a video-like format is disclosed in the
following exemplary description in connection with the accompanying
drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram schematic illustrating a technique for
time-compressing audio signals;
FIG. 2 is a detail block diagram schematic of the embodiment of
FIG. 1;
FIG. 3 is a basic block diagram schematic of scheme for replaying
in compressed form the information produced by the embodiment of
FIG. 1;
FIG. 4 is a basic block diagram schematic illustrating a scheme for
replaying in the original form the time-compressed information
produced by the embodiment of FIG. 1;
FIG. 5 is a detailed schematic of the embodiment of FIG. 4;
FIG. 6 is a waveform illustration comprised of waveforms A, B and C
illustrating the conversion of the normal audio input signal into a
frequency modulated time-compressed audio signal;
FIG. 7 is a waveform illustration consisting of waveforms A, B, C,
D and E illustrating the step-by-step technique for converting the
time-compressed audio waveform into a composite waveform suitable
for transmission and recording on conventional video equipment;
FIGS. 8 and 9 in their respective waveforms, A and B, illustrate
the vertical blanking waveforms produced by the embodiment of FIG.
1, including the addition of identification codes;
FIG. 10 is a basic block diagram schematic of the technique for
removing gaps in the time-compressed signal during the recording of
said signal;
FIG. 11 is a basic block diagram schematic of the technique for
removing gaps present in the time-compressed signals during
reproduction of the time-compressed signals from the recording
media;
FIG. 12 is a block diagram schematic of a system for recording
time-compressed audio and video information;
FIG. 13 is a block diagram schematic of a system for replay of the
information recorded in the system of FIG. 12; and
FIG. 14 is a schematic illustration of a typical implementation of
the pulse discrimination of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In order to use the FM recording process conventionally used for
video signals, it is necessary for the audio signal to be
time-compressed before application to the recording head. In order
to convert the audio signal from the time-compressed space to that
of a normal audio signal, it is necessary to time expand the time
compressed. A typical manner for achieving these objectives is
described herein through the use of a time buffer store and a
magnetic disc recorder.
Referring to FIG. 1 it is illustrated in block diagram form an
audio record system 10 comprising a buffer store 12, a magnetic
disc recorder 14, timing logic 15, phase comparator 16 and a TV
waveform generator 18. The timing logic 15 responds to clock pulses
prerecorded on a track of the magnetic disc recorder 14 by
controlling the audio sampling and buffer such that input audio is
supplied to the buffer store 12 for approximately 1 rotation of the
magnetic disc, i.e., 33 milliseconds, and then the timing logic
causes the information in the buffer store 12 to be transmitted in
a period equivalent to 1 active TV line, i.e., 53 micro seconds,
for recording on the magnetic disc 14. The cycle is repeated and
the second burst of compressed audio input is recorded on the same
track in a position immediately adjacent to the first burst of
compressed audio input. As shown in waveform A of FIG. 6 the audio
input is separated into bursts of approximately 33 milliseconds and
compressed as shown in waveform B through the operation of buffer
store 12 before being used to modulate the short burst of FM signal
as shown in waveform C. The buffer store 12 can be implemented in
numerous ways including the use of a parallel-bit digital buffer
store and the sampled analog type described in BBC Engineering
Monograph No. 63, August, 1966.
Since conventional U.S. TV waveforms for synchronizing purposes are
harmonically related to a 525 line rate the clock pulses recorded
on the magnetic disc recorder 12 can be any multiple of 525 pulses
for synchronizing with the output of the TV waveform generator 18.
The phase comparator 16 serves as a servo control for maintaining
the magnetic disc rotation speed at one revolution per TV frame
period as determined by the output from the TV waveform generator
18. The phase comparator circuit 16 can be implemented through
numerous techniques well known in the television art and
elsewhere.
The audio recording system illustrated in basic block diagram form
in FIG. 1 is illustrated in detail block diagram form in FIG. 2. An
audio source 101 such as a microphone-amplifier or an audio
recorder, supplies audio signals to a low-pass filter 102 having a
cut-off frequency typically less than 6 kilohertz. The bandwidth
must be less than half the sampling rate and 5 KHz is a commonly
used audio bandwidth for AM broadcast reception. The filtered
analog signal is then applied to analog-to-digital converter 103
which in turn converts the signal to a multiple-bit digital signal,
i.e., a 7 bit digital signal, which is supplied in parallel to a
shift register 104. The analog-to-digital converter 103 and the
shift register 104 are driven by clock pulses at a rate which is
the multiple of 525, i.e., 14,750 hertz. The digital representation
of the audio signals produced by the analog-to-digital converter
103 fills the shift register 104 to its capacity, which for this
discussion, corresponds to 526 samples each of the 7 bits. When all
the positions in the shift register 104 are filled, the clock rate
for the shift register and the digital-to-analog converter 105 is
established at approximately 10 megahertz by the output of crystal
oscillator 119 for a period of time corresponding to 526 pulses.
The clocking of the shift register at 10 megahertz for 526 pulses
causes the information to be transmitted to the digital-to-analog
converter 105 which in turn reconverts the digital signal to an
analog signal which is time-compressed into a time period of
approximately 53 microseconds. The time-compressed analog audio
signal is then applied to a low-pass filter 106 having a bandwidth
suitable for the removal of sampling structure while retaining
audio information in time compressed form. The bandwidth should be
less than 4 MH.sub.z which corresponds to the video baseband
frequency limit. The time-compressed analog audio output signal
from the filter 106 is applied to frequency modulate the output of
fm oscillator circuit 107. The output of fm oscillator 107 is
subsequently applied through gate circuit 108 to the magnetic disc
recorder 14. The fm oscillator 107 generates a waveform whose
frequency is modulated in response to the amplitude of the signals
provided by the low-pass filter 106. Gate circuit 108 functions to
gate output signals from the fm oscillator circuit 107 to the
recorder 14 when enabled by a 64 microseconds pulse, which
corresponds in duration to a television line, from the pulse
generator 116. Thus, in the time interval between two audio signal
samples from the analog-to-digital converter 103, as determined by
the respective clock rates, the shift register 104 is cleared
through the digital-to-analog converter 105 and is in condition to
accept another 526 samples before being cleared again.
The timing reference for the recoridng process may typically be in
the form of a predetermined pulse train on one track of the
magnetic disc recorder 14. The prerecorded clock pulse train may
typically contain 525 pulses of which one pulse is identifiable by
virtue of amplitude or width. The clock pulse train could, of
course, be any multiple of 525 pulses and dividers used to obtain
either 31,500 hertz or 15,750 hertz for synchronizing with the
output of the TV waveform generator 18. The clock pulse train as
derived from the magnetic disc recorder is applied to pulse
discrimination circuit 111 which divides the clock pulse train into
two waveforms, the first being f.sub.1 consisting of one pulse per
rotation and the other being f.sub.2 consisting of 525 pulses per
disc rotation. The pulse discrimination circuit 111 can be
implemented through the use of a circuit which is responsive to the
width or amplitude of the singularly identifiable pulse. The
function of circuit 111 is to identify the unique pulse and provide
two outputs, an f.sub.1 output which corresponds to one pulse each
disc rotation, and an f.sub.2 output which corresponds to a train
of 525 pulses per disc rotation.
A typical implementation of a pulse discriminator circuit is
illustrated in FIG. 14. Assume a pulse train from the disc recorder
of the type A wherein the single pulse P is distinguished from the
remaining pulses as being of twice the duration. The pulse train is
applied to a differentiator circuit 310 and an integrator circuit
312. The differentiator circuit 310 responds to the leading edge of
each pulse and develops an output pulse train P1 of positive pulses
which is subsequently clipped by threshold detector 314 to produce
the f.sub.2 pulse train. The integrator circuit 312 responds to the
pulse train from the disc recorder 14 by producing an output pulse
train P2. The integration of the long duration pulse P results in a
pulse P2' exhibiting a greater amplitude than the remaining pulses
P2". Threshold detector 316 is set to respond to pulse amplitudes
greater than that of the P2" pulses thus functioning to generate an
f.sub.1 pulse in response to the P2' pulse.
The phase comparator circuit 16 functions as a servo control for
maintaining the disc rotation speed at one revolution per TV frame
period as determined by the output from the TV waveform generator
18. The single pulse f.sub.1 is used in conjunction with a "start"
pulse from start pulse circuit 110 which is applied to AND gate
127. The simultaneous presence of a "start" pulse and an f.sub.1
pulse at AND gate 127 results in the gating of signal S which
serves to reset all the counters in the recording system 10 to
establish a reference time for commencement of the recording
operation. The f.sub.2 pulses which occur at television horizontal
sweep rates are supplied via gate circuit 113 and a gate circuit
114, the latter of which is normally open, to provide clock pulses
to the analog-to-digital converter 103, and via adder circuit 120
to shift register 104. Gate circuit 113 transmits the f.sub.2 train
of pulses to gate circuit 114 except when an inhibit pulse is
supplied to it from divider circuit 124 to which it responds by
deleting one pulse from the train. Gate circuit 113 can be
implemented through the use of an exclusive OR circuit or through
the use of an AND gate which has as one input the f.sub.2 pulse
train from circuit 111 and as a second input a waveform of, say
unity amplitude from divider circuit 124 under all conditions
except during the presence of an output pulse from divider circuit
124 at which time the amplitude to zero resulting in deletion of
one pulse from the f.sub.2 train transmitted to gate circuit 114.
This inhibit pulse from divider circuit 124 must coincide with one
of the pulses from separator circuit 111. Many other circuits can
perform this function, even a simple transistor or tube that is
caused to be biased below operation level when an inhibit output
pulse is transmitted by divider circuit 124. The divider circuits
may typically consist of counters. The use of divider circuits is
illustrated in reference U.S. Pat. No. 3,564,127.
Gate circuit 114 transmits the pulse train from gate circuit 113
which consists of 525 pulses per disc rotation and feeds them to
divider circuit 115, which in turn functions to divide by 526 and
provide an output once every 526 pulses of f.sub.2. The output of
divider circuit 115 is divided by 252 in divider circuit 122 and
causes a pulse waveform generated by pulse generator 123 to inhibit
ten pulses of f.sub.2 from passing through gate circuit 114. The
output of divider circuit 122 is also divided by 2 in divider
circuit 124, and the output of divider circuit 124 as described
above is used to inhibit one pulse of f.sub.2 from passing through
gate circuit 113 once every disc rotation. Divider 115 generates an
output pulse whenever 526 of the f.sub.2 pulses have been supplied
to the shift register 104 and opens gate circuit 118 via time delay
circuit 117 to permit f.sub.3 pulses at a 10 megahertz rate from
oscillator 119 to be supplied to the digital-analog converter 105
and the shift register 104.
Gate circuit 118 functions as a buffer amplifier that is normally
switched off except when a pulse from the delay circuit 117 enables
it to transmit the 10 MHz wave-form f.sub.3 from oscillator circuit
119 to digital-to-analog converter 105 and adder circuit 120.
Having been enabled, the gate circuit 118 remains open until it has
transmitted 526 pulses of f.sub.3, at which time the divider
circuit 121 divides by 526 and delivers a disenable pulse and
closes gate circuit 118. There are many ways such a circuit can be
implemented. As one example, a bistable circuit can produce and
enable voltage when set by the pulse from delay circuit 117 and
then reset by the pulse from divider circuit 121. The enable
waveform and the pulses from oscillator circuit 119 can then be AND
gated to produce the output for digital-to-analog converter 105 and
adder circuit 120.
Divider circuit 121 responds to the 526th pulse from the output of
gate circuit 118 by closing gate 118. The termination of f.sub.3
pulses from the output of gate 118 completes one cycle of the
operation of shift register 104. This cycle of operation of shift
register 104 is repeated for 252 cycles. In each cycle, the
magnetic disc of recorder 14 rotates precisely one rotation plus a
period of time corresponding to one TV line period of 64
microseconds. Each cycle ends with a compressed audio burst being
recorded on the magnetic disc in positions equivalent to the active
portion of the conventional TV lines of recording or that used for
visual information. The conventional televison wave-form comprises
a time-multiplexed and amplitude-multiplexed combination of visual
information and synchronizing information. The line periods contain
a portion of visual information that is often referred to as the
active portion (53 microseconds). The remainder of the line period
has the visual signal blanked out and replaced by synchronizing
information.
At the conclusion of 252 cycles, the divider 122 generates an
output which is applied to pulse generator 123, the latter of which
responds by closing the gate circuit 114 for a duration
corresponding to 10 f.sub.2 pulses. The closing of gate 114 causes
the analog-to-digital converter 103 to ignore 10 samples of the
audio input and causes the magnetic disc to rotate an additional 10
line period before the next audio burst is recorded. This
unrecorded 10 line period is provided for the insertion of vertical
blanking and synchronizing signals as required for processing on
conventional TV apparatus. Following the recording of a second set
of 252 lines on the first track of the magnetic disc, the divider
124 responds to the output of divider 122 by generating a track
switch signal which is applied to the magnetic disc recorder 14 to
transfer the recording process to a second track on the magnetic
disc. The transfer of the recording process can be achieved by
supplying a pulse to a stepping motor to move the recording head to
another track or, if multiple fixed heads are used, to switch the
recording signal from the output of gate circuit 108 to a second
head on a second track. Simultaneous with the generation of a track
switch signal, divider 124 causes one pulse from the pulse train
f.sub.2 to be inhibited by providing a pulse in coincidence at the
input to the gate 113. The initiation of this inhibit signal by
divider 124 following the completion of recording on one track will
cause the gate 114 to remain closed for a duration corresponding to
eleven f.sub.2 pulses.
This recording process will result in the recording of 504 lines on
each track of the magnetic disc with two gaps, one of a 10 line
period and one of an 11 line period. The 505th line of audio will
be recorded on the first line position of the second track.
In the operation just described, the pulse generator 116 generates
the 64 microsecond pulse to open the gate circuit 108 and permit
the frequency modulated carrier resulting from the operation of fm
oscillator 107 to pass to the magnetic disc recorder 14 each time
the shift register 104 transfers its stored information to the
digital-to-analog converter 105. The time delay circuit 117
functions to establish an appropriate time delay such that the
signal from the digital-to-analog converter 105 and the filter
circuit 106 initiates frequency modulation of the carrier in the fm
oscillator circuit 107 approximately 7 microseconds after the gate
circuit 108 is opened by the output from pulse generator 116. The
operation of the pulse generator 116 and the delay circuit 117
results in the recording of a signal format on the magnetic disc
which consists of adjacent carrier bursts of 64 microseconds
duration, each burst frequency-modulated for approximately 53
microseconds with a 7 microsecond unmodulated carrier preceding
each burst and a 4 microsecond duration of unmodulated carrier at
the conclusion of each burst. This format of each burst, i.e.,
unmodulated-modulated-unmodulated, permits the phase discontinuity
occurring at the junction of the adjacent carrier bursts to be
positioned in the blanked portion on the track of the magnetic disc
which is to be occupied by horizontal syncs, thus providing
adequate time for any transients caused by the discontinuity to
decay before the modulation commences.
It is apparent that many other circuit arrangements can be devised
to perform the logic functions described in reference to the
functional operation of the embodiment disclosed in FIG. 2. One
variation would involve the changing of the rate of magnetic disc
rotation from that corresponding to a TV frame rate and
establishing it as a TV field rate and utilizing two tracks for
accomplishing the recording process described above. Furthermore,
the rate of sampling the audio input signal can be changed to
provide greater compression of audio signals having a narrower
bandwidth and less compression for a audio signals of a wider
bandwidth. A wider band audio signal can be recorded by sampling
more frequently, and the track will be filled in a shorter time.
Conversely, the track can record a narrower band audio signal by
reducing the sampling rate and the bandwidth of the audio low-pass
filter. The frequency expansion achieved with the arrangement
described above is given by the ratio of the input supply time and
the output transmission time of the shift register 104 and can be
represented as:
(33,333 .times. 10.sup.-.sup.3 + 64 .times. 10.sup.-.sup.6)/53
.times. 10.sup.-.sup.6 = 630/1
The time compression achieved is equal to the number of rotations
of the magnetic disc for one complete track recording. In the
example described above, this corresponds to 504/1.
The information thus recorded in compressed form on the magnetic
disc recorder 14 can be replayed in compressed form for
re-recording on a broadcast tape recorder by means of the
arrangement typically illustrated in block diagram form in FIG. 3.
The recorded tracks are replayed in sequence as a continuous signal
with each track requiring one thirtieth of a second for playback.
The output from the magnetic disc recorder 14 on the arrangement 20
of FIG. 3 is supplied to a processing amplifier 22 which inserts
blanking and synchronizing signals supplied from the TV waveform
generator 18 to which the magnetic disc recorder 14 is phase-locked
through the phase control 16. The processing amplifier is a circuit
used in many TV applications as, for example, camera control units
where the camera output waveform is accepted as a time sequence of
active line periods containing video information with random noise,
spikes, etc., between these periods. The processing amplifier
removes all content between the active line periods and adds
composite synchronizing and blanking waveforms to the video
information from the camera. The output is a conventional video
composite waveform. A typical processing amplifier is the Model
1085 of the Ampex Corp.
The waveforms illustrating the input audio signals leading up to
the recording on a magnetic disc are illustrated in waveforms A, B
and C of FIG. 6. The waveforms A, B, C, D and E of FIG. 7
illustrate the processing of the signal replayed from the magnetic
disc recorder and processed through the processing amplifier 22 of
FIG. 3 resulting in the composite waveform as illustrated in
waveform E of FIG. 7. The demodulation of the fm carrier is
provided by the disc recorder A which supplies an output waveform
illustrated in waveform B of FIG. 7 as an input to the processing
amplifier 22. In the waveforms A and B of FIGS. 8 and 9 there is
illustrated vertical blanking waveforms resulting from the replay
process described with reference to FIG. 3. It will be noted that
an identification code can be added at a prescribed location such
as line 525. The waveform shown in FIG. 8 represents the output
from the processing amplifier 22. FIG. 9 represents the same
waveform at the adjacent vertical synchronizing period. The
complete waveform for one TV frame includes 525 lines and two
vertical intervals.
The material recorded on the video tape recorder 24 of FIG. 3 can
consist of a large number of programs that have been individually
assembled on the magnetic disc recorder 14 and transferred at
different times. Each program can consist entirely of compressed
audio or can be comprised of interspersed frames of video and audio
to form the content of an audio-visual presentation. These programs
can be replayed from the tape for distribution over closed circuit
cable or transmitted by conventional transmitters. Individual
programs can be re-recorded on separate remote disc recorders by
arranging that each program be preceded by an identification code
which enables each program to be recognized by conventional logic
circuits.
Once the required program has been recorded on a disc recorder, or
for that matter any appropriate recording device such as magnetic
disc recorder 14, video tape recorder 24 or the remote recording
devices referred to above, the program can be replayed in its
original time duration through the use of circuit implementation
which functions in a manner essentially opposite to that
illustrated and described with reference to FIG. 2. A simple block
diagram schematic of a typical embodiment for providing the
reconstruction of the original information is illustrated as the
apparatus 30 of FIG. 4 wherein the information present on the
recording device 32 is played back through a buffer store
arrangement 34 in response to signals from timing logic 36
producing an output signal from the buffer store 34 which is
supplied to the audio output circuit 38 which presents the audio
information in an uncompressed form corresponding to the audio
information provided by the audio source 101 of FIG. 2. The buffer
store 34 can be of the sampled-analog form or can be implemented in
a manner illustrated in FIG. 5 as comprising circuits 103', 104',
and 105'. The circuits 102' and 106' can be considered part of the
buffer store or as necessary filters to be coupled to the input and
output of the buffer store 34.
A detailed implementation of the arrangement 30 of FIG. 4 is
illustrated in block diagram form in FIG. 5. It is noted that the
units identified in FIG. 5 correspond essentially to the units
illustrated and described in reference to FIG. 2 with the
difference being that instead of having audio information flowing
from an audio source to the magnetic disc recorder the flow is
reversed and information recorded on the magnetic disc recorder in
time-compressed form is returned through gate 108', the fm
oscillator 107', filter circuit 106', analog-to-digital converter
103', shift register 104', digital-to-analog converter 105', filter
circuit 102', and audio output circuit 38. The fm oscillator
circuit 107 of FIG. 2 is replaced with demodulator circuit 107' in
FIG. 5 and the positions of the digital-to-analog converter 103 and
analog-to-digital converter 105 of FIG. 2 have been switched in the
playback schematic of FIG. 5. The low-pass filter circuit 106 is
the same as that illustrated in FIG. 2 apart from the reversal of
connections. The analog-to-digital converter 103' is the same as
the analog-to-digital converter 103 of FIG. 2 except for the change
of position and the sampling frequency is now increased to 10
megahertz. The shift register 104' is identical to that of FIG. 2,
but now the register is filled by 526 samples at a 10-megahertz
rate and discharged through the digital-to-analog converter 105' at
the lower rate of 15,750 hertz. The low-pass filter circuit 102' is
the same as the low-pass filter 102 of FIG. 2, but instead of
accepting signals from an audio input source it supplies audio
output signals to the audio output circuit 38 which may be
comprised of an audio amplifier and a loudspeaker, a headset, etc.
One significant difference between the embodiment of FIG. 5 and
that of FIG. 2 is that the TV waveform generator 18 of FIG. 2 can
now be replaced by a simple reference signal source to provide
synchronization of the magnetic disc recorder 14 at a stable
frequency. In fact, a 60 hertz power line supply may be utilized to
synchronize the rotation speed of the magnetic disc recorder.
The control logic utilized to provide this reverse flow of
information from the recorder to the audio output circuit 38 is the
same as that illustrated in FIG. 2 to control the flow of
information from the audio source 101 to the recorder 14.
Assume that the magnetic disc recorder 14 has a set of tracks each
filled with compressed audio information. The operation of the
"start" switch circuit 110' initiates the playback process when
such signal coincides with a frame frequency signal f.sub.1 at the
AND circuit 127'. The AND circuit 127' responds to coincidence of
these input signals by generating an S signal to reset all the
counters. The magnetic disc rotates one revolution plus the
duration of one TV line period, and the gate circuit 108' opens to
transmit the first compressed audio burst, which is frequency
modulated on the carrier from the magnetic disc 14, to the
demodulator circuit 107' which in turn transmits the demodulated
output, which presents the compressed audio waveform to the
analog-to-digital converter 103' via the low-pass filter circuit
106'. The shift register 104' is filled with 526 samples at a 10
megahertz f.sub.3 rate before the readout commences at the 15,750
hertz rate which corresponds to the f.sub.2 signal. From this point
in time the shift register 104' will be continuously read out at a
rate of 15,750 hertz and subsequently refilled by a short audio
burst at a 10 megahertz rate between readout pulses 526 n and 527
n, where n is the number of disc rotations since the playback
process was initiated. When 252 audio bursts have been fed into the
shift register 104', the magnetic disc will rotate an additional
ten TV line period before the 253rd burst is supplied to the shift
register 104'. This lapse of time without without transmission of
audio information will produce a gap in the audio output of
approximately 640 microseconds in duration. This brief
discontinuity in the audio output produced by this gap, which gap
occurs approximately every 8.4 seconds, is of such a short duration
as not to be discernible. Following 504 rotations of the magnetic
disc, the head of the magnetic disc will commence reading out from
a second track. This process will continue until the entire
recorded program is completely reproduced. The completion of the
recorded program can be recognized by establishing a predetermined
number of tracks per program, thus requiring means for counting the
tracks, or if more flexibility is required, a coded signal can be
added at the end of each program to identify the conclusion of the
program at which time the apparatus 40 of FIG. 5 is returned to a
quiescent state. The gap in recording and reproduction that occurs
every 8.4 seconds can be eliminated if desired by various
techniques which are obvious to those skilled in the art. The use
of a small shift register to handle the overflow, the use of a
change of sampling rate during the 253rd rotation, or the use of
twin buffer stores as shown in FIGS. 10 and 11 are but a few
techniques available. If the system were used for high fidelity
musical appreciation programs, it is possible that the higher
sampling rate necessary would cause the gaps to occur at closer
intervals of time. In that case, the gaps would become more
noticable and it may be desirable to take steps to remove them. The
use of two buffer stores, or, more economically, the use of two
shift registers between the A/D and D/A converters provides one
method of achieving this. In FIGS. 10 and 11 the buffer stores
refer to the shift registers only. By providing two stores in FIG.
10, No. 1 and No. 2, the need for the high frequency unloading to
occur between two low frequency samples is removed. When one store
is filled, the input can be switched to the second store and the
first store emptied at any time during the filling of the second
store. Similarly in FIG. 11, by alternate use of the two stores No.
1 and No. 2, when the 252nd burst has been fed into one shift
register, it can commence discharging to provide the output audio.
The 253rd can be fed into second shift register 10 lines later and
before the first register has discharged. By this time, continuous
audio output can be achieved in spite of the timing discrepancies
that occur in reading the bursts from the magnetic disc.
If the embodiment described above is to be utilized for color
visuals, it is necessary to use a color TV waveform generator in
the embodiment illustrated in FIG. 2. Furthermore, the frequencies
referred to for f.sub.1 and f.sub.2 will be slightly changed to
those conventionally used for the NTSC waveforms. It is possible to
use a frequency for the high rate sampling signal f.sub.3 which is
harmonically related to the low rate f.sub.2 and to derive the
color subcarrier frequency of 3.579,545 megahertz from the signal
f.sub.3. As an example, signal f.sub.3 can be 9.9,124,938 megahertz
which is 630 times NTSC line frequency of 15,734.26 hertz.
The addition of the time-compressed audio and video signals in
time-multiplex can be achieved in various ways. In simplest form,
the process could consist of first determining the appropriate
places in the audio presentation at which visuals should be
displayed. If the visuals are arranged to change at intervals that
are integrals of the time period to replay one track of compressed
audio (i.e., 16 seconds), the first track of the multiple track
disc recorder 14 in FIG. 2, can be recorded with the first visual
frame. The audio recording sequence can then commence from track 2
onwards until the next visual is required. At this point the next
unoccupied track can be recorded with the next visual frame and so
on to the end of the presentation. FIG. 12 shows a schematic
arrangement for recording video information from video store 200
and time-compressed audio information from source 201. The video
store 200 comprises a source of visual frame waveforms. It can
consist of a separate set of tracks on the same disc as is used for
recording the complete program, or it could consist of a television
slide scanner. When the slide change signal is generated by means
of an actuator 202, the control logic 204 connects the input from
the video store 200 to the disc recorder 206 through a modulator
208 which is enabled by a pulse lasting for one rotation of the
disc. Each visual waveform must contain an identification code in
the waveform.
To display the visuals at the same time as the audio is reproduced
in the terminal processor, an arrangement such as FIG. 13 can be
utilized. A code recognition unit 210 will recognize when the
output frame from the disc recorder 200 represents a visual. A
switch circuit 210 connects the output for the duration of one
rotation to a refresh memory 212 that can be one track on the same
disc having its own record/replay head. The output from this track
is displayed on the monitor 214 continuously until another visual
replaces it. Once the visual has been transferred to the refresh
memory 212, the audio expansion and reproduction continues in
circuit 216 for presentation on headset 218. Each slide change will
cause a break of 33 milliseconds in the reproduced audio, but this
will not be noticeable.
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