U.S. patent number 3,622,714 [Application Number 04/854,712] was granted by the patent office on 1971-11-23 for telephonic transmission using complementary comb filters.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to David A. Berkley, Jeofry S. Courtney-Pratt.
United States Patent |
3,622,714 |
Berkley , et al. |
November 23, 1971 |
TELEPHONIC TRANSMISSION USING COMPLEMENTARY COMB FILTERS
Abstract
This disclosure describes a system for assuring circuit
stability in a telephonic link having substantial acoustic coupling
at stations therein, as well as eliminating remote end echo in such
circuits. The system uses complementary comb filter banks in which
the passband center frequencies are selected to reduce the
incidences of harmonic relations among the passband at any one of
the filter banks. The system is adapted also to the reduction of
electrical circuit echo.
Inventors: |
Berkley; David A. (Red Bank,
NJ), Courtney-Pratt; Jeofry S. (Locust, NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, Berkeley Heights, NJ)
|
Family
ID: |
25319372 |
Appl.
No.: |
04/854,712 |
Filed: |
September 2, 1969 |
Current U.S.
Class: |
379/392;
379/406.01; 381/66 |
Current CPC
Class: |
H04Q
1/453 (20130101); H04M 9/087 (20130101); H04B
3/21 (20130101) |
Current International
Class: |
H04B
3/21 (20060101); H04Q 1/453 (20060101); H04M
9/08 (20060101); H04Q 1/30 (20060101); H04B
3/20 (20060101); H04m 009/08 () |
Field of
Search: |
;179/1HF,170.2,170.4,170.6,170.8,15.55 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Stewart; David L.
Claims
What is claimed is:
1. In a voice telephone system, apparatus for processing speech
signals between first and second stations comprising:
first and second filter banks disposed in the transmission paths
between the respective said stations, each bank comprising a
plurality of passband filters, the passbands of the respective
banks being complementary, and the passband center frequencies
between adjacent complementary comb teeth of the combined filter
banks being spaced by a selected odd simple fraction of an octave.
2The system of claim 1, wherein said odd fraction
is substantially 1/3. 3. The system of claim 2, wherein said first
and
second filter banks are located respectively at said stations. 4.
The system of claim 2, wherein said filter banks are located at a
switching center common to said stations.
Description
FIELD OF THE INVENTION
This invention relates to telephonic communications, and broadly
concerns the suppression or elimination of an unwanted recurrence
of a desired speech signal in such systems.
BACKGROUND OF THE INVENTION
A number of instances exist in telephony where speech quality and
intelligibility are seriously reduced by the recurrence somewhere
in the system of a desired speech signal.
In a loop comprising two "hands-free" stations, for example, there
can be a substantial direct acoustic coupling between the local
receiver and the transmitter at each station. If at any time the
net loop gain is greater than unity, the loop becomes unstable and
may oscillate. The undesired speech signal recurrence can be viewed
as a talker's voice returning to his own transmitter via the two
direct acoustic paths, one at the remote station and the other at
the talker's station.
In such loops, even when overall gain is low, there still is the
problem of remote end echo, which stems from a speaker's voice
returning to his ear, at a reduced but discernible level, after
traveling around such a loop. For remote end echo to occur requires
only one hands-free station in the loop; and both the direct and
indirect acoustic paths at such station contribute to the echo.
The usual countermeasure has been to employ various voice-switching
schemes which in effect make the loop transmission direction one
way at a time. Although improving stability and remote end echo
problems, this expedient creates the problem that only one party
can talk and be heard at a given time. If both parties attempt to
talk at once, this "double talk" will lead to lockout. If, further,
the loop has relatively long delays for any reason, the
conversation can break down altogether when lockout occurs.
In addition, voice switching at best will on the average degrade
the quality of received speech by clipping initial message
requests. Not always can the listener supply the missing parts from
the context, and thus requests by users of such loops for the
speaker to repeat himself are frequent. With such circuits there is
also a significant risk of a listener misunderstanding what was
said altogether.
Besides the signal recurrences already mentioned, there also is
electrical echo, particularly on long telephone circuits. This echo
is a talker's signal returning to his ear, say, from a remote
hybrid. With large enough levels and round-trip delays, this echo
too has the psychoacoustic effect on a speaker of drastically
interrupting his free-cadenced generation of speech.
Accordingly, the following are important objects of the
invention:
TO ASSURE LOOP STABILITY IN A TWO-WAY HANDS-FREE VOICE
COMMUNICATIONS CHANNEL WITHOUT DEGRADING THE QUALITY OR MESSAGE
CONTENT OF RECEIVED SPEECH;
TO ELIMINATE THE REQUIREMENT OF VOICE SWITCHING ON SUCH
CHANNELS;
TO ELIMINATE WHEREVER PRESENT, THE PROBLEM OF REMOTE END ECHO;
TO REDUCE THE PROBLEM OF ELECTRICAL ECHO;
TO PERMIT SIMULTANEOUS, TWO-WAY CONVERSATION ON CIRCUITS NOW
REQUIRING VOICE SWITCHING DUE EITHER TO THE NEED FOR CIRCUIT
STABILITY OR THE NEED TO REDUCE THE EFFECT OF ECHO;
TO PRESERVE THE NATURALNESS OF RECEIVED SPEECH ON SUCH CHANNELS
WITHOUT USING VOICE SWITCHING AND WITHOUT REQUIRING ADDED
BANDWIDTH; AND
IMPORTANTLY, TO ASSURE THE CAPABILITY OF HANDS-FREE AUDIO
COMMUNICATIONS ON A VIDEO TELEPHONE LINK WITHOUT THE NEED FOR A
HANDSET.
In connection with echo suppression, it has previously been
proposed to use complementary comb filters at, for example, the two
ground stations in a synchronous satellite telephone system, to
allocate the available bandwidth of a single-voice channel between
a send path and a receive path. Each path includes a filter bank
consisting of several filters. In terms of attenuation vs.
frequency characteristic, certain of the filters of the send path
bank are complementary to certain of the filters of the receive
path bank. When a "double talk" occurs, this feature is turned to
account by providing in effect acceptable two-way transmission with
adequate loss inserted in the echo path.
SUMMARY OF THE INVENTION
In broadest terms, the present invention utilizes two banks of comb
filters in a voice telephone system. Each bank has several comb
teeth, or filters. The filter passbands of the first bank are
stopbands in the second bank, and vice versa; thus, no passband in
one filter bank overlaps any significant portion of a passband in
the other filter bank. The passbands of the two filter banks are in
this sense complementary.
The passbands at each bank are in addition frequency spaced
pursuant to a system that reduces the incidence of harmonic
relations among the passbands at that bank. In general, this is
achieved to advantage by a logarithmic spacing of passband center
frequencies of the successive complementary comb teeth of the two
banks. In a particular embodiment, adjacent passband center
frequencies are logarithmically spaced by a selected odd simple
fraction of an octave for example, one-third octave. In either of
the filter banks then, the blocking of both a fundamental and its
second harmonic thereby is avoided, which preserves highly useful
recognitional features of a speaker's voice.
A suitable incorporation of the two filter banks as, for example,
in the circuitry of two communicating hands-free telephone stations
of the system, creates a stable loop. Specifically, at either of
the stations, the signal received includes frequencies only within
the passband set of the sending station. As the two passband sets
are mutually exclusive, no closed feedback path exists in the loop
to cause instability.
Further, if acoustic coupling of a received signal to the
transmitting microphone does occur at one of the stations, that
signal is for the same reason precluded from returning as a remote
end echo to the originator's ear. To the listener's ear however,
the received signal despite the filtering is substantially
undegraded, owing to the unique harmonic relations of the passbands
at each bank.
Using conventional modulation techniques, an improvement in speech
quality is achieved by multiplexing additional voice frequency
passbands, which fall outside the normal telephone channel band
limits, into the stopband regions of each comb filter. The added
speech quality is in some instances an advantageous trade-off for a
certain amount of remote end electrical echo which might then be
passed by the system.
By concentrating the complementary comb filters of the present
invention at a central point such as a central office the problem
of suitably matching sets of combs is simplified.
The invention and its further objects, features and advantages will
be readily discerned in detail from a reading of the descriptions
which follow of its illustrative embodiments.
THE DRAWING
FIG. 1 is a block diagram embracing the basic invention;
FIG. 2 is a schematic diagram of frequency allocation between two
filter banks;
FIGS. 3 and 4 are block diagrams of two systems using the
invention;
FIG. 5 is a block diagram depicting an added speech processing
features; and
FIG. 6 is a block diagram showing a further inventive system using
the feature of FIG. 5.
ILLUSTRATIVE EMBODIMENTS
The basic invention is broadly depicted in FIG. 1 in which two
telephone stations, A and B, are connected through a central office
10. For simplicity's sake, only relevant station and transmission
elements are shown. Stations A and B are hands-free telephones for
which there exists a strong acoustic coupling between transmitter
11 and receiver 12 of station A, and between transmitter 13 and
receiver 14 of station B.
Pursuant to the invention, transmitter 11 is connected to a first
filter bank consisting of several voice frequency band-pass filters
15, 17, 19, 21, 23, 25. Similarly, transmitter 13 is connected to a
second filter bank consisting of several voice-frequency band-pass
filters 16, 18, 20, 22, 24, 26.
The passbands of the first filter bank are, in the second filter
bank, unused or blocked portions of the channel spectrum.
Similarly, the passbands of the second filter bank are the unused
or blocked portions of the first filter bank. In this sense, the
two filter banks are complementary.
Pursuant to an important general aspect of the invention, the
passband frequencies in the several filters of each bank are so
selected that at least some harmonic of a blocked fundamental is
passed if it lies within the band limits. This is achieved by
spacing the center frequencies in accordance with odd logarithmic
relations between the center frequencies. Thus, considering both
the first and the second filter bank, the center frequencies of
adjacent passbands are separated by a selected odd simple fraction
of an octave such as one-third, one-fifth, one-seventh, etc. In the
present embodiment, the spacing is one-third octave.
FIG. 2 depicts schematically the meshing of complementary comb
filter teeth wherein the passband width, within each bank, and the
center-frequency spacing with respect to all passbands, is
one-third octave. For 1/3 -octave bands, if a fundamental is passed
by the bank comprising filters 15, 17, 19, 21, 23, 25, so also is
at least its fourth and tenth harmonics if they are within the
limits of the bank. If a fundamental doesn't pass, its fourth and
10the harmonics must pass the bank comprising filters 16, 18, 20,
22, 24, 26. The fundamental must, of course, be not less in
frequency than the lowest passband limit.
Also with respect to 1/3 -octave bands, as to fundamentals below
the lowest passband limit, if its second harmonic passes, so may
its third, fifth, sixth, seventh, eighth, ninth, 11th and so on. If
the second harmonic does not pass, at least the fourth and 10th
harmonics will pass. The selection of 1/3 -octave bands appears
particularly advantageous. A bandwidth of 1 octave is too wide to
enable a sufficient number of teeth to be built into the
complementary combs to obtain good transmission in both directions,
due to the limited bandwidth available in a telephone voice
channel. On the other hand, ringing of the filters becomes
significant for one-fifth octave and smaller teeth.
Linear bands can, of course, be constructed to pass harmonics, but
not within the limited telephone channel bandwidth. The excessive
bandwidth required at lower frequencies in such case would prevent
building a complementary filter with good transmission quality. The
choice of 1/3 -octave bands guarantees the passage of some
harmonics of any given signal, while retaining good complementary
characteristics.
In the particular embodiment of FIG. 2, if an input signal to the
filter at station B has fundamental acoustic energy density
centered at 400 Hz. (roughly the center frequency of filter 18 but
a no-transmit region for said first bank) then although the
fundamental is blocked, the second harmonic, 800 Hz., of that
signal is transmitted through filter 19. In addition, the third,
fifth, and eighth harmonics, 1,200, 2,000, and 3,200 Hz., likewise
are transmitted through filters 21, 23, and 25. Further, if the
input signal centers at about 505 Hz., the center frequency of
filter 17, it will pass, as will its fourth and sixth harmonics,
2,020 and 3,030 Hz., via filters, 23 and 25.
The harmonic richness of human voice assures the broad workability
of the above scheme. It has been found to preserve the
recognitional quality of the transmitted voice as well as
intelligibility. The invention substantially precludes the
occurrence of a closed loop in the paths connecting stations A and
B. In the example of FIG. 1, acoustic coupling unavoidably exists
between transmitter 11 and receiver 12 as well as between
transmitter 13 and receiver 14. But by virtue of the complementary
comb filter banks, no significant amount of the signal energy
passing through one filter bank--even when acoustically coupled
directly or through room reverberation into the remote
receiver--will pass through the other filter bank associated with
said remote receiver.
The invention finds further use as a suppressor of electrical echo.
In this embodiment, a complete set of filters is provided at each
of the two stations.
In FIG. 3 telephone stations C and D are shown connected through
transmission system 40. The filter banks 30 and 32 at stations C, D
each contain, for example, the filters 15, 17, 19, 21, 23, 25
referred to earlier in connection with FIGS. 1 and 2. The filter
banks 31 and 33 similarly each contain the filters 16, 18, 20, 22,
24, 26. Conventional antisidetone networks 38, 39 are provided
respectively at stations C and D. Transmission system 40 includes
typically four-wire terminal sets 41, 42 and transmission channels
43, 44. The transmission path between station C and station D thus
includes transmitter 34, filter bank 30, network 38, hybrid 41,
channel 43, hybrid 42, network 39, filter bank 32, and receiver 36.
Similarly, the transmission path between station D and station C
includes transmitter 37, filter bank 33, network 39, hybrid 42,
channel 44, hybrid 41, network 38, filter bank 31, and receiver
35.
For same reasons stated above with respect to the FIG. 1
embodiment, isolation is afforded between the two transmission
paths, in consequence of which circuit quality and stability are
not affected by acoustic coupling, if any exists, of receiver and
transmitter at the respective stations C and D.
Additionally, however, neither station when transmitting will
experience a remote end echo of its own transmission. For example,
if an echo develops at hybrid 42 of a transmission from station C,
such echo--being frequency-limited to the subbands passed by filter
bank 30--will not be passed by the filter bank 31. The same
situation obtains as to transmission from station D.
In a variation of the inventive embodiment of FIG. 1, the first and
second filter banks instead of existing at the separate station
sets, are centrally located at a common switching point. FIG. 4
shows such a system. The station sets E and F are connected through
a central office 60, which includes hybrid networks 61, 62 and
filter banks 63, 64. Bank 63 comprises filter 15, 17, 19, 21, 23,
25 as described in FIG. 2; and similarly, bank 64 comprises filters
16, 18, 20, 22, 24, 26.
The transmission path to station F includes the transmitter 66 at
station E, antisidetone network 67, hybrid 61, filter bank 63,
hybrid 62, and at station F, antisidetone network 68, and receiver
69. The transmission path to station E includes the transmitter 70
at station F, network 68, hybrid 62, filter bank 64, hybrid 61, and
at station E, network 67, and receiver 71.
Circuit stability in the FIG. 4 embodiment is realized for the same
reasons stated with respect to the FIG. 1 embodiment. In addition,
pursuant to the invention, voice quality is maintained by the use
of particular passbands in each filter bank, such as has been
described with respect to FIG. 2. Moreover, by placing the comb
filter centrally with respect to the communicating station sets,
the proper allocation and matching of passbands is readily
realized.
A further aspect of the invention relates most particularly to
speech quality. As is known from long-time power density spectrum
analysis, continuous speech exhibits maximum power density in the
region centering on about 500 Hz. Relatively high-power levels for
both men and women are also present in the 125-200 Hz. region,
however. Further, voice energy is generated also in the frequency
range 4,000-8,000 Hz., particularly by women. Although relatively
much lower in power, the latter region nevertheless contains signal
components valuable in the present scheme for speech recognition,
intelligibility, and quality.
Telephone bandwidth, however, is limited in voice telephone systems
to the range of about 225-3,500 Hz., which normally precludes
transmission of the information in the regions adjoining.
Accordingly, in the following inventive embodiment, speech power in
the frequency regions adjoining this normal voice-frequency band
are, in each transmission direction, frequency-multiplexed into
unused spectral regions between the comb teeth. FIG. 5 depicts such
speech processing. Speech input, for example, from a microphone and
having a bandwidth of, say, 180 to 5,100 Hz., is to be transmitted
over a telephone channel. The channel band extends from 280 Hz. to
3,550 Hz. approximately. The channel is equipped, for one of the
above-mentioned inventive purposes, with a complementary comb
filter. The in-band filters 15, 17, 19, 21, 23, 25 are those shown
in FIG. 2. An additional filter 80 with a passband of 180-224 Hz.
is here provided to receive speech in this region; and two
additional filters 81, 82 with contiguous passbands of 4,500-4,800
Hz. and of 4,800-5300 Hz. respectively are provided in the region
beyond the upper channel edge for speech in that region.
The bandwidth of the filter 80 is the one-third octave that obtains
with respect to the filters 15, 17, 19, 21, 23, 25. The combined
bandwidths of filters 81, 82 however are slightly less than
one-third octave.
Speech is processed in filters 15, 17, 19, 21, 23, 25 as already
described; and in this instance also by passband filters 80, 81,
82. The respective outputs of filters 80, 81, 82 are
frequency-shifted by the respective modulators 83, 84, 85 into
selected unused regions. These regions are in the present
embodiment, respectively, between 355 and 450 Hz.; between 1,400
and 1,800 Hz.; and between 2,240 and 2,800 Hz. These particular
unoccupied spaces are preferred because they minimize the number of
modulators required. It is, of course, possible to use additional
space for further multiplexing of speech or data.
All signals are then conventionally transmitted to a remote point
where the frequency-shifted signals are demodulated and, along with
the unshifted signals, are connected to a receiver, not shown.
The processing principle illustrated in FIG. 5 is advantageously
incorporated in a two-way telephone link in the general fashion
shown in FIG. 6. Except as mentioned below, the components of FIG.
6 are identical to those described with respect to FIG. 3; and like
numerals are accordingly used.
In FIG. 6, the lines at station C leading from filter 30 to
antisidetone network 38 represent the output bands from the filters
80, 15, 17, 19, 21, 23, 25, 81, 82 which are contained therein.
Similarly, the lines at station D leading from filter 33 to
antisidetone network 39 represent the output bands from the filters
86, 16, 18, 20, 22, 24, 26, 87, 88 contained therein.
Filters 86, 87, 88 of bank 33 have band-pass regions selected in
the manner already described with respect to filters 80, 81, 82 in
the principle's illustration in FIG. 5. The outputs of filters 80,
81, 82 are respectively frequency-shifted as described by
modulators 83, 84, 85, the outputs of which in FIG. 6 are
symbolically placed between the appropriate filter outputs to
connote the frequency multiplexing that has occurred. Similarly,
the outputs of filters 86, 87, 88 are respectively
frequency-shifted by modulators 89, 90, 91.
After transmission through system 40 from station C and passage
through network 39, the modulated portion of the signal is
demodulated back to the regions embraced by the filters 80, 81, 82
by the respective demodulators 92, 93, 94. The signal components
then pass to filter bank 32 which is substantially the same as
filter bank 30. Similarly, after transmission through system 40
from station C and passage through circuit 38, the modulated
portion is demodulated by demodulators 95, 96, 97 back to the
regions embraced by the filters 86, 87, 88. The signal components
then pass to filter bank 31 which is substantially the same as
filter bank 33.
The system just depicted is effective in achieving circuit
stability without voice switching, while recovering a high degree
of voice quality. Additionally, given reasonably good hybrids, the
system is also effective in reducing some of the electrical echo
that might be present, depending on how much of the unused spectrum
at each filter bank is utilized to transmit out-of-band voice
energy.
The addition of center-clipping stage, not shown, following the
filter of each passband reduces filter ringing, if present.
Clipping distortion then is removed by following each clipping
stage by a second filter (none shown) substantially embracing the
same frequency band as the first. The center clippers, when used,
supply the additional benefit of reducing room reverberations in
the manner described in the U.S. Pat. application of D. A. Berkley
and O. M. M. Mitchell, Ser. No. 854,457 filed concurrently with the
present application, and which is hereby incorporated by reference
into the instant application. The further use and implementation in
the present application of the center-clipping teachings of that
invention will be readily apprehended by a reading of that
disclosure.
The spirit of the invention is embraced in the scope of the claims
to follow.
* * * * *