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BACKGROUND OF THE INVENTION
The present invention relates to the control of echo on telephone circuits
and to voice-activated switches which have been used to block noise and
activate channels in telephone communications systems. (As used herein a
"circuit" includes all the paths for transmission and reception of signals
between talkers or telephone users in a conversation, while a "channel" is
the transmission path for signals in a single direction. A circuit
therefore can be comprised of at least two channels, and a channel can be
comprised of all media, such as cables and satellite links, often called
"trunk lines", which provide one-way transmission for signals.) The
invention is particularly relevant to those systems which employ speech
interpolation techniques for combining speech signals from several
circuits in a single channel.
Noise, which typically is present on all communications channels, but at a
significantly lower level than speech, must be minimized to provide
efficient and coherent voice communication. Typically, voice-activated
switches are used to recognize speech in the presence of noise in
telephone communication circuits; they allow those signals that exceed a
predetermined threshold level to pass into the circuit. The threshold
level of the voice-activated switch is selected to be responsive to
signals above the level of noise expected to be encountered on a telephone
circuit. In some types of voice-activated switches the threshold level is
fixed at a predetermined value, while other types, called adaptive voice
switches, employ associated control circuits which enable the threshold to
be continuously adjusted to remain at a level just above the noise. The
inability of voice switches to detect the presence of all speech results
in a "clipping" of the conversation when speech signals fall below the the
predetermined noise threshold. The more complex adaptive threshold
switches tend to "clip" the transmitted signals less than those having
fixed thresholds since the adaptive threshold is always no higher than is
necessary above the instantaneous noise level. Examples of these several
types of voice-activated switches may be found in U.S. Pat. Nos. 4,008,375
(Lanier) entitled "Digital Voice Switch for Single or Multiple Channel
Application"; 3,890,467 (Scuilli) entitled "Digital Voice Switch for Use
with Delta Modulation" and 3,832,491 (Scuilli et al) entitled "Digital
Voice Switch with an Adaptive Digitally-Controlled Threshold". Also see
U.S. Pat. No. 4,052,568 (Jankowski) entitled "Digital Voice Switch".
In addition to apparatus for reducing noise, systems for controlling echo
in telephone circuits have been developed because perceived echo is
offensive to telephone users and, in certain types of communications
systems, all echo signals (including those that may not be noticeable to
telephone users) will tend to degrade communications system performance in
other ways. This is particularly true of speech interpolation
communications systems which will be discussed below.
Echo is generated at the two-to-four wire-interface, called a hybrid, which
couples individual telephones into a telephone circuit. Because of
impedance mismatches, typically created by variation in lengths of the
wires connecting individual telephones to the hybrids, partial reflections
of signals arriving at the hybrid will occur. The reflection of received
signals back to a transmitting telephone results in echo on the circuit
connecting two telephones. If the physical length of the channels which
interconnect the hybrids in a telephone circuit are sufficiently long, the
propagation time for signals in the circuit will be large enough for the
echo to be noticed by the telephone user. If the physical channel (trunk
line) length is short, some echo may still be present, although it may go
unnoticed by the telephone user.
In telephone communication systems, echo signals which may be present tend
to resemble low level speech signals. The echo signals are frequently at
an amplitude level greater than the voice switch threshold level set to
reject noise. Thus, to a conventional voice switch, many echo signals
would be indistinguishable from low level speech signals. Because
conventional voice switches must be designed to be sensitive to a varied
range of speech levels in order to accommodate high and low level
speakers, they generally will respond to an echo signal as if it were
speech.
In order to deal with the problems associated with echo, telephone
communications systems have employed echo suppressors in each telephone
circuit. Echo cancellers which provide superior performance to echo
suppressors, also can be used. However, use of either cancellers or
suppressors adds significant cost to the construction of a telephone
system.
Echo suppressors in general block echo signals by inserting an attenuation
in the transmit channel for a telephone in a telephone circuit to reduce
the echo. They are commonly used in both the transmit and receive channels
of a long distance communications circuit. These devices are successful in
reducing echo. However, because echo suppressors attenuate transmit speech
signals when signals are present on the receive channel, when both
speakers on a telephone circuit are talking simultaneously (doubletalk),
there are noticeable changes in level of the speech signals which can
prove disturbing to the talkers. For example, during doubletalk, when one
speaker starts or stops talking, a distinct and frequently annoying change
in the speech level of the other speaker can be heard. Because of these
frequent changes in channel attenuation during doubletalk situations, use
of suppressors can result in confusing and offensive speech transmission.
Echo cancellers, on the other hand, do not present this difficulty since
they remove echo from the transmit channel by generating a signal from the
receive speech, which is a replica of the echo. The canceller then
subtracts this echo replica signal from the transmit signal so as to
cancel the echo. Echo cancellers have fewer of the limitations of the
suppressors, but are substantially more complex and expensive.
In addition to being useful in controlling noise, voice switches also are
useful in communications systems employing forms of speech interpolation,
wherein each trunk line may carry speech signals from parties to more than
one conversation. Interpolation of speech signals from several
conversations is permitted by the usual pauses or breaks in speech
activity by one or both parties to each conversation. One technique of
interpolation currently used in telephone communications is called digital
speech interpolation (DSI). An article entitled "Digital Speech
Interpolation", by S. J. Campanella published in the Comsat Technical
Review, Vol. 6, No. 1, and U.S. Pat. Nos. 3,542,956 (Sekimoto) entitled
"PCM Telephone Communication System"; 3,927,268 (Sciulli et al) entitled
"Speech Predictive Encoded Communication System"; and 3,988,674 (Sciulli)
entitled "Frame Synchronization for Speech Predictive Encoded
Communications Systems" furnish descriptions of several types of DSI
implementations.
In interpolation systems, voice switches serve to sense the presence or
absence of speech signals in several conversations and thus control the
transmission of signals on a single shared circuit. In many interpolation
systems, including DSI, a voice-activated switch is used not only to
detect the presence of speech for transmission but also to perform its
usual function of blocking noise, especially that typically found on the
idle channels. However, since echo signals are generally of a higher
magnitude than the noise level in a circuit, they may exceed the
noise-proof threshold of the switch. Since echo signals would be passed by
a conventional voice-activated switch used in interpolation systems, the
interpolation system will experience extra loading due to the transmission
of the echo signal. During periods of heavy traffic, the overall system
performance will be degraded due to the artificial activation of channels
by voice switches responding to echo signals. Even those echo signals
which are not noticeable to telephone users can also act to degrade
interpolation system performance. These inaudible echo signals can be
encountered in long distance circuits (such as those provided by cable or
satellites) and can be present in relatively short transmission paths as
well.
There is therefore a need to provide a means whereby the voice switches
used in interpolation systems are able to block echo signals while at the
same time being sufficiently sensitive to low level talkers. Further, all
telephone communications systems, not just interpolation systems, require
a new means of echo control which operates during doubletalk conditions in
a less offensive manner than echo suppressors, but which also is less
expensive and less complex than an echo canceller.
The present invention seeks to meet these needs by means of a circuit for
appropriately controlling a voice switch during those circuit conditions
which allow echo to occur. Although particularly useful in interpolation
system applications such as DSI, it provides a simple, effective means of
echo control in a great number of communications circuit applications,
providing an alternative to echo cancellers and echo suppressors.
SUMMARY OF THE INVENTION
The present invention provides echo control in communications circuits by
utilizing a voice-activated switch in the transmit circuit path or channel
associated with each telephone in the circuit. The voice switch utilized
in the invention may have a lower "noise-proof" threshold, but in all
cases utilizes a higher "echo-proof" threshold. In various embodiments of
the invention, either of these thresholds or both of them may be adjusted
in an adaptive manner to compensate for changes in noise level on the
channels. The invention employs a means for desensitizing the voice switch
to low level signals in those cases where there are signals present on the
receive circuit path or channel of the telephone terminal. This is
accomplished by increasing the threshold from the lower (e.g.,
"noise-proof") threshold level for which the voice switch is normally
rendered conductive to the higher "echo-proof" level. As a result, low
level echo signals caused by reflections of the received signals at the
hybrid interface for the telephone terminals do not activate the switch,
but normal speech above the threshold will be transmitted. In general, low
level transmitted signals also will be clipped when the higher threshold
is operative, however this clipping would occur only during doubletalk.
Under doubletalk conditions the amount of clipping would not be as readily
noticeable nor as offensive to a listener as the changes in channel
attenuation by echo suppressors which occur for all levels of speech
during doubletalk. Further, the implementation of the invention in the
environment of a digital speech interpolation system is simple and
inexpensive when compared to either echo cancellers or echo suppressors.
Additional features and advantages of this invention will appear from the
following description, taken with the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is illustrative of a prior art, digital speech interpolation (DSI)
system forming an environment for embodiments of the invention.
FIG. 2 illustrates in graphic form the principles of operation of
embodiments of the present invention for control of echo, for a
predetermined fixed "noise-proof" threshold level in FIG. 2A, or for an
adaptive "noise-proof" threshold level in FIG. 2B.
FIG. 3 illustrates in schematic form one embodiment of an echo control
circuit according to the invention utilizing a threshold voice switch in
conjunction with a DSI system as shown in FIG. 1.
FIG. 3A shows a variation of an embodiment of the invention shown in FIG. 3
with an adaptive voice switch threshold.
FIG. 4 is a general illustration of the employment of the invention in any
telephone circuit using a two-to-four wire hybrid interface.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention, in preferred embodiments described herein, comprises
a voice-activated switch for use as an echo control in each transmission
channel of a two-way telephone circuit that utilizes a threshold means of
voice detection that is responsive to speech in the receive channel of the
circuit. The invention in several of these embodiments is described as
incorporated in a DSI system because this appears to be a particularly
suitable application. It is to be understood that the invention has
advantages in telephone systems other than DSI systems and with other
voice switch algorithms; it will therefore also be described with regard
to more general applications than DSI systems. It should also be noted
that the invention need not necessarily require a lower "noise-proof"
threshold, but may be configured such that it need only be employed for
echo control using a single "echo-proof" threshold under the appropriate
circumstances. However, because the addition of noise control features
appear advantageous, the embodiments described herein include such
features.
A generalized DSI system is illustrated in FIG. 1 which has a single
telephone circuit incorporating a pair of two-to-four wire hybrid couplers
15a and 15b. The hybrid couplers 15a and 15b are each connected to a
respective telephone 10a and 10b, corresponding to two talkers, A and B.
This structure would normally be found in any two-to-four wire telephone
communication circuit arrangement. The illustrated DSI system also uses,
on the receive side for talker B, a pulse code modulation (PCM) analog to
digital (A-D) encoder 20a, a transmitter 30a, a receiver 40a and a PCM
decoder 50a, comprising a receive channel for digital encoding and
decoding of analog speech signals originating from telephone 10a. (As used
herein, if not otherwise indicated, "transmit" and "receive" will refer to
signals transmitted and received respectively by talker B at telephone
10b.) Similarly, on the transmit side, comprising a transmit channel of
the circuit, a PCM encoder 20b, a transmitter 30b, a receiver 40b, and a
PCM decoder 50b are provided. (In the simplest type of telephone circuit,
to which the present invention is also applicable, the transmit and
receive channels shown within the dashed lines in FIG. 1 as comprising the
DSI encoder/decoder and DSI transmitter/receiver apparatus would be
comprised of some direct hard wire equivalent, such as standard telephone
cables.)
Typically, echo is noticed when a speaking party (e.g., talker A in FIG. 1)
in a two-way telephone conversation hears his own voice returned to him,
delayed in time. The echo is caused by a reflection of the speakers'
signals at a hybrid interface, such as 15b at the other party's (e.g.,
talker B's) location. Thus for echo to exist at telephone 10a and be heard
by talker A in the system illustrated in FIG. 1, talker A's transmitted
signals must be received (such as at receiver 40a associated with talker
B's receive channel) at telephone 10b (talker B) for reflections to take
place at hybrid 15b and echo to be propogated back to talker A via talker
A's receive channel (talker B's transmit channel). Similarly, the hybrid
15a causes the speech from talker B to be returned as echo at telephone
10b.
Voice switches have been used to block noise, and in interpolalation
systems to activate a transmitter as well, (such as 30b in talker B's
transmit channel). Because the threshold level for these switches have
been set near the noise level, echo signals have not been blocked since
they generally are at a higher level than the noise. Thus in the prior
art, without some additional means for echo control, echo of one talker's
speech, such as A's may be returned to him in the transmit channels of the
other party, such as B's transmit channel (A's receive channel) since the
voice switch in this channel will not block echo.
In order to make the voice switch in a transmit channel responsive to echo,
the present invention utilizes the fact that in a telephone circuit like
that of FIG. 1, for echo to be created by talker A, for example, a hybrid
coupler such as 15b must receive talker A's voice signals via the receive
channel (i.e., talker B's receive channel) and reflect a portion of such
received signals into talker B's transmit channel. Therefore in those
situations where receive signals (i.e., A's voice signals) would be
encountered by hybrid 15b, for example, in accordance with the present
invention, a voice switch in talker B's transmit channel would employ a
threshold higher than the noise-proof threshold in order to block any of
A's voice signals reflected as echo by the hybrid 15b in talker B's
transmit channel. When none of talker A's voice signals would be present
at hybrid 15b, the voice switch in talker B's transmit channel would
employ the lower, noise-proof threshold. The same arrangement would be
provided for hybrid 15a at speaker A's side of the circuit. This general
approach would be similar for both interpolation systems and conventional
systems, that is, in any telephone circuit employing a two-to-four wire
hybrid interface. In a DSI system application, the threshold of the
threshold-sensitive voice switch normally utilized for noise rejection by
a DSI transmitter, (e.g., 30b of FIG. 1), is upwardly adjusted in order to
prevent encoding of echo signals by the transmitter upon receipt of
received signal samples by its associated receiver (e.g., 40b of FIG. 1).
Such DSI applications and general applications of the invention will be
described in greater detail below.
In the prior art, the threshold level for threshold-sensitive voice
switches has been set either at a fixed value, or at an adaptive value
which is always to be above the noise level present in a channel. All
signals above the threshold level activate the switch and are passed.
Those signals whose values are below the threshold level cannot activate
the switch and are blocked. The threshold level may be fixed at a level
which is a good compromise between low level speech sensitivity and noise
immunity. In adaptive switches it may be varied so as to be continuously
adjusted to remain slightly above the noise level; this tends to reduce
the amount of clipping to which transmitted voice signals are subjected.
Examples of an adaptive voice-activated switch can be obtained with
reference to the aforementioned U.S. Pat. Nos. 4,008,375; 3,832,491; and
4,052,568.
In accordance with the invention, since echo can only exist if received
speech signals are present at a hybrid such that they can be reflected as
echo into the transmit channel, echo control in a telephone circuit may be
accomplished by increasing the threshold level of a threshold-sensitive
switch in the transmit channel only when received speech signals are
present in the receive channel. Of course, the level of the increased
threshold should be such that it entirely blocks only the lower-amplitude
level echo signals. It should not be so high as to entirely block or
substantially clip the speech normally originating at the telephone
corresponding to the hybrid with which the switch is associated.
FIGS. 2A and 2B illustrate the operation of the present invention employing
different noise level thresholds. FIG. 2A illustrates the case of a lower
predetermined (fixed) voice switch "noise-proof" threshold; FIG. 2B
illustrates the case of a lower noise level-adjusted adaptive threshold.
In both the two illustrated cases, low level echo signals would not
typically exceed the higher level echo-proof thresholds set for the switch
and would thus be blocked by the voice switch. Normally transmitted speech
above the echo-proof threshold may encounter only occasional clipping of
the low level sound during the doubletalk condition. This is due to the
fact that the average echo signal level is typically 16 db (measured as
echo return loss) lower in level than the speech signal. An occasional
clip of the transmitted speech during the doubletalk situation often
remains imperceptible to the listener either due to the brevity of the
clip, depending both upon clip duration and the particular sound being
clipped, or because speech is dominated by one speaker or the other during
doubletalk in normal conversation. It should be noted in the cases
illustrated in FIGS. 2A and 2B, dependent on whether the initial threshold
level is adaptive or set to a predetermined value, that the higher
threshold may either be itself adaptive or be increased by a fixed amount
so that during the presence of received speech the threshold level exceeds
the additive level of the echo and the noise.
FIG. 3 illustrates in a block diagram an example of how the principles of
the invention may be implemented. This figure shows an embodiment which
employs a voice switch having a fixed "noise-proof" threshold which
operates in accordance with the invention as illustrated in FIG. 2A. This
embodiment particularly is shown in a manner which may be used in
conjunction with the DSI telephone communication system shown in FIG. 1.
Throughout the following description therefore, reference will be made to
the DSI system and its components which are shown in FIG. 1. It is to be
further understood that the embodiment of the invention shown in FIG. 3 is
to be considered as being employed by the terminal associated with
telephone 10b of FIG. 1, and that a similar arrangement would be provided
at the terminal associated with telephone 10a of that Figure to eliminate
the echo heard by talker B. To avoid redundancy, therefore, implementation
of the invention at only one terminal is described. It will also be
understood that the embodiment described in FIG. 3 may be considered for
use with DSI systems wherein speech signals are sampled and PCM (pulse
code modulation) encoded on a periodic basis, and where adjacent telephone
terminals utilize PCM encoder/decoder and transmitter/receiver apparatus
on a time-shared basis and where the shared apparatus operates with a
plurality of transmit and receive channels as discussed in the
aforementioned article of Campanella and noted U.S. patents. (The PCM
encoded signals represent a digital word obtained by analog to digital
conversion, made typically at the Nyquist rate of once every 125 usec.)
Accordingly it should be understood that the operation of the
voice-activated switch employed in the invention may be considered as also
being utilized on a time-shared basis corresponding to the arrangement
employed for the other time-shared apparatus in such DSI system where time
shared apparatus is used. Since these arrangements differ among the
various DSI systems known in the art, they will not be discussed in detail
herein. In general it is necessary that the switch be activated by
appropriate received PCM sample signals and transmitted PCM sample signals
properly coordinated for the interpolated conversations in the DSI
systems. Examples of the incorporation of voice switches in such
multiplexed arrangements may be found in U.S. Pat. Nos. 3,649,766
(LaMarche) entitled "Digital Speech Detection System"; 3,794,763
(Boudewijns et al) entitled "Speech-Controlled Switching Arrangements" and
3,832,491 (Scuilli et al) entitled "Digital Voice Switch with an Adaptive
Digitally-Controlled Threshold".
Shown in FIG. 3 is a speech detection circuit which uses a threshold-type
voice switch in conjunction with a DSI transmitter such as 30b shown in
FIG. 1, to sense the presence of a speech signal samples prior to
transmission. Since some DSI systems employ a voice switch in the DSI
transmitter of each channel in the system to control the operation of that
transmitter for purposes of the description of the embodiment of FIG. 3,
the voice switch apparatus may be considered that normally associated with
a transmitter such as 30b for noise rejection, but being operated in
accordance with the principles of the invention. Examples of the
incorporation of such noise switches in DSI systems for noise rejection
may also be had by reference to patents cited in the preceeding paragraph.
The embodiment shown in FIG. 3 employs a voice switch adapted to receive
PCM encoded speech signal samples from PCM A-D encoder 20b of FIG. 1, and
regulate the transmission of such signal samples to the DSI transmitter
30b by means of a set of gates 110. Gates 110 enable the transmission of
the PCM signals to DSI transmitter 30b under control of a circuit
responsive to the amplitude of the PCM signals from encoder 20b. PCM
signals that exceed a threshold, selected according to the invention, will
cause the gates 110 to be opened and will be transmitted. To this end,
speech signal samples from encoder 20b are also applied to a conventional
digital threshold comparator 120. This device digitally compares the
encoded signals from encoder 20b to one of two selected threshold levels.
One of the two levels is selected on the basis of the occurrence of
received speech signal samples at the DSI receiver 40a associated with
telephone 10b. To this end, multiplexer 130 is provided which selects one
of the two levels and applies the selected level to comparator 120. It is
adapted to respond to the presence of received speech signal samples at
DSI receiver 40a, and, depending on whether or not these signals samples
are present, would apply either a larger echo-proof or smaller noise-proof
threshold level to the comparator 120. The larger echo-proof threshold is
applied when receiver 40a indicates the presence of received speech signal
samples and the lower noise-proof threshold is used in all other
instances. If only received samples are present it is still necessary to
utilize the higher echo-proof threshold even when no speech is to be
transmitted from telephone 10b since it is advantageous in this situation
to prevent unnecessary false encoding and transmission of echo signals by
the system.
In the simplest case detection of the receive signal may be accomplished by
a conventional current sensor whose output may control multiplexer 130,
however the detection in a DSI system is more complex. Many DSI systems
use a time-sharing arrangement for utilizing the tranmitter/receiver and
encoder/decoder pairs at each side of the communications system (e.g.,
30b, 40a and 20b, 50a, respectively at the talker B side in FIG. 1) for a
multiplicity of transmit and receive channels so that transmit and receive
signal samples may be properly routed and assigned among the several
talkers in the system. Various arrangements of effecting such assignments
in DSI are discussed in the aforementioned article of Campanella and the
noted U.S. Patents. In all such arrangements it is necessary that the
appropriate output of the receiver 40a for each conversation be
coordinated for the gates 110 in the transmit path so that the gates 110
receive the appropriate signal samples and are activated for the
appropriate conversation. Thus the input to multiplexer 130 from receiver
40a should itself be multiplexed to coordinate the appropriate received
signal samples from the receiver 40 b with the appropriate signal samples
applied by encoder 20b to gates 110 for a given conversation. The way in
which this is done, of course, is dependent upon the way in which the
transmitter/receiver apparatus is time-shared among the multiple channels
used in the DSI system. However, it would be similar to the way in which
the received and transmitted signal samples are coordinated by the DSI
system.
In DSI systems the indication by receiver 40a of present receive signal
samples for multiplexer 130 can be obtained in a relatively
straightforward way. In many DSI schemes, DSI receiver 40a produces in
appropriate sequence for channel assignment, digital words corresponding
to a PCM-coded receive signal sample for each channel, or an indicator of
which channels are idle (no received sample). In this type of DSI system,
an appropriate logic arrangement may be used to sense these digital words,
such as a shift register with at least the number of positions as bits of
the PCM code for the signal samples, and an OR gate with inputs
corresponding to each position in the register. The shift register of this
arrangement may receive the sequential PCM samples from the receiver 40a,
and be reset before each new sample is applied to it. The OR gate in this
way indicates the presence or absence of a receive speech sample in the
sequence. The OR gate may thus supply the control signal indicative of
received speech to multiplexer 130. Alternatively, in speech predictive
encoded communications DSI systems (SPEC) as described in U.S. Pat. Nos.
3,988,674 and 3,927,268, the sample assignment work (SAW) itself furnishes
the receive channel usage information directly.
The two threshold levels to be selected by multiplexer 130 are provided by
level signal sources 131 and 132. As the presently described embodiment
relates to DSI systems where the speech samples are in a PCM code, these
sources would respectively apply appropriate digital words corresponding
to the upper "echo-proof" signal threshold and the lower "noise-proof"
signal threshold. The operation of threshold comparator 120 is thus
dependent upon the presence or absence of received speech signal samples
at the receiver 40a; it thus compares the transmit signal samples to
either a higher or lower threshold supplied by the multiplexer 130 to
determine whether the speech signal samples produced by encoder 20b are
found to exceed the selected threshold.
The threshold comparator 120 produces a digital signal indicating whether
or not the encoded speech samples from encoder 20b exceed whichever of the
lower (132) or higher (131) threshold which has been selected by
multiplexer 130. In order that these digital signals may control the
conduction of gates 110, a shift register 140 is provided to receive the
digital indications produced by comparator 120. The shift register 140 is
adapted to count the number of successive times the signal samples applied
to detector 120 exceeds the selected threshold within a given amount of
time, such as, for example, a period of three or four times the sampling
period of the DSI encoder 20b, corresponding to multiples of the Nyquist
sampling period of 125 .mu.sec. The choice of a number, such as three or
four, of such successive samples above the threshold as a precondition to
actuation of gate 110 can be established by empirical testing. It
represents a compromise choice among competing criteria: noise "spike"
rejection, threshold level, and speech "clipping". For a discussion of how
such a choice may be made, see the article, "A New Digital Voice-Activated
Switch" by J. A. Jankowski, Jr., Comsat Technical Review, Vol. 6, No. 1,
Spring 1976, pp. 159-178 and U.S. Pat. Nos. 4,008,375 and 4,052,568 noted
above.
AND gate 145 is provided to receive the output of the shift register 140.
The input to AND gate 145 from shift register 140 is so configured that if
the aforementioned predetermined number of successive samples (the
configuration of four successive samples is illustrated) are found to
exceed the selected threshold, an output signal is generated from AND gate
145 and applied to hangover circuit 150 which allows gates 110 to remain
open for a suitable period for hangover delay. Typically, this delay is
about 150 milliseconds and allows the gates 110 to remain open subsequent
to the removal of the output signal from AND gate 145. Switch gates 110
are normally non-conductive to prevent transmission of the signal samples
applied to it but with the presence of the control signals provided by the
AND gate 145, gate 110 is rendered conductive. It thus allows the samples
obtained from the encoder 20b to be applied to the DSI transmitter 30b. A
delay circuit 100 is also provided between encoder 20b and gates 110 to
delay the encoder output from reaching the gates prior to the completion
of operation of comparator 120, register 140 and gate 145 which determine
the state of gates 110. This delay period typically would be on the order
of 4-5 milliseconds.
Thus, when echo conditions are present (the detection of receive speech
signal samples) switch gates 110 inhibit the transmission of any signal
samples supplied by the encoder 20b that fail to exceed the higher
"echo-proof" threshold supplied by source 131. It will be understood that
if the normal threshold 132 is an adaptive one which is continually
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