 |
Description  |
|
|
TECHNICAL FIELD
The present invention relates to a technique for providing improved
subjective performance in a communication system or network. More
particularly, during moments when no speech or other information signal is
being transmitted to an end user during a call, a noise-fill level is
provided to a connected end user which is at a predetermined level below
that normally found on a channel when there is both an information signal
and a noise signal being received.
DESCRIPTION OF THE PRIOR ART
In certain communication systems it has been found advantageous to insert
noise when a speech or data signal is not present. More particularly, in,
for example, a packet communication system, an access interface (AI)
generates voice packets only when its speech detector determines that
there is activity present on a trunk. When gaps are encountered in the
packet stream, the receiving AI inserts noise at a level corresponding to
the continuous real-time estimates of the background noise present in the
incoming trunk. These estimates can, for example, be made at the
transmitting AI and sent to the receiving AI in the packet headers or
preambles. Noise insertion is performed to prevent degradations in voice
quality that could result from obvious discontinuities in the background
noise level as the speech transmission is switched on and off. The term
"noise pumping" is frequently used to describe the sudden changes in noise
associated with inadequate background noise matching.
An exemplary communication system that uses a noise-matching technique is
described, for example, in (1) the article "TASI-E Communication System"
by R. L. Easton et al. in IEEE Transactions On Communications, Vol.
COM-30, No. 4, April 1982, at pages 803-807, and in particular at pages
804 and 805, and (2) in U.S. Pat. No. 4,408,324 issued to D. H. A. Black
et al. on Oct. 4, 1983. In such communication system, a channel-checking
arrangement is used to periodically measure, inter alia, the noise on the
channels of the system, the measured channel noise then being used along
with the measured background noise on the incoming trunk in a
noise-matching operation during silent periods on the channel. In other
words, when a trunk is not connected to a channel, such communication
system inserts noise at the transmitting or receiving end of that channel
to make the total noise at the channel output equal to the same value as
when the trunk is connected to the channel and a signal is being
transmitted over the channel, thereby avoiding various effects such as
noise-pumping.
Similar techniques are also used in digital conferencing arrangements as
described, for example, in U.S. Pat. No. 4,482,998 issued to M. A. Marouf
et al. on Nov. 13, 1984. There, when no one is momentarily speaking during
a conference connection, which is formed from a plurality of ports on a
bridge connection, a minimum number of selected ports are maintained in a
holdover state to provide background noise on the bridge. This ensures
that each conferee receives some minimum background noise to eliminate the
feeling that the conferee is cut off from the conference.
The problem remaining in the prior art is to provide a technique which can
further improve, if possible, the subjective performance of a
communication system, which includes speech interpolation and may or may
not include noise-matching.
SUMMARY OF THE INVENTION
The foregoing problem in the prior art has been solved in accordance with
the present invention which relates to a technique in a communication
system such as, for example, a Wideband Packet Technology (WPT) Access
Interface (AI) with speech interpolation, wherein a noise measurement at
the transmitter end, or the reproduced noise-fill at the receiver end, is
attenuated or reduced by a predetermined amount from the average monitored
level of noise normally received over a channel when communication is
taking place before being provided to an end user during non-information
transmission periods.
Other and further aspects of the present invention will become apparent
during the course of the following description and by reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of pertinent parts of an exemplary communication
system employing the reduced noise-fill technique of the present
invention;
FIG. 2 depicts an exemplary arrangement for a combination of a speech
detector, noise measuring means and attenuator in the transmitter of FIG.
1; and
FIG. 3 is a graph representing exemplary result of providing improved
subjective performance of speech transmission by reducing the amount of
noise fill provided during gaps in the speech transmission below the point
(0 dB) where the matched noise level is exactly equal to the input noise
level.
DETAILED DESCRIPTION
FIG. 1 shows only pertinent parts of a transmitter 10 and a receiver 20
within a communication system for practicing the present noisematching
technique, where information signal detection and noise measurements are
performed at transmitter 10 and used at both the transmitter 10 and the
associated receiver 20. For purposes of explanation hereinafter, it will
be assumed that the information signal used as an input to transmitter 10
is a speech signal, but it should be understood that the information
signal could comprise any other information signal, such as a music
signal, that a person associated with a remote receiver might be listening
to. Additionally, it should be understood that any form of information
signal transmission can be used for practicing the present invention of
noise matching, as, for example, analog, digital or packet transmission
with a wideband or narrow band spectrum, since the form of transmission is
arbitrary.
More particularly, during a connection between two end users in voice
communication, the input speech signal from a first end user at input 11
is directed to each of (a) a first input terminal 13 of a switching means
12, (b) a speech detector 16, and (c) a noise measuring arrangement 17.
Another input signal, such as data signals, that might be sent over the
same communication channel 18 between transmitter 10 and receiver 20 is
provided as an input to a second input terminal 14 of switching means 12.
Speech detector 16 monitors input 11 to determine whether a speech signal
is active (present) or inactive (not present) and provides an output
control signal which is representative of the speech activity and is
received by both switching means 12 in transmitter 10 and remote receiver
20 via communication channel 18. The control signal from speech detector
16 causes switching means 12 in transmitter 10 to (a) connect input
terminal 13 to output terminal 15 when a speech signal is being detected
at input 11 in order to transmit the detected speech signal to receiver 20
over communication channel 18, and (b) to connect input terminal 14 to
output terminal 15 when a speech signal at input 11 is not detected in
order to transmit the other input signal, when present, arriving at second
input terminal 14 of switching means 12 to receiver 20 over communication
channel 18. The other input signal on input 14 of switching means 12 can
be, for example, a packet signal which has its packets stored in a memory
(not shown) for transmission by a gating means (not shown) which is
responsive to the same speech detector output control signal that causes
switching means 12 to connect its input 14 with its output 15. In this
manner other signals can be transmitted on communication channel 18 when
it is not being used for the speech signal transmission and thereby
provide a Speech Interpolation technique.
Noise measuring arrangement 17 is used to determine the level of the
background noise in the speech signal at input 11 and to generate a
background noise level control signal for transmission either directly to
receiver 20 over communication channel 18 or indirectly to receiver 20 via
optional attenuator arrangement 19. In accordance with the present
invention, when attenuator arrangement 19 is present in transmitter 10, it
functions to reduce the value of the determined background noise level by
a predetermined amount before being transmitted to receiver 20 over
channel 18. It is to be understood that attenuator arrangement 19 is an
optional element and (a) is, therefore, shown by dashed lines, and (b)
when present in transmitter 10 can form a part of noise measuring
arrangement 17. Alternatively, attenuator arrangement 19 can be disposed
in receiver 20 as an optional attenuator arrangement 22 as will be
described hereinafter. It is to be understood that the concurrent
information, active/inactive, and noise value signals transmitted on
communication channel 18 are transmitted as separate portions of an
overall communication signal and, therefore, can be concurrently
transmitted in any suitable manner such as, for example, on separate leads
or in a composite signal in, for example, the header and information
portions of a packet or in different frequency subbands of the composite
signal. It is to be understood that speech detector 16 and noise measuring
arrangement 17 may actually be formed as part of one circuit as will be
shown hereinafter in FIG. 2, but is described here as separate elements
for ease of description.
At receiver 20, (a) the active/inactive control signal portion is received
by each of a first and a second switching means 23 and 27 in order to
control the path through these switching means; (b) the information signal
portion is received at an input terminal 28 of second switching means 27;
and (c) the noise value control signal portion is received by a noise
generator 21. Noise generator 21 is responsive to the background noise
level control signal transmitted by noise measuring arrangement 17 in
transmitter 10 for generating a level of noise which corresponds to the
level of background noise indicated by the received background noise level
control signal. In accordance with the present invention, the background
noise signal produced by noise generator 21 is provided to the input
terminal 24 of first switching means 23 either (a) directly when the noise
value signal from noise measuring arrangement 17 has been previously
attenuated by the predetermined amount in attenuator 19, or (b) indirectly
via attenuator 22 when the noise value signal from noise measuring
arrangement 17 has not been previously attenuated before being
transmitted. It is to be understood that optional attenuator 22 can be a
separate circuit, disposed before or after noise generator 21, or form a
part of noise generator 21. Regardless of which attenuator 19 or 22 is
used, the resultant noise signal provided to input terminal 24 of first
switching means 24 is a signal which has been attenuated or reduced in
level by a predetermined amount from the background noise level which was
determined for the signal at input 11 of transmitter 10.
In operation, when speech detector 16 at transmitter 10 detects the
presence of a speech signal, including background noise, at input 11, it
generates a control signal having first value which is transmitted to
receiver 20 while simultaneously causing switching means 12 to connect
input terminal 13 to output terminal 15 and thereby transmit the speech
signal, and included background noise, to receiver 20. Noise measuring
arrangement 17, which has been continuously determining the background
noise level received at input 11, transmits a noise value, which either
has been attenuated (when attenuator 19 is present) or has not been
attenuated (when attenuator 19 is not present) to receiver 20. At receiver
20, the received first value control signal from speech detector 16 causes
first switching means 23 to close the path between input terminal 24 and
output terminal 26 to divert any noise signal from noise generator 21 away
from a first output path 31 of receiver 20, while simultaneously causing
second switching means 27 to close the path between input terminal 28 and
output terminal 30 to direct the received speech signal, and included
background noise, to first output path 31.
When speech detector 16 does not detect a speech signal at input 11 of
transmitter 10, it generates a control signal having a second value which
is transmitted to receiver 20 over communication channel 18 while
simultaneously causing switching means 12 to connect input terminal 14 to
output terminal 15 and thereby transmit other input signals to receiver
20. At receiver 20, Noise Generator 21 generates a noise signal at a level
specified by the current measured background noise level of the signal at
input 11, or by a prior measured background noise level value measured
during a last period when speech signal was not detected, this noise level
being determined in transmitter 10 by noise measuring arrangement 17 with
or without optional attenuator 19. The received second value control
signal from speech detector 16 causes first switching means 23 to close
the path between input terminal 24 and output terminal 25 to direct the
attenuated noise fill signal obtained from noise generator 21 and optional
attenuator 22 (when present) onto first output path 31 from receiver 20,
while simultaneously causing second switching means 27 to close the path
between input terminal 28 and output terminal 29 to direct the received
other information signals onto a second output path 32 from receiver 20.
By the above technique, when a speech signal, including background noise,
is detected at input 11 of transmitter 10, the speech plus background
noise signal is transmitted to receiver 20 via switching means 12 and
communication channel 18, and directed onto first output path 31 from
receiver 20 by second switching means 28. No additional attenuated noise
fill signal is provided to first output path 31 from first switching means
23 because the path between input terminal 24 and output terminal 25 is
not closed. When no speech signal is detected at input 11 of transmitter
10, then another input signal is transmitted via switching means 12 and
communication channel 18 to receiver 20 in place of the normally
transmitted speech signal, where this other information signal is directed
by second switching means 27 onto second output path 32 while only an
attenuated noise fill signal is transmitted over first output path 31 to
the listening, or possibly speaking, end user. It is to be understood that
first and second switching means 23 and 27 at receiver 20 can have any
suitable arrangement to realize comparable interconnections.
In an exemplary wideband packet technology transmitter 10, which is also
known as an access interface (AI), with digital speech interpolation, an
exemplary arrangement for providing noise matching in transmitter 10
according to the present invention is shown in FIG. 2. In the arrangement
of FIG. 2, it will be seen that speech detector 16, noise measuring
arrangement 17 and optional attenuator 19 of FIG. 1 can all be formed as
part of speech detector 16. Noise matching involves two functions, noise
level estimation and noise generation. Noise level estimation is performed
as part of the speech detection function in speech detector 16 by the
following exemplary digital circuits. The speech signal at input 11 is
high-pass filtered in HP filter 40 to reduce hum and remove any DC
component. The resultant signal is full-wave rectified in rectifier 41 and
then low-pass filtered in LP filter 42. The resulting envelope signal is
monitored in peak monitor circuit 43 for the peaks and minima levels.
These peak and minima levels are taken to be measures of the speech level
and the background noise level, respectively, and are used by speech
detector 16 in setting its speech threshold. In accordance with the
present invention, when the noise value to be transmitted to receiver 20
is to be attenuated before transmission to receiver 20, optional
attenuator 19 would be used with a noise level translator 44 to provide a
digital noise value which is at the predetermined reduced or attenuated
value. It is to be understood that attenuator 19 can be either before or
after or a part of noise level translator 44.
In operation, the noise level measure from speech detector 16 can be made
available as, for example, an 8-bit quantity for transmission to noise
level translator 44 and/or attenuator 19. The range of noise levels this
represents could be divided into 16 exemplary parts and translated to a
4-bit noise level value by noise level translator 44. This 4-bit noise
level value would be transmitted in, for example, the header of each
speech packet during the period when the end user associated with input 11
is active and providing a speech signal. When the end user associated with
input 11 becomes inactive and, therefore, packets are no longer sent, the
noise level value transmitted in the last packet, and received at receiver
20, is used to generate an appropriate random noise signal by noise
generator 21, which noise signal is inserted in the speech gap at output
31 by switching means 23. In an exemplary arrangement of noise generator
21, for each of the 16 noise levels, noise generator 21 can produce a
random or pseudorandom sequence of Pulse Code Multiplex (PCM) samples with
the desired noise powers.
In accordance with the present invention, the noise fill signal to be
provided during gaps in the output speech signal at output 31 of receiver
20 to provide improved subjective performance of speech signals was found
to occur when the noise signal reduced below the 0 dB noise match value.
More particularly, in an illustrative example shown in FIG. 3, the
subjective performance of speech transmission is found to be improved, in
accordance with the concept of the present invention, as the noise fill
signal is reduced below the 0 dB noise match value. From the graph of FIG.
3, it can be seen that the subjective performance was found to
increasingly improve as the background noise fill is decreased from the 0
dB noise match value until it reaches a maximum between the -3 and -6 dB
noise match value depending on the actual noise level at input 11. The
subjective performance was found to decrease at noise match values below
-3 dB. Therefore, it is shown that the end users find the subjective
performance of speech transmission better when the noise match level is
reduced below the 0 dB, level with the best subjective performance
occurring between the -3 and -6 dB noise match levels.
* * * * *
|
|
 |
|
 |
Description |
|
