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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to digital voice communication; and, more
particularly, to a digital voice communication system and method that
involves the radio transmission of synthesized speech.
Although the present invention is suitable for many different voice
communication systems that involve switching "on " and "off" of voice
transmission during periods of silence, it is particularly advantageous
for use in cellular digital telephone systems and is described in
connection therewith.
2. Discussion of Related Art
A cellular communication system is a mobile telephone service where radio
coverage is divided into cells; and each cell is assigned a number of
available radio frequencies. A mobile telephone station transmits and
receives control and voice communication information from a base station
within the same cell. The base stations are controlled by a cellular
system switching and control network that provides connection with the
world wide telecommunication system.
In digital communication systems, assigned frequencies are divided into
individual channels of communication, with the transmit and receive
frequencies being separated from each other. Each channel of information
has a frame format, that is, each channel transmits a succession of
frames, which has a duration typically of forty milliseconds, and
constitutes one cycle of a regularly recurring series. Each frame of
information is transmitted in one of six time slots. Each slot includes
one hundred sixty-two symbols, and has a duration of approximately 6.67
milliseconds. Each slot corresponds to a burst of RF energy that includes
compressed digital speech signals, which are decompressed at the receiving
station and converted to analog speech.
An encoder is provided in each transmitter, both at a base station and a
mobile station, which synthesizes the speech signals before modulation and
transmission thereof. One type of cellular communication system includes a
technique for low rate speech coding, referred to as Codebook Excited
Linear Prediction (CELP), which involves searching a table or codebook of
randomly distributed excitation vectors for that vector which, when
filtered through pitch and linear predictive coding short term synthesis
filters, produces an output sequence which is closest to the input
sequence. This output sequence of synthesized speech codes occurs upon
excitation of the input sequence which, in turn, occurs upon the
introduction of the digital equivalent of analog speech.
Upon the detection of voice inactivity, which occurs between words,
sentences, or pauses in conversation, for example, the input to the
encoder is switched off, which interrupts transmission of the RF energy.
This switching on and off of the transmitter during a conversation
produces audible switching artifacts, which at times leads the listener to
believe the connection is being inadvertently interrupted, and at the very
least, causes the listener substantial annoyance and discomfort.
Heretofore, it has been proposed to produce an artificial background noise
during periods of voice inactivity. This was in the form of background
noise that was encoded and generated independently of the conversation
preceding the inactivity. Although suitable for the purposes intended, the
proposed background noise generation was at times substantially different
from the background noise of the conversation during periods of voice
activity, which may be unpleasant and disconcerting to the listener.
SUMMARY OF THE INVENTION
One of the objects of the present invention is to alleviate the annoyance
and discomfort to a listener caused by on and off switching artifacts
between intermittent periods of voice activity during a conversation over
a digital communication system.
Another object of the present invention is to provide background noise for
a discontinuous transmission and receiving system during periods of voice
inactivity that has the attributes of background noise during periods of
voice activity.
Additional objects and advantages of the invention will be set forth in the
description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention will be realized and attained by means of
the elements and combinations particularly pointed out in the appended
claims.
To achieve the objects and in accordance with the purpose of the invention,
as embodied and broadly described herein, the invention is a method of
generating background noise during intervals of voice inactivity in a
digital communication system, having a transmitter with an encoder for
encoding and transmitting discontinuous frames of digital information, and
a receiver with a decoder for receiving and decoding the discontinuous
frames of transmitted information, comprising, detecting in the
transmitter, transitions between voice activity and voice inactivity,
discontinuing transmission of digital information a predetermined time
following detection of voice inactivity, resuming transmission upon
detection of voice activity, decoding digital output data received from
the transmitter, detecting in receiver transitions between voice activity
and voice inactivity of the transmitter, processing the decoded digital
output data including data received after the detection of voice
inactivity in the receiver to generate data having attributes of
background noise transmitted during the predetermined time following
detection of voice inactivity, and applying an analog equivalent of the
generated data continuously to an output speaker of the receiver during
discontinuance of transmission by the transmitter.
In another aspect, the present invention is a digital communication system
comprising a transmitter having an analog to digital converter for
converting analog input speech to digital data, a voice encoder for
encoding the digital data, a voice activity detector for detecting a
transition between voice activity and inactivity, a switch for
discontinuing transmission of the encoded data a predetermined time period
subsequent to the detection of voice inactivity, a receiver disposed
remote from the transmitter having a decoder for decoding the received
data, a speaker for outputting an analog equivalent of the decoded data, a
comfort noise generator at the receiver for outputting digital signals
corresponding to noise having a spectral shape and loudness level similar
to the received data decoded by the decoder, and a switch at the receiver
for connecting the generator output to the speaker at the expiration of
the predetermined time period following detection of voice inactivity.
In still another aspect, the present invention is a system for generating
background noise for a digital communication system, comprising means for
receiving synthesized noise, means for deriving an average loudness level
of the received noise, means for deriving filter coefficients from the
received noise, a synthesis codebook having a table of values
corresponding to long term estimates of background noise, an excitation
codebook having a table of values corresponding to long term spectrally
flattened background noise estimates, an infinite impulse response filter
responsive to the excitation table values in accordance with the derived
filter coefficients to output signals having spectral shape attributes
corresponding to the received noise, means for scaling the synthesized
background noise estimate signals to produce a first series of signals
having a loudness level corresponding to average RMS level over a
predetermined time period following detection of voice inactivity and
means for scaling the filtered spectral shape signals to produce a second
series of signals each having a spectral shape corresponding to long term
spectral shape of the background noise having said loudness level, means
for weighting the first and second signals to vary the loudness level and
spectral shape periodically, and means for combining the weighted first
and second series of signals to generate the comfort noise.
It is to be understood that both the foregoing general description and the
following detailed description are exemplary and explanatory only and are
not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part
of this specification, illustrate one embodiment of the invention and
together with the description, serve to explain the principles of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of the transmitting portion of the
communication system incorporating the present invention;
FIG. 2 is a schematic block diagram of the receiving portion of the
communication system incorporating the present invention;
FIG. 3 is a functional block diagram of the comfort noise generator of FIG.
2 in accordance with the present invention;
FIG. 4 is a schematic diagram of a filter used in the comfort noise
generator of the present invention; and
FIG. 5 is a flow chart of the comfort noise generator of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made in detail to the present preferred embodiment of
the invention, an example of which is illustrated in the accompanying
drawings. Wherever possible, the same reference numerals will be used
throughout the drawings to refer to the same or like components. When
using the term connected or electrically connected herein, it is not
intended to mean directly connected but may mean ultimately connected,
where components may be connected therebetween but are omitted in that
they do not aid in the understanding of the invention. Also, when using
the term switch herein, it is understood that it can be any device or
method for connecting inputs and outputs of software or hardware
components.
The system of the present invention comprises a transmitter with a
microphone input, an analog to digital converter, a delay/instantaneous
switch circuit, a voice encoder, a voice forward error correction encoder,
a voice activity detector, a modulator, and an RF power amplifier.
As herein embodied and shown in FIG. 1, a transmitter generally referred to
at 10 has a microphone 12 for inputting analog speech. Connected to the
microphone is an analog to digital converter 14 for converting the analog
input speech to digital data. Electrically coupled to the output of the
A/D converter over line 15 through switch 16 of switching circuit 18 is a
voice encoder 20 for compressing digital speech signals. A voice FEC
encoder 22 has an input coupled to the output of voice encoder 20 for
providing parity bits, for example, to protect against transmission
errors. A modulator 24 has an input coupled to output 26 of voice FEC
encoder 22 for modulating the digital speech signals. Power amplifiers 28
are connected to modulator 24 over output line 30. A voice activity
detector 32 has an input coupled to output line 15 of A/D converter 14 and
an output 34 coupled to voice FEC encoder 22. Output line 34 represents a
voice activity flag that is high as long as a voice is detected and goes
low when a voice ceases. Switch circuit 18 includes a delay component 36
having an input connected to line 34 through a NOT gate 38 and an output
40 connected to switch 16 through AND gate 42. Line 34 is also connected
via a NOT gate 46 directly to AND gate 42 in parallel with delay component
36 over line 44.
When input 40 of gate 42 is low and input 44 is low, switch 16 is closed.
When input 44 goes high and input 40 goes high, switch 16 opens. This
causes a delay of eighty milliseconds upon the cessation of voice activity
before switch 16 opens. Upon the resumption of voice activity, line 34
goes "high" which causes input 44 to go low, which immediately causes the
switch 16 to close without delay. The changing of input 40 to low after
eighty milliseconds does not change the operated state of the switch.
Thus, there is a delay in opening switch 16 upon the detection of voice
inactivity, but no delay in closing switch 16 upon the detecting of voice
activity.
The system of the present invention comprises a receiver, having RF power
amplifiers, a demodulator, a voice FEC decoder, a voice decoder, a
delay/instantaneous switch, a digital to analog converter, an output
speaker, and a comfort noise generator.
As herein embodied and shown in FIG. 2, a receiver generally referred to as
50, comprises power amplifiers 52 for amplifying incoming signals, a
demodulator 54 having an input connected to amplifiers 52, and an output
connected to voice FEC decoder 56. Decoder 56 is connected at its output
to voice decoder 58 over lines 60 and 62. Voice decoder 58 is connected at
its output 64 to one terminal of switch 66 of delay/instantaneous switch
circuit 68. Switch 66 has a common terminal 69 connected to D/A converter
over input line 72. An output speaker 74 is connected to output 64 of the
D/A converter. A comfort noise generator 76 has an output connected to
terminal 78 of switch 66 and in input 80 connected to the output of voice
decoder 58, and another input over line 82 connected to line 60 at the
input of voice decoder 58. Line 60 changes from "one" to, "zero" upon the
transition from voice activity to voice inactivity. Output line 80 of
voice decoder 58 outputs synthesized speech from voice decoder 64 to the
input of comfort noise generator 76. Delay/instantaneous switch 68
includes a delay component 84 having a NOT gate 86 disposed in the input
of delay component 84 and an AND gate 88 connected in the output of delay
component 84. Upon the detection of a transition from voice activity to
voice inactivity, line 60 goes from one to zero which changes both input
90 and 92 of switch 68 to high. After a delay of eighty milliseconds
through delay component 84, output line 94 of gate 88 goes high which
connects switch 66 to terminal 78 of generator 76 and disconnects switch
66 from voice decoder 84. Upon transition from voice inactivity to voice
activity, line 60 goes high which immediately causes input 92 of AND gate
88 to go low, and change the position of switch 66 to disconnect switch 66
from the output of the comfort generator and connect it to output 64 of
voice decoder 58. A delay of eighty milliseconds will have no effect.
When input to gate 88 from delay component 84 goes low switch 66 will
remain connected to voice decoder 58 until line 92 goes low.
Thus, similar to the transmitter 10, a transition from voice activity to
inactivity causes a delay of eighty milliseconds before the output of
comfort noise generator 88 is connected to input line 72 of D/A converter
70; and a transition from voice inactivity to voice activity causes an
immediate connection of voice decoder 58 to input line 72 of the D/A
converter.
In operation, during each pause in the conversation, background noise
corresponding to two frames of information is transmitted and received
prior to discontinuing transmission. Thus, in the transmitter 10 that is
communicating with this receiver 50, eighty milliseconds of background
noise is being transmitted after the transition from voice activity to
voice inactivity. During this eighty millisecond delay in the receiver,
ten, separate eight millisecond samples of the transmitted background
noise are input to comfort noise generator 76 over line 80 and
simultaneously output through switch 66, terminal 68, over line 72 to D/A
converter 70.
Referring to FIG. 3, and as herein embodied, comfort noise generator 76
comprises an excitation codebook 100 containing a table of floating point
numbers that correspond to long term estimates of spectrally flattened
background noise and a synthesis codebook 102 containing a table of values
corresponding to long term estimates of background noise. Codebooks 100
and 102 preferably each has approximately 4k of random entries, and
include a clock that preferably reads out the codebook entries every eight
milliseconds, for example.
An infinite impulse response filter 104 is connected to output 106 of
codebook 100; and a demultiplexer 108 accepts the decoded synthesized
noise from line 80 (See FIG. 2) of the receiver, and derives filter
coefficients from the background noise received during the eighty
milliseconds or two frames of delay over lines 110 and 112. The loudness
level for each eight millisecond sample is obtained also by averaging the
loudness level over the eighty millisecond periods.
A multiplier 114 normalizes each sample of an eight millisecond block of
samples on line 115 corresponding to the output from filter 104 to the
average RMS level or loudness derived from the final eighty milliseconds
of transmission at the end of the speech spurt. The normalized scale
factor is compared in block 116. A multiplier 120 similarly normalizes
each entry of an eight millisecond block of samples from synthesis
codebook 102 from line 121 to the average RMS level or loudness of the
final eighty milliseconds of transmission at the end of the speech spurt.
The normalized scale factor is compared in block 122.
The averaged outputs on lines 118 and 124 are summed at 126 through
multipliers 128 and 130, to output on line 32, comfort noise which has the
attributes of the final eighty milliseconds of transmission subsequent to
detection of voice inactivity.
Prior to combining the signals on lines 118 and 124, they are multiplied by
a weighting factor on lines 134 and 136, respectively. Weight factor
.varies. on line 134 for each block of sixty-four samples starts with a
value 1.0 and decrements once every sixty-four samples by a small number
0.0 D until it reaches zero. Weight factor 1-.varies. on line 136 starts
at zero and increments once every sixty-four samples by the same small
number 0.0 D until it reaches "1;" the sum of the two weighting factors
always equalling "1 ". This changes the mix of the loudness level and
spectral shape of the comfort noise to more closely resemble reality and
alleviate the feeling of artificiality during long periods of voice
inactivity of a conversation.
Referring to FIG. 4, filter 104 has ten summing stages X1 through X10. The
entries from excitation codebook 100 enter the filter at X1. The output of
the filter is moved successively every sample or 125 microseconds, similar
to a shift register. These outputs are called state variables and are
denoted by SV1 to SV10. At each summing stage, the state variables are
multiplied by filter coefficients al through a10 at respective multipliers
M1 through M10. These filter coefficients are derived from synthesized
speech samples over two frames of information following the end of voice
activity. The products of each of the multipliers M1 through M10 are
summed at each step one cycle of the filter and output on line 115.
Referring to FIG. 5, an algorithm, which may be installed in a fixed point
digital signal processor, is illustrated as implementing the method and
system of the present invention. As previously mentioned, the synthesized
noise is input over line 80, as indicated at block 149, and is initialized
by setting .varies. to "1", deriving an average loudness level L, and
converting the background noise autocorrelation lags representative of the
spectral shape of the input noise to filter coefficients a, and setting
state variables to zero, as indicated at block 142. Once the system is
initialized, it is operating both during periods of voice activity as well
as inactivity. Since switch 66 does not close until eighty milliseconds
after the cessation of voice activity, filter 76 will have filter
coefficients that correspond to background noise only.
Every eight milliseconds or five times each frame, a series of sixty-four
sample entries are simultaneously read from excitation codebook 100 and
synthesis codebook 102 as indicated at blocks 144 and 146 respectively.
The entries from codebook 100 are passed through filter 104 having
coefficients corresponding to the last two frames transmitted as indicated
at block 148. Each sample entry from synthesis codebook 102 is scaled to
have a value corresponding to a two frame average of the loudness level L
as shown at block 150. Also, the outputs of the filter 104 are scaled to
have a loudness level averaged over the last two frames of received data
as shown at block 152. Each RMS value from block 150 is weighted with at
block 154; and each RMS value from block 152 is weighted with at block
156. Every 64th sample .varies. is decremented by 0.00 D and 1-.varies. is
incremented as illustrated at blocks 158 and 160. The scaled and weighted
synthesized values Y.varies. and X.(1- .varies.) are combined to produce
the comfort noise Z at block 162. The codebook pointers are updated in
block 164 at the end of the eight MS interval. If there is still no voice
activity, the process is repeated as indicated at decision block 166 to
commence as indicated by line 168.
Having described the presently preferred system embodiment and method of
the invention, additional advantages and modifications will readily occur
to those skilled in the art. For example, the sampling times could be
varied as well as the frequency with which the weights are incremented or
decremented. Also, the switch could provide for a greater or lesser delay
before discontinuing transmission upon detection of voice inactivity, or
the number of stages of the filter could be increased or decreased, if
desired, for example. Accordingly, the invention in its broader aspects is
not limited to specific details, representative apparatus, and
illustrative examples shown and described. Departure may be made from such
details without departing the spirit or scope of the general inventive
concept as defined by the appended claims and their equivalents.
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