Patent Publication Number: US-6708147-B2

Title: Method and apparatus for providing comfort noise in communication system with discontinuous transmission

Description:
BACKGROUND OF THE INVENTION 
     The invention disclosed and claimed herein generally pertains to a method and apparatus for producing comfort noise in a system for transmitting information, wherein transmissions are intermittently discontinued. More particularly, the invention pertains to a system for transmitting human speech, wherein transmissions are discontinued during speech pauses. Even more particularly, the invention pertains to a communication system which is disposed to transmit information through an air interface. 
     There is increasing interest in providing mobile phones and other small electronic devices with the capability to connect and communicate wirelessly, over short ranges, with one another. Such capability would eliminate or substantially reduce the need for cables between devices such as phones, PC cards, headsets and laptops computers. Moreover, a number of such devices could thereby be joined together, very readily, to form small networks. As an example of this interest, the assignee herein, a major supplier of mobile telecommunication equipment and systems, has initiated a program known as the Bluetooth air interface to develop wireless communication capability of the above type. 
     In interconnecting small devices over a short range air interface, it is very important to restrain costs, and also to reduce the overall interference level on the interface, in order to improve RF spectrum efficiency. Accordingly, in an air interface such as the Bluetooth interface, a transmitter disposed to receive speech is provided with a discontinuous transmission (DTX) capability, which causes the transmitter to be switched off during speech pauses. Such capability reduces costs, by minimizing transmitter power requirements and also reduces signal interference level. The benefits of DTX may be particularly significant in connection with telephone transmissions, in view of the fact that during a normal phone conversation, the participants alternate so that, on average, each direction of transmission is utilized only about 50% of the time. 
     Notwithstanding the benefits and advantages of discontinuous transmission, background noise, which is transmitted together with speech in a system of the above type, disappears during silent or speech absent periods when the transmitter is switched off. This results in modulation of the background noise. More particularly, since switching of the transmitter between its transmit and transmit discontinue modes can be quite rapid, a very annoying noise effect can be produced at the receiver. 
     A principal technique of the prior art, to avoid such annoying noise, has been to model the background noise by speech encoder model parameters which are updated at low rate intervals. The model parameters are then used to generate comfort noise signal on the receiver side of the communication system, during periods when the transmitter is turned off. Such a scheme is generally suitable when both a model-based speech coder and a high performance DSP (Data Processor?) are available, in order to generate the required comfort noise at the receiver side. However, in short range air interface systems of the type described above, such as Bluetooth, these assumptions are not applicable. Neither the model parameters for the background noise, nor a DSP of sufficient computational capability, are typically available. Moreover, the voice coder in Bluetooth is a waveform coder with a one-bit quantization scheme, called the continuously variable slope delta modulation (CVSD). The 64 kb/s CVSD is a non-linear waveform coder with no model information available, apart from instantaneous waveform amplitude and slope information. These parameters are not amenable for modeling background noise, due to their rapidly varying nature. 
     SUMMARY OF THE INVENTION 
     In the invention, it has been recognized that background noise perception is based primarily on power level or amplitude of the noise information. This is true because the amplitude modulation (between silence and noise-containing speech components) of the background noise is the principal cause of the annoying noise which occurs during DTX switching of the transmitter. Accordingly, comfort noise generated at the receiver, at an amplitude which is substantially equal to the transmitter side amplitude, enhances speech quality and significantly diminishes the annoying effects described above. In accordance with the invention disclosed herein, adequate comfort noise may be provided, simply by judicious adjustment of a comfort noise generator located at the receiver side of the transmission interface. Thus, the invention provides a comparatively simply technique based on auditory perception of background noise, relative to the speech signal. 
     In one embodiment, the invention is generally directed to apparatus for providing comfort noise in a communication system used in connection with an audio voice signal comprising interspersed speech present periods and silent periods, wherein the communication system comprises a transmitter and receiver at opposing sides of a transmission path or interface, and the transmitter is provided with the aforesaid DTX capability. Thus, the transmitter is disposed to transmit the audio signal to the receiver during speech present periods, and to discontinue transmissions during silent periods. The apparatus comprises a comfort noise generator at the receiver side of the interface for producing comfort noise of adjustable amplitude. The apparatus further comprises a first subsystem for detecting silent periods at the transmitter side, and for providing notice to the receiver when the transmitter discontinues transmission of the audio signal. A second subsystem is provided for transmitting a succession of amplitude parameters through the interface to selectively adjust the amplitude of the generator, in corresponding relationship with a noise level detected at the transmitter. 
     In a preferred embodiment of the invention, the first subsystem includes a voice activity detector incorporated into the transmitter for producing speech present and speech absent flags in response to speech present and silent periods, respectively, occurring in the audio signal. In a useful embodiment, the speech present and speech absent flags respectively comprise first and second digital bits, which are transmitted across the interface to the receiver. In an alternative embodiment, wherein voice information is transmitted across the interface in digital packets, the occurrence of a particular silent period which causes transmission to discontinue, is indicated to the receiver by preventing transmission of a voice information packet which corresponds to the particular silent period. 
     Usefully, the second subsystem comprises a device for providing periodic estimates of the noise level at the transmitter, and further comprises means for transmitting amplitude adjustment parameters, which respectively represent the noise level estimates, across the interface to the comfort noise generator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a communication system which is provided with an embodiment of the invention. 
     FIG. 2 is a waveform diagram showing an audio signal containing speech parts and silent parts as indicated by P sp  and P sa  respectively. 
     FIG. 3 is a block diagram showing components for a modification of the embodiment of FIG.  1 . 
     FIG. 4 is a state diagram depicting operation of the embodiment of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown a communication system  10  for transmitting a voice signal x(n) through or across an air interface  12 , from the transmitter side  14  of the interface to the receiver side  16  thereof. Communication system  10  comprises a transmitter  18  and components associated therewith, located on transmitter side  14 , and further comprises a receiver  20  and components associated therewith, located on receiver side  16 . The audio signal x(n) contains voice or speech components. Accordingly, FIG. 1 further shows transmitter  18  provided with a Continuously Variable Slope Delta modulation encoder  22  (CVSD-enc), usefully operable at 64 kb/s, which implements a voice encoder algorithm to encode the speech component of the incoming signal. The encoded signal x′(n) is transmitted across air interface  12  by a transmission circuit  24  or the like of transmitter  18 , and received by a reception circuit  26  of receiver  20 . The received signal is then decoded, by CVSD decoder (CVSD-dec)  28 . While communication system  10  usefully comprises the Bluetooth system referred to above, the invention is by no means limited thereto. 
     Referring to FIG. 2, there is shown audio signal x(n) comprising successive signal samples  30 , the nth sample being of magnitude x(n). FIG. 2 further shows signal x(n) comprising periods P sp  which contain speech components  32 , interspersed between speech absent or silent periods P sa , which contain only background noise components  34 . It will be appreciated that x(n) represents a signal typically generated by a phone conversation or the like, that is, periods of speaking separated by speech pauses while listening to the other party. 
     As stated above, it is very advantageous, both to prolong transceiver battery life and to reduce interference over the air interface, to transmit signal x(n) only during speech present periods P sp . Accordingly, transmission is discontinued during silent periods P sa . To achieve such operation, FIG. 1 shows transmitter  18  further provided with a voice activity detector (VAD)  36  and a mechanism to discontinue transmission (DTX)  38 . The VAD  36  senses the presence or absence of speech components  32  in the voice signal, and thus recognizes the occurrence of successive silent periods. Usefully, the voice activity detector  36  comprises a VAD invented by the inventors herein. However, it is not intended to limit the scope of the invention thereto. Such detector generates a notification flag comprising a digital 1 to indicate a speech component in the voice signal, and generates a digital 0 to indicate a period of silence therein. The digital flags are coupled to DTX mechanism  38 , which operates to discontinue transmission of signal x(n) across interface  12  during silent periods P sa , while enabling transmission during speech present periods P sp . 
     In accordance with the invention, and to significantly diminish the annoying effects at the receiver described above, a comfort noise generator  40  is located on receiver side  16 . When transmitter  18  is in a transmission mode, a switch  42 , shown in FIG. 1, couples the output of CVSD decoder  28  to provide the receiver output y(n). However, during periods when transmission is discontinued, switch  42  couples a comfort noise signal produced by generator  40  to the receiver output. Thus, receiver output y(n) comprises speech components  32  of signal x(n), interspersed between periods of comfort noise from generator  40 . To operate the switch  42 , a flag F n  is transmitted across interface  12 , from transmitter  18  to receiver  20 . The flag F n  provides notice of the transmitter mode, whether transmission is occurring or discontinued, and is thus used to control operation of switch  42 . 
     In one useful embodiment, flag F n  is the notification flag produced by VAD  36  to operate DTX  38 , as described above. In such embodiment F n  would be a digital 1 to indicate transmission of a speech component, and would be a digital 0 to indicate a silent period in the voice signal and a corresponding discontinuation of transmission. 
     In a communication system such as Bluetooth, voice information is transmitted across an air interface in packets of digital bits, during assigned time slots. Accordingly, as an alternative technique for providing notice to the receiver of a silent period and consequent non-transmission, the flag F n  would simply comprise the absence of a packet corresponding to the silent period, so that the packet would not be received during its anticipated time slot. This technique would eliminate the need to transmit one or more additional bits across the interface, and could be implemented by structuring the transmission circuit  24  to be responsive to the corresponding digital 0 flag generated by VAD  36 . 
     As stated above, comfort noise provided by generator  40  must be at substantially the same amplitude as the background noise at the transmitter. Accordingly, FIG. 1 further shows a comfort noise level estimator (CN_l_E)  44  on transmitter side  14 , which is connected to transmitter  18 . The noise level estimator monitors noise level at the transmitter, and periodically computes a noise amplitude level parameter A p . Successive values of amplitude A p  are coupled across interface  12  to receiver  20  and generator  40 , and the amplitude of comfort noise produced by generator  40  is successively adjusted to such values. In one useful embodiment, values of amplitude parameter A p  are sent across the interface every 0.375 seconds, in digital form having a maximum size of two bytes. 
     Referring to FIG. 3, there is shown an alternative embodiment which limits the size of parameter A p  transmitted across the interface to three bits. FIG. 3 shows communication system  10  as described above. In addition, however, FIG. 3 shows a look-up table  46  on receiver side  16  which is coupled to generator  40 , and also to reception circuit  26 . In one embodiment of the invention, it has been recognized that quite adequate comfort noise may be provided by generator  40  if the amplitude of comfort noise is set to one of eight discrete predetermined levels. Accordingly, each of these levels is stored in look-up table  46 . A particular level is selected by noise amplitude parameter A p , and the amplitude of comfort noise provided by generator  40  is adjusted thereto. It will be readily apparent that only a three-bit parameter is required to select one of eight amplitude levels. 
     From the information provided by amplitude parameters A p , comfort noise generator  40  computes white noise with the right noise amplitude level. The generated white noise can be colored to conform to the band limited nature and frequency characteristics of the noise at the transmitter side. This is achieved by applying a comfort noise filter (CN_Filter)  48 , shown in FIG. 1, to the white noise. In a system such as Bluetooth, the extra information bits of the amplitude adjustment parameter A p  can be transmitted to the receiver side by utilizing a combined data-voice (DV) packet. The amplitude adjustment parameter can be placed in a data field which is mapped on bits 32-150 near the MSB of the DV packet. 
     Referring to FIG. 4, there is shown a state diagram describing the operation of an embodiment of the invention in such system, for the two states of switch  42  described above. More particularly, diagram  50  depicts the states at transmitter side  14  and diagram  52  depicts the states at receiver side  16 . The transmitter  18  is switched off during VAD=0 and stays in the idle state, periodically sending DV or noise level information to the receiver side  16 . During this time, the receiver side  16  generates comfort noise and sends it as a pulse code modulation (PCM) replacement to the background noise. During VAD=1, transmitter  18  is switched on, and the signal x(n) is CVSD encoded, transmitted across interface  12 , and then decoded at receiver side  16 . 
     Obviously, other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise than as specifically described.