Abstract:
A short period of background noise is recorded during a call and then played back during non-speech intervals, thereby matching as nearly as possible the spectrum and amplitude of actual background noise during the call. Segments of the recording are played back in random order to mask repetition. Recording can take place more than once during a single call or take place in more than one session. In accordance with another aspect of the invention, a small amount of white noise is added to the recorded noise to improve the randomness of the sound.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a noise generator for use in speaker phones and other communication devices wherein it is desired to avoid complete silence during a communication.  
           [0002]    Anyone who has used current models of speaker phones is well aware of the cut off speech and the silent periods during a conversation caused by echo canceling circuitry within the speaker phone. Such phones generally operate in what is known as half-duplex mode, which means that only one person can speak at a time. While such silent periods assure that the sound from the speaker is not coupled directly into the microphone within a speaker phone, the quality of the call is poor.  
           [0003]    Whether or not to receive (listen) or transmit (talk) is not easily resolved in the particular application of telephone communication. Analyzing a voice signal in real time and deciding whether or not a person has finished speaking is a complex problem despite the ordinary human experience of doing it unconsciously or subconsciously. A variety of voice activity detectors have been proposed in the art.  
           [0004]    Once it is decided that there is no voice activity in a channel, systems of the prior art imposed a silence in an attempt to eliminate acoustic and electronic echoes. The silence was interpreted by consumers as the connection having been interrupted and a party to a call would mistakenly hang up. This problem has been solved by providing so-called “comfort noise” in which a low level noise signal is applied to a line rather than silence. U.S. Pat. No. 6,122,611 (Su et al.) describes a system that not only adds noise during periods of silence but also adds a little noise during conversation to avoid changes in the apparent loudness of the speech.  
           [0005]    While one might think that all noise is the same, such is not the case. An automobile produces quite a different background noise from an office or a living room full of people. Adding “white” (essentially purely random) noise produces yet another background sound. U.S. Pat. No. 5,657,422 (Janiszewski et al.) discloses processing the noise to make it sound more “natural.” Switching from natural background noise during speech to artificially generated noise during non-speech intervals can be annoying because the sounds are different.  
           [0006]    In view of the foregoing, it is therefore an object of the invention to provide an improved generator of comfort noise.  
           [0007]    Another object of the invention is to provide a more natural sounding comfort noise.  
           [0008]    A further object of the invention is to provide a comfort noise that matches actual background noise as closely as possible.  
         SUMMARY OF THE INVENTION  
         [0009]    The foregoing objects are achieved in this invention in which a short period of background noise is recorded during a call and then played back during non-speech intervals, thereby matching as nearly as possible the spectrum and amplitude of actual background noise during the call. Segments of the recording are played back in random order to mask repetition. Recording can take place more than once during a single call or in more than one session. In accordance with another aspect of the invention, a small amount of white noise is added to the recorded noise to improve the randomness of the sound. In accordance with a further aspect of the invention, actual background noise is attenuated but not eliminated and is added to the output signal as a third component. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1 is a block diagram of a comfort noise generator constructed in accordance with a preferred embodiment of the invention;  
         [0012]    [0012]FIG. 2 is a block diagram of a comfort noise generator constructed in accordance with another aspect of the invention; and  
         [0013]    [0013]FIG. 3 is a block diagram of a telephone incorporating a comfort noise generator constructed in accordance with a preferred embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    An audio input signal is converted into digital samples by suitable apparatus (not shown in FIG. 1) and the samples are coupled to input  11 , which is the data input for random access memory (RAM)  12 . An eight kHz clock signal is coupled to input  14  of counter  15  to control timing. Alternate sixteen-bit samples are read into RAM  12 , effectively sampling at a four kilohertz rate. One kilobyte of memory was used in one embodiment of the invention, providing approximately two hundred and fifty milliseconds of audio. A larger memory provides a longer recording time. The output from counter  15  is a ten bit address at which data is stored. During recording, data is stored in consecutive addresses, beginning at address zero, under the control of a read/write signal on input  16 .  
         [0015]    A second input to counter  15  is a ten-bit random number from random number generator  18 . During a read operation, random number generator  18  applies data to counter  15 , which is coupled to RAM  12  as the ten-bit start address of a read. Data is read sequentially until another count is received from random number generator  18 . The result is that small segments of the recorded signal are randomly reproduced. The random read significantly reduces any periodic artifacts in the recorded signal from appearing in the data stream while emulating the spectral content and amplitude of the actual background noise very well.  
         [0016]    In one embodiment of the invention, the recorded signal was divided into either eight or sixteen segments. The result is that the data stream from RAM  12  does not repeat for four or eight seconds. Using a one kilobyte RAM, eight segments each include one hundred twenty-eight bytes. A segment can begin anywhere. The segments do not align with the beginning or end of the address space of the RAM, except by chance. For example, a segment can begin at 3E8H (1000 decimal) and continue past zero to address 067H (103 decimal). Address counter  15  simply rolls over. It does not stop at 3FFH (1023 decimal) or at zero.  
         [0017]    Recording actual background noise during a telephone call can take place more than once during a single call and can be made in more than one recording session, under the control of the read/write signal on input  16 . Recording is stopped by voice activity detectors (not shown), known per se in the art, coupled to suitable logic for generating the read/write signal. If RAM  12  is not filled, more background noise is recorded at the next opportunity. Usually, 250 ms of background noise is easily obtained and one can omit the logic or software, not shown, for determining whether or not RAM  12  was filled. In accordance with another aspect of the invention, multiplex circuit  21  receives data from register  22  and from RAM  12  and couples either one or the other to the signal input of digital gain attenuator  25 . The gain of attenuator  25  is controlled by register  26 . The output from attenuator  25  is either the comfort noise from RAM  12  or a zero level signal from register  22 .  
         [0018]    Periodic background noise, such as music, is noticeably different even from a random signal incorporating a recording of the background sound in accordance with the invention. Thus, the circuit shown in FIG. 1 includes enable input  28  coupled to multiplex circuit  21 . If loud, periodic background noise is detected by suitable apparatus (not shown), the comfort noise signal is preferably disabled and a low level signal is coupled to attenuator  25 . The signal from comfort noise circuit  10  is ultimately added to the signal on either the line out side of a telephone or on the speaker out side, or both (two comfort noise circuits). Using the all zero data from register  22  essentially passes the actual background noise and, in effect, disables the comfort noise circuit.  
         [0019]    [0019]FIG. 2 illustrates another aspect of the invention in which white or colored (filtered) noise is added to the recorded noise to provide a comfort noise signal. Noise recorder  10  is the circuit illustrated in FIG. 1. White noise generator  33  provides a sixteen bit random signal that is coupled to attenuator  35 , which preferably sets the amplitude of the signal to −40 dB relative to a system level of zero dB. The signal from attenuator  35  is added to the signal from comfort noise circuit  10 , which is at a level determined by register  26  (FIG. 1), in summation circuit  36 .  
         [0020]    The combined signal further randomizes the comfort noise while closely replicating the actual incoming background noise. Read/write input  37  preferably switches both the input and the output of noise recorder  10  for recording or playback (the position shown for ganged, single pole double throw switches  41  and  42 ). Switches  41  and  42  are actually implemented in logic rather than the physical switches as illustrated.  
         [0021]    A signal on input  31  representing the actual background noise is coupled through programmable attenuator  37  to another input of summation circuit  36 . The actual background noise can be fully or partially attenuated and is preferably reduced to approximately the same level as the signals from attenuator  35  and recorder  10 . The result is a very natural sounding background noise matching the amplitude and spectral content of the actual noise. The resulting comfort noise signal on the output of summation circuit  36  is sent to the line output or the speaker output of a telephone.  
         [0022]    [0022]FIG. 3 is a block diagram of a telephone incorporating a comfort noise generator constructed in accordance with a preferred embodiment of the invention. Telephone  50  includes microphone input  51 , line out  52 , line in  53 , and speaker out  54 . A comfort noise generator is interposed in at least the receive side of the telephone, between line in  53  and speaker out  54 . Comfort noise circuit  61  is constructed in accordance with either FIG. 1 or FIG. 2 and receives a signal from the line input, preferably after some processing (not shown) such as line echo cancellation. The output from comfort noise circuit  61  is combined with a signal from line input  53  in summation circuit  62  and coupled to speaker output  54 .  
         [0023]    Preferably, comfort noise is also used in the transmit side of the telephone. Comfort noise circuit  65  is constructed in accordance with either FIG. 1 or FIG. 2 and receives a signal from the microphone input, preferably after some processing (not shown) such as acoustic echo cancellation. The output from comfort noise circuit  65  is combined with a signal from microphone input  51  in summation circuit  66  and coupled to line out  52 . Whether for transmit or receive, the comfort noise can be used continuously or only when no voice activity is detected.  
         [0024]    The invention thus provides an improved circuit for generating comfort noise. The circuit provides a more natural sounding comfort noise that matches the spectrum and amplitude of the actual background noise as closely as possible while enabling one to fully control the signals within a system. The circuit is easily implemented in integrated circuit form and much of the control logic can be implemented in software rather than in hardware. Thus, the circuit is extremely flexible and can be adapted to existing telephones or incorporated into new designs.  
         [0025]    Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, one can provide random segment lengths by having a first address from generator  18  be a start address and a second number from the generator be a stop address for each segment. A problem with this approach is that, on average, the segments will be as long as half of memory, which is not preferred. Rather than summing, one can switch between comfort noise and a line in signal or switch between comfort noise and a microphone input signal. The term “random” is not used herein in its strictly mathematical sense of a completely unpredictable event. Any finite counter can only produce a pseudo random number. As used herein, “random” means sufficiently random for its intended purpose. Although speaker phones are specifically identified for using the invention, the invention can be incorporated into any type of telephone or communication system where the ambiguity of silence must be eliminated. Although the data lines to the RAM are illustrated as separate for input and output, it is understood in the art that data lines to memory are typically bi-directional. Instead of summation circuits  62  and  66 , one can use single pole double throw switches (or the equivalent logic) to switch comfort noise in or out of the circuit.