Patent Publication Number: US-5896576-A

Title: Audio mute for digital cordless telephone

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates generally to communication apparatus and, more particularly, to an improved audio mute method and apparatus for a digital cordless telephone. 
     2. Description of Related Art 
     Analog cordless telephones are known in the prior art. While the voice quality of such telephones degrades relatively rapidly with distance, they have a noise response which exhibits a relatively gradual cumulative degradation of the signal. While digital cordless telephones utilizing spread spectrum techniques promise much improved voice quality and range, the noise response of such systems is abrupt, annoying to the user, and can exhibit large &#34;booms.&#34; 
     Proposals for reducing such abrupt noise effects have included a simple algorithm according to which the output signal to the user is simply turned off when the channel is known to be bad; i.e. a simple switch. Other approaches use complex algorithms which look for &#34;non&#34; speech-like audio signals that occur when the channel goes bad and perform complex audio signal conditioning to reduce audio artifacts. The first approach is undesirable because the telephone user experiences a complete &#34;dropout&#34; or interruption of the voice to which he is listening. The second approach is undesirable because of high complexity and expense. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to improve telephone communication systems; 
     It is another object of the invention to improve cordless telephone communication systems; 
     It is another object to provide an improved audio mute technique for digital cordless telephones; 
     It is another object to provide such a technique which is relatively simple to implement and yet yields improved effectiveness over &#34;simple switch&#34; techniques; 
     It is another object to provide an audio mute technique which avoids complete dropout or interruption of the voice or other signal provided to the telephone user; and 
     It is another object of the invention to provide an audio mute technique for digital cordless systems which is particularly applicable to adaptive delta pulse code modulation (ADPCM) techniques. 
     According to the invention a signal indicating the quality of the received data is monitored and a transition from good to poor quality is detected. In response to this detection, a first attenuation level is applied to the received signal selected to maintain the intelligibility of the signal to the user while lowering the volume thereof. This attenuation level is then reduced to at least a second level. This second level is preferably maintained over a selected time to accommodate for the known error propagation time of the ADPCM signal. An additional useful feature according to another aspect of the invention is the implementation of a clipping circuit to limit the excursion of the audio signal. This clipping circuit is again put into operation upon detection of poor data quality and may last for the duration of the error propagation time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings, of which: 
     FIG. 1 is a circuit block diagram of a monitoring system according to the preferred embodiment; and 
     FIG. 2 is a timing diagram further illustrative of the structure and operation of the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventors of carrying out their invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the generic principles of the present invention have been defined herein specifically to provide a particularly useful and readily implementable monitoring circuit for an ADPCM digital cordless telephone system. 
     The preferred embodiment muting system 11 is illustrated in FIG. 1. This system is particularly adapted for use in a wireless voice transmission system employing conventional adaptive delta pulse code modulation (ADPCM). As known to those skilled in the art, this modulation scheme exhibits memory, such that the effect of the occurrence of a bit error lasts for a period of time and therefore requires a period of time to propagate out of the system. Such systems typically employ modems such as the modem 13 of FIG. 1, which provides an indicator or flag indicating that the received data is of good quality or not. 
     FIG. 1 further illustrates an ADPCM decoder 15 which receives a digital baseband ADPCM signal D s  from the modem 13 and produces a digital representation of an analog signal denoted A s . The signal A s  may then be converted to an analog signal by a digital-to-analog converter 18 and then to an audio output signal by a speaker 19. The decoder 15, converter 18, and speaker 19 are typical components of conventional ADPCM systems and their design and use is well-known to those skilled in the art. In particular, the decoder 15 may provide an output which is a 16-bit value representing the analog value of the signal at discrete times. 
     According to the preferred embodiment, a multiplier 17 is inserted into the signal path between the decoder 15 and the audio output device 19. The multiplier 17 is arranged to multiply the signal A s  by a factor a(t). The factor a(t) is provided over a signal line 22 by an algorithm generator 21. 
     The algorithm generator 21 may further optionally provide a CLIP signal over a signal line 23 to a clipping circuit 25. The clipping circuit 25 per se is conventional and of a design well-known to those skilled in the art. The clipping circuit 25, when employed, serves to limit the maximum excursion of the audio signal to within selected limits for audio volume limit control. It may be noted that the D/A converter 18 can be located elsewhere in the signal path between the decoder 15 and the speaker 19, for example, between the decoder 15 and the multiplier 17. 
     The a(t) signal and CLIP signal are illustrated in FIG. 2 with respect to the DATA GOOD signal. As shown, when the DATA GOOD signal rises at 27, indicating data quality is poor, a maximum attenuation level a 1  is applied by the algorithm generator 21 to lower the signal level. At the same time, the CLIP signal may be applied by the algorithm generator 21 to limit the maximum excursions of the signal, if the CLIP option is employed. 
     After the DATA GOOD signal drops back to &#34;good&#34; quality at 29, the attenuation signal a(t) ramps or steps back up to &#34;less&#34; attenuation. FIG. 2 particularly illustrates two step-ups of the attenuation level to respective attenuation levels a 2  and a 3  and then to zero attenuation or normal signal level a 0 . This step-up of attenuation factors accommodates the memory and error propagation period inherent in ADPCM systems. The period of data corruption or error propagation is a known characteristic of any particular ADPCM system. 
     Two or more attenuation levels, e.g. a 1 , a 2 , may be used. Typical attenuation values are a 1  ≅30 dB, a 2  ≅20 dB, a 3  ≅6 dB. Attenuation level a 1  is applied as long as the channel is bad, while the other attenuation levels a 2 , a 3  may be applied for durations of 4-8 milliseconds (ms) and 50 ms, respectively. The CLIP signal is preferably employed to improve limiting during the entire time attenuation is applied, as shown in FIG. 2. 
     The algorithm illustrated in FIG. 2 can be implemented in software or hardware, using, for example, a programmed digital processor or discrete componentry or a combination thereof. In one software embodiment, a micro-controller implements the algorithm generator 21. In such an embodiment, the micro-controller reads the &#34;DATA GOOD&#34; indication of the modem 13 and, upon a transition from good to poor signal quality, extracts the a(t) contour from memory and imposes it on the output of the decoder 15. The a(t) contour may be so imposed by employing a multiplier 17 already used for volume control in typical circuits. 
     In operation, application of the a(t) signal weakens or lowers the volume of the signal heard by the user, while avoiding a complete dropout. This operation has been found to be more pleasing to the user than complete dropout of the signal. Intelligibility and generated noise during a dropout is thus maintained at comfortable listening levels. In addition, application of the a(t) contour is straight-forwardly implementable with a minimum of, or no additional, componentry. 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.