Transcoders and mixers for voice-over-ip conferencing

Abstract

Transcoders and mixers having reduced algorithmic delay and processing complexity. An improved mixer for signals having encoded speech parameters wherein the parameters obtained through decoding are used by a parameter estimator to improve the encoding by providing a parameter estimate for the mixed signal. In the case of pitch parameters, the mixer uses the principle of strong-pitch-domination. The mixing of wideband signals is simplified by performing mixing of individual lower and upper sub-bands. A transcoder and a mixer that converts a wideband signal into a narrowband signal relies upon high frequency suppression. A transcoder and a mixer that converts a narrowband signal into a wideband signal relies upon filter combination.

1 . A transcoder for converting an input signal to an output signal, the input signal comprising a wideband signal having an upper sub-band and a lower sub-band, and the output signal comprising a narrowband signal, the transcoder comprising:
 a lower sub-band decoder operable to decode the input signal to provide a decoded lower sub-band signal; and   a narrowband encoder operable to encode the decoded lower sub-band signal to produce the output signal.   
     
     
         2 . The transcoder of  claim 1 , further comprising a low pass filter operable to low pass filter the decoded lower sub-band signal to provide a filtered lower sub-band signal to the narrowband encoder, the narrowband encoder being operable to encode the filtered lower sub-band signal. 
     
     
         3 . The transcoder of  claim 2 , wherein the low pass filter has a cutoff frequency substantially equal to 8 kHz. 
     
     
         4 . The transcoder of  claim 2 , wherein the low pass filter has a passband operable to suppress high frequency aliasing within the filtered lower sub-band signal. 
     
     
         5 . The transcoder of  claim 1 , wherein the lower sub-band decoder is operable to decode the input signal encoded according to a G.722 protocol. 
     
     
         6 . The transcoder of  claim 1 , wherein the narrowband encoder is operable to encode the decoded lower sub-band signal according to a G.711 protocol. 
     
     
         7 . The transcoder of  claim 1 , further comprising:
 a decoder operable to decode the narrowband signal to produce a decoded narrowband signal; and   a mixer operable to mix the decoded narrowband signal with the decoded lower sub-band signal to produce a mixed signal;   wherein the narrowband encoder is operable to encode the decoded lower sub-band signal by encoding the mixed signal to produce the output signal.   
     
     
         8 . The transcoder of  claim 7 , further comprising a low pass filter operable to low pass filter the decoded lower sub-band signal to produce a filtered lower sub-band signal, wherein the mixer is operable to mix the decoded narrowband signal with the decoded lower sub-band signal by mixing the decoded narrowband signal with the filtered lower sub-band signal to produce the mixed signal. 
     
     
         9 . The transcoder of  claim 7 , wherein a low pass filter is operable to suppress high frequency aliasing within the decoded lower sub-band signal. 
     
     
         10 . The transcoder of  claim 7 , wherein the narrowband encoder is operable to encode the mixed signal according to a G.711 protocol. 
     
     
         11 . A method for converting an input signal to an output signal, the input signal comprising a wideband signal having an upper sub-band and a lower sub-band, and the output signal comprising a narrowband signal, the method comprising:
 decoding the lower sub-band of the input signal to produce a lower sub-band signal; and   encoding the lower sub-band signal to produce the output signal.   
     
     
         12 . The method of  claim 11 , further comprising a step of low pass filtering the lower sub-band signal to produce a filtered lower sub-band signal, wherein encoding the lower sub-band signal comprises encoding the filtered lower sub-band signal. 
     
     
         13 . The method of  claim 12 , wherein the step of low pass filtering comprises suppressing high frequency aliasing within the lower sub-band signal. 
     
     
         14 . The method of  claim 12 , wherein the step of low pass filtering comprises low pass filtering with a cutoff frequency substantially equal to 8 kHz. 
     
     
         15 . The method of  claim 11 , wherein the input signal is encoded according to the G.722 protocol. 
     
     
         16 . The method of  claim 11 , wherein encoding the lower sub-band signal comprises encoding the lower sub-band signal according to a G.711 protocol. 
     
     
         17 . The method of  claim 11 , further comprising:
 decoding a narrowband signal to produce a decoded narrowband signal; and   mixing the decoded narrowband signal with the decoded lower sub-band signal to produce a mixed signal;   wherein encoding the lower sub-band signal comprises encoding the mixed signal to produce the output signal.   
     
     
         18 . The method of  claim 17 , further comprising a step of low pass filtering the lower sub-band signal to produce a filtered lower sub-band signal, wherein mixing the decoded narrowband signal with the decoded lower sub-band signal comprises mixing the decoded narrowband signal with the filtered lower sub-band signal to produce the mixed signal. 
     
     
         19 . The method of  claim 18 , wherein the step of low pass filtering includes suppressing high frequency aliasing within the lower sub-band signal. 
     
     
         20 . The method of  claim 17 , wherein encoding comprises encoding the mixed signal according to the G.711 protocol.