This means using for example the Speex Wide-band (16 kHz sample-rate) or even Ultra-wide-band (32 kHz sample-rate) with Asterisk is useless. Asterisk will resample all codecs internally to 8000 samples/sec.This L16 PCM codec is the same as the G.711 codec, however the L16 PCM codec samples to 16 bits samples instead of the 8 bits samples in the case of the G.711 codec.
From the maximum voice quality point of view, when enough bandwidth is available, the best quality can be obtained by using the L16 PCM codec.G723 - Requires 6.4+Kbps (I and O) continuous data stream and also needs room for the SIP signal, 28.8Kbps connection has shown to be the minimal connection.A 128Kbps connection has shown to be the minimal connection needed in our testing. G711u/a - Requires 64+Kbps (inbound and outbound) continuous data stream for a quality call, plus the SIP signal.NEB = Nominal Ethernet Bandwidth (one direction) Mulptiple Maximum Likehood Quantization (MPMLQ) Low Delay Code Excited Linear Prediction (LD-CELP)Ĭonjugate Structure Algebraic Code Excited Linear Prediction (CD-ACELP) The following sites were used to build the following comparison:Īdaptive Differential Pulse Code Modulation (ADPCM) If one wants to increase the quality of the voice by increasing the used bandwidth (> 64 kbit/s) one has to use another codec. Because the sampling rate of 8000 samples/second and the encoding to 8-bit samples are intrinsic properties of the codec, the bitstream can not exceed 64 kbit/s.
Note that this is the datastream without information send in the headers of the packages, so the actual datastream will be higher. This will lead to a (8000 times 8-bit=) 64 kbit/s bitstream. The G.711 codec encodes the pulse-code modulation samples (sampling rate of 8000 samples/second) to logarithmic 8-bit samples. The G.711 standard has two algorithms: the A-law (mostly used in Europe) and the U-law algorithm (used in North America and Japan). It samples the analogue voice input signal using pulse-code modulation (PCM) at a sampling rate of 8000 samples/second. G.711 is a codec primarily used in telephony, released in 1972.