Mahmut

SPEECH CODECS

      Speech codecs differ from Audio codecs in that they are optimized for speech signals. Human speech can be modeled easier than generic audio. This means that high quality speech can be coded with specific methods better than generic audio. On the other hand humans are very good at hearing problems with human speech signal. This means that otherwise good quality generic audio codecs perform poorly with speech signal even at quite high bit rates.

      -G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation (PCM) of voice frequencies. It can be also used in one of methods for fax communication over IP networks. G.711 represents logarithmic pulse-code modulation (PCM) samples for signals of voice frequencies, sampled at the rate of 8000 samples/second. G.711 defines two main compression algorithms, the µ-law algorithm (used in North America & Japan) and A-law algorithm (used in Europe and the rest of the world). Both are logarithmic, but A-law was specifically designed to be simpler for a computer to process. The standard also defines a sequence of repeating code values which defines the power level of 0 dB. G.711 μ-law tends to give more resolution to higher range signals while G.711 A-law provides more quantization levels at lower signal levels. When using μ-law G.711 in networks where suppression of the all 0 character signal is required, the character signal corresponding to negative input values between decision values numbers 127 and 128 should be 00000010 and the value at the decoder output is -7519. The corresponding decoder output value number is 125......

      -G.723 is a ITU-T standard speech codec using extensions of G.721 providing voice quality covering 300 Hz to 3400 Hz using Adaptive Differential Pulse Code Modulation (ADPCM) to 24 and 40 kbit/s for digital circuit multiplication equipment (DCME) applications....