 applications Voice codec Application Bit rate (kbps) G.729 ADPCM Wireless 3G and 4G standards 4.75-12.2 AMR Wideband Wireless 3G and 4G standards 6.6-23.85 The synthesis process reproduces the components of the voice and passes them through the filter with parameters from the received information. This voice coding technique can yield very high compression rates, and is used in many applications. An example is AMR, the voice coder technology used in GSM, 3G and 4G standards. AMR encodes voice at variable rates from 12.2 kbps down to 4.75 kbps, showing significant compression compared with a full digital rate of 64 kbps. This comes however at the expense of sound quality, that is, the lower the bit rate is the higher the loss of information. Loss of information leads to lowering of the quality of the voice. Moreover, the quality loss is accentuated when reception errors occur. A 64 kbps lossless voice coding is very robust in the presence of large reception errors in that the quality is not significantly degraded. However, a 4.75 kbps coded stream can become unintel- ligible very quickly as transmission errors increase. A number of voice coders have been developed by a number of organisations and stan- dardised by ITU. Table 5.3 lists a number of commonly used codecs and their applications. Most codecs convert a voice stream filtered at 4 kHz, except for AMR Wideband (AMR-WB) which encodes a voice stream filtered at 7 kHz [7]. Comparison of Voice Coding Techniques As discussed above, different voice coding techniques result in different bit rates, and therefore transmission requirements. Clearly a lower rate is beneficial from a business point of view as transmission resources can be used more efficiently. However, this reduced Broadband Telecommunications Technologies and Management Table 5.4 Perceptual quality of a number of voice codecs [10] Narrowband AMR Wideband AMR Rate (kbps) Rate (kbps) 12.65 10.20 15.85 12.20 18.25 transmission rate comes at the expense of perceptual voice quality. A number of voice qua