A little bit of history…
The entire process on G.723.1 was carried out with more progress (urgent mode) than G.729. First commercial very-low-bit rate videophones were announced in 1992. The G.729 was not selected for the videophone (visual telephony) coder because the progress on this coder was thought to be very laggard. In 1994 the solution of a dual-rate coder was agreed. Finally both coders G.729 and G.723.1 were completed simultaneously. Low-bit-rate videophones only send a few frames per second over a telephone bandwidth modem, consequently they have relatively high delays. Therefore the frame size of the coder was set at 30ms. Because of the video bitstream a decrease of the audio bitstream was necessary. The upper bit-rate bound was set at 6.3kbit/s. Also the complexity was improved compared with the G.728. About 16 MIPS and 2200 words of RAM are currently needed to implement G.723.1. Below the performance is outlined. Performance for random bit errors and bursty frame erasures was not tested because these are not typical of the communication channels provided by wire line telephone bandwidth modem. The frame size was set at 30ms and the required look ahead delay is 7.5ms. There exist several annexations to the main recommendation. Annex A describes a single user silence compression scheme in order to reduce the bit rate further. A voice activity detector and a comfort noise generator is pointed out. Furthermore annex B describes the interoperable floating point version of the coder. Therefore this code can be implemented on a host processor, such as a PC. Annex C point out the possibility of scalable channel coder. Scalable means that the channel coding rate can be adjusted to match the channel bandwidth available. It uses punctured convolution coding to provide unequal error protection – the most protection for the most sensitive bits.