9, and G.722, in addition to G.723.1, where the latter is often favored for videophone applications. G.711 coders are mainly known for their low complexity and delay while resulting in toll-quality voice. Appendix I in G.711 defines a Packet Loss Concealment (PLC) algorithm to help hide transmission losses in a packetized network. G.729, on the other hand, provides a toll- quality voice with acceptable delay level but at the lower rate. G.729 operates at 8 kbit/s, but there are extensions that provide 6.4 kbit/s and 11.8 kbit/s rates. G.722 is an ITU stan- dard codec that provides 7-kHz wideband audio at datarates from 48 to 64 kbit/s. This is especially useful in fixed-network VoIP applications, where the required bandwidth is typi- cally not prohibitive. G.722 provides a significant improvement in speech quality compared to other narrowband codecs such as G.711. Finally, G.723.1 can operate at 6.3 kbit/s (using 24-byte frames) or at 5.3 kbit/s by using 20-byte frames. Human Auditory Perception and Subband Coding In another method of speech coding, the speech signal is divided into separate frequency bands called subbands. Each subband can be coded by using either a waveform-based or an AbS technique (or a combination of both). Because not all the speech signal is relevant to the human ear, Subband Coding (SBC) enables the human auditory perception model to be considered as well. As an example, the Absolute Threshold of Hearing (ATH), which is the minimum sound level that an average ear with normal hearing can perceive, varies from one frequency to another. The ATH can be approximated using the following equation (Painter and Spanias, 1997): (1.1) where T(f) is the minimum sound pressure level and f represents the frequency (see Figure 1.5). Notice that the value of ATH depends on the listener's age and gender as well. In addi- tion, human hearing performs space selection of different frequency bands called critical bands. In other words, the human inner ear behaves as a bank of bandpass filters with nonuniform 