bandwidth. Table 1.1 shows the critical bands of the human inner ear. This means the human ear cannot distinguish between two sounds of the same fre- quency band. This property is referred to as the frequency masking effect. Now, by using a subband coding technique, the coder can dynamically allocate the bits to different subbands. In this case the coder analyzes the relevance of different bands and allocates the bits accord- ingly (i.e., more important subbands are quantized with more bits). One of the disadvan- tages of subband coding is the overlap between the adjacent subbands due to the imperfection of the bandpass filters. However, this can be compensated by using Quadrature Mirror Filter (QMF) banks. Pulse Code Modulation Fig. 1.5 Absolute threshold of hearing. Frequency (kHz) Table 1.1 Critical Band of the Human Ear Center Center Critical Band Frequency (Hz) Bandwidth (Hz) Critical Band Frequency (Hz) Bandwidth (Hz) 0-100 1720-2000 100-200 2000-2320 200-300 2320-2700 300-400 2700-3150 400-510 3150-3700 510-630 3700-4400 630-770 4400-5300 770-920 5300-6400 920-1080 6400-7700 1080-1270 7700-9500 1270-1480 10500 9500-12000 1480-1720 13500 12000-15500 Source: Adapted from Painter and Spanias (1997). Because PCM is one of the most common speech coding techniques with many applica- tions and variations, the next section is devoted to describing the PCM process in more detail, although most of the fundamental concepts such as sampling and quantization are common between all speech coding methods. Pulse Code Modulation PCM is mainly used for converting analogue speech signals into discrete digital bits. Sam- pling, quantizing, and coding are considered to be the foundation of any PCM system. Sampling Sampling is the first step in any pulse modulation system. In fact, by sampling, a signal is represented by a set of discrete samples. If the frequency of the sampling is high enough, the original signal can be recovered from the samples. This concept will be examined in more detail later. Chapter 1 Voice Transmissio