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MPEG-2 encoder paths. (Courtesy of Bhatt et al., 1997. IEEE.)
4 for Y, and ratios commonly encountered with digital TV are 4:4:4, 4:2:2 and 4:2:0. These are explained next. 4:4:4 means that the sampling rates of Y, Cb, and Cr are equal. Each pixel would get three digital words, one for each of the component signals. If the words are 8-bits (commonly called a byte, but see Sec. 15.2) then each pixel would be encoded in 3 bytes. 4:2:2 means that the Cb and Cr signals are sampled at half the rate of the Y signal component. Every two pixels would have two bytes for the Y signal, one byte for the Cb signal and one byte for the Cr signal, resulting in 4 bytes for the 2-pixel block. 4:2:0 means that Cb and Cr are sampled at half the Y sampling rate, but they are sampled only on alternate scan lines. Thus vertical as well as horizontal resolution is reduced by half. A 2 2 pixel block would have 6 bytes, 4 bytes for Y, 1 byte for Cb and 1 byte for Cr. Following the digitizer, difference signals are formed, and the discrete cosine transform (DCT) block converts these to a spatial frequency domain. The familiar Fourier transform transforms a time signal g(t) to a frequency domain representation G(f ), allowing the signal to be filtered in the frequency domain. Here, the variables are time t and frequency f. In the DCT situation, the input signals are functions of the x
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(horizontal) and y (vertical) space coordinates, g(x, y). The DCT transforms these into a domain of new variables u and v, G(u, v). The variables are called spatial frequencies in analogy with the time-frequency transform. It should be noted that g(x, y) and G(u, v) are discrete functions. In the quantizer following the DCT transform block, the discrete values of G(u, v) are quantized to predetermined levels. This reduces the number of levels to be transmitted and therefore provides compression. The components of G(u, v) at the higher spatial frequencies represent finer spatial resolution. The human eye is less sensitive to resolution at these high spatial frequencies; therefore, they can be quantized in much coarser steps. This results in further compression. (This step is analogous to the nonlinear quantization discussed in Sec. 10.3.) Compression is also achieved through motion estimation. Frames in MPEG-2 are designated I, P, and B frames, and motion prediction is achieved by comparing certain frames with other frames. The I frame is an independent frame, meaning that it can be reconstructed without reference to any other frames. A P (for previous) frame is compared with the previous I frame, and only those parts which differ as a result of movement need to be encoded. The comparison is carried out in sections called macroblocks for the frames. A macroblock consists of 16 16 pixels. A B (for bidirectional) frame is compared with the previous I or P frame and with the next P frame. This obviously means that frames must be stored in order for the forward comparison to take place. Only the changes resulting from motion are encoded, which provides further compression. An estimate of the compression required can be made by assuming a value of 200 Mb/s for the uncompressed bit rate for SDTV (see Table 16.1), and taking 5 Mb/s as typical of that for a TV channel, the compression needed is on the order 200/5 40:1. The 5 Mb/s would include audio and data, but these should not take more than about 200 kb/s. Audio compression is discussed later in this section. The whole encoding process relies on digital decision-making circuitry and is computationally intensive and expensive. The decoding process is much simpler because the rules for decoding are part of the syntax of the bit stream. Decoding is carried out in the integrated receiver decoder (IRD) unit. This is described in Sec. 16.8. In DBS systems, MPEG-1 is used for audio compression, and as discussed earlier, MPEG-2 is used for video compression. Both of these MPEG standards cover audio and video, but MPEG-1 video is not designed for DBS transmissions. MPEG-1 audio supports mono and twochannel stereo only, which is considered adequate for DBS systems currently in use. MPEG-2 audio supports multichannel audio in addition to mono and stereo. It is fully compatible with MPEG-1 audio, so the IRDs, which carry MPEG-2 decoders, will have little trouble in being upgraded to work with DBS systems transmitting multichannel audio.
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