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The need for audio compression can be seen by considering the bit rate required for high-quality audio. The bit rate is equal to the number of samples per second (the sampling frequency fs) multiplied by the number of bits per sample n: Rb fs n (16.3)
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For a stereo CD recording, the sampling frequency is 44.1 kHz, and the number of bits per sample is 16: Rb 44.1 103 16 2 1411.2 kb/s
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The factor 2 appears on the right-hand side because of the two channels in stereo. This bit rate, approximately 1.4 Mb/s, represents too high a fraction of the total bit rate allowance per channel, and hence the need for audio compression. Audio compression in MPEG exploits certain perceptual phenomena in the human auditory system. In particular, it is known that a loud sound at one particular frequency will mask a less intense sound at a nearby frequency. For example, consider a test conducted using two tones, one at 1000 Hz, which will be called the masking tone, and the other at 1100 Hz, the test tone. Starting with both tones at the same level, say, 60 dB above the threshold of hearing, if now the level of the 1000-Hz tone is held constant while reducing the level of the 1100-Hz tone, a point will be reached where the 1100-Hz tone becomes inaudible. The 1100-Hz tone is said to be masked by the 1000-Hz tone. Assume for purposes of illustration that the test tone becomes inaudible when it is 18 dB below the level of the masking tone. This 18 dB is the masking threshold. It follows that any noise below the masking threshold also will be masked. For the moment, assuming that only these two tones are present, then it can be said that the noise floor is 18 dB below the masking tone. If the test-tone level is set at, say, 6 dB below the masking tone, then of course it is 12 dB above the noise floor. This means that the signal-tonoise ratio for the test tone need be no better than 12 dB. Now in a pulsecode modulated (PCM) system the main source of noise is that arising from the quantization process (see Sec. 10.3). It can be shown (see Roddy and Coolen, 1995) that the signal-to-quantization noise ratio is given by a S b N q 22n (16.4)
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where n is the number of bits per sample. In decibels this is c S d N q 10 log 22n > 6n dB
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(16.5)
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This shows that increasing n by 1 bit increases the signal-to-quantization noise ratio by 6 dB. Another way of looking at this is to say that a 1-bit decrease in n increases the quantization noise by 6 dB. In the example above where 12 dB is an adequate signal-to-noise ratio, Eq. (16.5) shows that only 2 bits are needed to encode the 1100-Hz tone (i.e., the levels would be quantized in steps represented by 00, 01, 10, 11). By way of contrast, the CD samples taken at a sampling frequency of 44.1 kHz are quantized using 16 bits to give a signal-to-quantization noise ratio of 96 dB. Returning to the example of two tones, in reality, the audio signal will not consist of two single tones but will be a complex signal covering a wide spectrum of frequencies. In MPEG-1, two processes take place in parallel, as illustrated in Fig. 16.4. The filter bank divides the spectrum of the incoming signal into subbands. In parallel with this the spectrum is analyzed to permit identification of the masking levels. The masking information is passed to the quantizer, which then quantizes the subbands according to the noise floor. The masking discussed so far is referred to as frequency masking for the reasons given earlier. It is also an observed phenomenon that the masking effect lasts for a short period after the masking signal is removed. This is termed temporal masking, and it allows further compression in that it extends the time for which the reduction in quantization applies. There are many more technical details to MPEG-1 than can be covered here, and the reader is referred to Mead (2000), which contains a detailed analysis of MPEG-1. The MPEG Web page at http://www.mpeg.org/MPEG/audio.html also provides a number of articles on the subject. The compressed bit rate for MPEG-1 audio used in DBS systems is 192 kb/s. MPEG-4 (part 10) was developed jointly by the Video Coding Experts Group (VCEG) of the International Telecommunication Union (ITU), Telecommunication Standardization Sector (ITU-T) which uses the designation H.264, and the MPEG of the ISO/IEC. As noted in Sullivan et al. (2004), this version of MPEG is known by at least six different
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