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Audio Compression

Audio compression is a companion to video compression, but the techniques are very different. First, the audio signal requires much less bandwidth than the video signal. For example, a stereo audio pair sampled at 48 KHz and using 16-bit samples requires 1.536 Mbps (compared to 124 Mbps for the video signal). It would be quite possible to send uncompressed audio. Moreover, audio compression cannot achieve the same compression ratio as video; a typical rate for the stereo audio pair is 192 Kbps, an 8:1 compression ratio.

Nevertheless, it is easy to make an economic argument for audio compression. For example, in a 40 Mbps transmission link, an additional two video channels can be carried if audio compression is used (assuming 4 Mbps video).

There are two leading contenders for audio compression for entertainment quality audio: MPEG audio compression and Dolby AC-3.

MPEG-1 Layer 2 (Musicam)

MPEG-1 Layer 2 audio compression, also known as Musicam, is specified in ISO/IEC IS 13818-3. MPEG audio compression delivers near–CD quality audio using a technique called sub-band encoding. MPEG audio compression is used mainly in Europe and is used by most direct-to-home satellite providers in the United States.

MPEG audio compression is a two-channel system, but it can encode a Dolby Pro-Logic signal, which includes two additional channels for rear and center speakers. (This is analogous to the way in which a Dolby Pro-Logic signal is carried by a BTSC [Broadcast Television System Committee] signal as part of an NTSC transmission).

Dolby AC-3

Dolby AC-3 is a more advanced system than MPEG audio compression. AC-3 was selected as the audio compression system for digital television in North America and is specified by ATSC Standard A/52. AC-3 encodes up to six discrete channels: left, right, center, left-rear, right-rear, and sub-woofer speakers. The sub-woofer channel carries only low frequencies and is commonly referred to as a 0.1 channel because it has such a limited frequency range. Thus, AC-3 5.1 gets its name from five full channels and a 0.1 channel. In addition, AC-3 has a two-channel mode that can be used to carry stereo and Dolby Pro-Logic encoded signals.

AC-3 also uses sub-band encoding but provides a number of advantages over MPEG audio compression:

  • In AC-3 5.1, surround-sound effects are reproduced much more faithfully than is possible with Dolby Pro-Logic because of the additional channels.

  • AC-3 5.1 distributes the available transmission link capacity between the 5.1 channels so that more bits are used for those channels containing more information at any particular time. This method effectively makes them statistically multiplexed discrete digital channels.

  • AC-3 5.1 is a consumer-grade version of the theatrical Digital Theater Sound (DTS) system, so sound tracks may be directly transferred from the theatrical release.

AC-3 has been chosen as the audio system for digital terrestrial broadcasting and for DVD in the United States. It also has been selected by OpenCable.

Other Audio Compression Algorithms

As with video, many other audio compression algorithms exist. For example, RealAudio is commonly used for sending audio over the Internet, and MP3 has caused a stir in recordable audio. MP3 uses MPEG-1 Layer 3 audio compression, which is considerably more sophisticated than MPEG-1 Layer 2. Many audio compression algorithms have special characteristics that make them suitable for very low bit rate facilities, for software decoding on a PC, and so on. However, it is unlikely that they will pose a significant threat to Dolby AC-3 encoding for entertainment-quality audio in the near future.

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