Waht's the difference between PCM and AC3?

Question

When using SPDIF, what is the difference between PCM and AC3?

Answer 1

Pulse-code modulation (PCM) is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a numeric (usually binary) code. PCM has been used in digital telephone systems and 1980s-era electronic musical keyboards. It is also the standard form for digital audio in computers and the compact disc "red book" format. It is also standard in digital video, for example, using ITU-R BT.601. However, straight PCM is not typically used for video in standard definition consumer applications such as DVD or DVR because the bit rate required is far too high.

Dolby Digital, or AC-3, is the common version containing up to six discrete channels of sound, with five channels for normal-range speakers (20 Hz – 20,000 Hz) (right front, center, left front, right rear and left rear) and one channel (20 Hz – 120 Hz) for the subwoofer driven low-frequency effects. Mono and stereo modes are also supported. AC-3 supports audio sample-rates up to 48kHz. Batman Returns was the first film to use Dolby Digital technology when it premiered in theaters in Summer 1992. The LaserDisc version of Clear and Present Danger featured the first Home theater Dolby Digital mix in 1995.

This codec has several aliases, which are different names for the same codec:

    * Dolby Digital (promotional name, not accepted by the ATSC)

    * DD (an abbreviation of above, often combined with channel count: DD 5.1)

    * Dolby Surround AC-3 Digital (second promotional name, as seen on early film releases and on home audio equipment until about 1995/1996)

    * Dolby Stereo Digital (first promotional name, as seen on early releases, also seen on True Lies LaserDisc)

    * Dolby SR-Digital (when the recording incorporates a Dolby SR-format recording for compatibility)

    * SR-D (an abbreviation of above)

    * Adaptive Transform Coder 3 (relates to the bitstream format of Dolby Digital)

    * AC-3 (an abbreviation of above)

    * Audio Codec 3, Advanced Codec 3, Acoustic Coder 3 (These are backronyms. However, Adaptive TRansform Acoustic Coding 3, or ATRAC3, is a separate format developed by Sony)

    * ATSC A/52 (name of the standard, current version is A/52 Rev. B)

Answer 2

PCM is usually two-channel audio (stereo), uncompressed

AC3 or Dolby Digital is 5.1 channel compressed.