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 What
is AC-3 ?
AC-3 (Audio Code number 3) refers to a multichannel music compression
technology that has been developed by Dolby Laboratories. The goal
of this digital compression technology is to produce a digital representation
of an audio signal which, when decoded and reproduced, sounds the
same as the original signal, while using a minimum of digital information
(bit-rate) for the compressed (or encoded) representation, providing
true surround sound.
Over the past years Dolby Laboratories has
used the term Dolby Digital to refer to this digital system in the
film and theatre industries, and has used the term Dolby Surround
AC-3 to refer to the system in the home theatre market. In practice
both the cinema and home theater applications are variations of
the same basic technology (slightly different data rates). So to
minimize the possibility of confusion by consumers the Company has
decided that Dolby's consumer multichannel digital format will be
known by the same name used in the film and theatre industries ie
Dolby Digital.
The AC-3 can carry from 1 to 5.1 channels.
It provides five full range channels (3 Hz to 20,000 Hz) in what
is sometimes referred to as a "3/2" configuration: three front channels
(left, center, and right), plus two surround channels. A sixth bass-only
effects channel (3 Hz to 120 Hz), also called sometimes "low
frequencies enhancement channel" (LFE), is also provided, giving
rise to the term "5.1" channels. As AC-3 is mainly designed for
providing true surround sound, it also includes information about
the room size and differences in dB between the channels levels.
How
does AC-3 work ?
Like MP3 or AAC, AC-3 uses masking properties of sounds to achieve
its compression. Input uncompressed PCM samples must be 32, 44.1
or 48 kHz on up to 20 bits.
- The first step in the encoding process
is to transform the representation of audio from a sequence of
PCM time samples into a sequence of frequencies coefficients blocks.
This is done in the analysis filter bank. Overlapping blocks of
512 time samples are multiplied by a time window and transformed
into the frequency domain. Due to the overlapping blocks, each
PCM input sample is represented in two sequential transformed
blocks. The frequency domain representation may then be decimated
by a factor of two so that each block contains 256 frequency coefficients.
The individual frequency coefficients are represented in binary
exponential notation as a binary exponent and a mantissa.
- The set of exponents is encoded into
a coarse representation of the signal spectrum which is referred
to as the spectral envelope.
- This spectral envelope is used by the
core bit allocation routine which determines how many bits to
use to encode each individual mantissa.
- The mantissa is quantized according
to the bit allocation informations.
- The spectral envelope and the coarsely
quantized mantissas for 6 audio blocks (1536 audio samples) are
formatted into an AC-3 frame.
- The AC-3 bit stream (from 32 to 640
kbps) is a sequence of AC-3 frames.
AC-3
utilizations
Dolby AC-3 is used intensively in the cinema at 640 kbps data rate.
The THX quality label is also in 95% of the cases based on some AC-3
installations. It is used on laserdiscs at 384 kbps bitrate, and now
in DVD at similar bitrates. The 5 channels of the old Dolby Pro-Logic
are extracted from the 2 stereo channels, so they only reproduce parts
of the audio spectrum range. As AC-3 provides only full range channels,
its sound is really much better in terms of quality and spatialisation.
You can notice that an AC-3 bitstream can carry a Pro-Logic signal
in its two front channels for compatibility with old systems.
One of the main advantages of AC-3 is its
utilization: since several years, it has intensively been used, so
it became a real world "standard".
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