Dolby AC-3 |

- 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
utilizationsDolby 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". |

© 1999-2001 Gabriel Bouvigne for MP3'Tech - www.mp3-tech.org