Grass Valley Imagestore 750 v.3.0.1 - Manual (Page 122)

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Advanced Audio
Dolby
Dolby Data Types
Dolby Digital (AC-3)
Dolby Digital is a mechanism that carries multi-channel audio in a stereo audio signal. It is some-
times called “AC-3,” after the coding technology used. The high-quality compression achieved
takes advantage of human hearing. The resulting decoded audio is not meant for re-encoding,
rather for listening; hence its use in transmission. The Dolby Digital bitstream is a transmission
bitstream, intended for delivery to the consumer at home.
Dolby E
Dolby E is a high quality, video-frame-synchronized audio compression coding scheme that
enables broadcasters to distribute surround sound and multi-lingual audio using an existing
stereo infrastructure. Up to 8 audio channels plus metadata can be carried on a stereo channel.
Dolby E can tolerate the multiple encoding and decoding cycles encountered during broadcast
contribution, post-production, and distribution stages. Dolby E encoders and decoders convert
to and from compressed audio.
Dolby Metadata
A Dolby data stream contains information about the Dolby-encoded audio bit stream itself. This
information is called metadata. Metadata is different for Dolby Digital and Dolby E. It is used to
control aspects of the audio decoding and reproduction.
Dolby E metadata contains Dolby Digital metadata for every audio program carried in the Dolby
data stream, plus information about the transport stream. Dolby Digital metadata contains data
to be used by consumer electronics.
The metadata comprises of a set of parameters, for example:
Audio coding mode Describes a program’s associated audio channels.
Dynamic range Used to compress audio dynamic range at the consumer equipment.
Dialog normalization Adjusts consumer equipment audio output for consistent viewer loud-
ness; for example, across commercials, films, local news.
Dolby Handling on Legacy Systems
An Imagestore 750 without Dolby modules can pass and switch between Dolby E data-streams.
However it can only pass, not switch, Dolby Digital data.
Audio processing is always carried out in parallel with the video processing. The diagram below
shows the basic engine:
Fig. 6-1: Basic Audio Engine
Audio Processing Engine
A/B
Mix
Voice-
Over
Video A
Video B
PGM Video
AES AES
Video Processing (A/B Mix, Keyers, etc.)