Dolby Laboratories DP503 - Manual (Page 17)

SECTION 2A
DOLBY FAX - AN OVERVIEW
2A.1 Introduction
The Dolby Fax system allows high quality audio to be sent over ISDN telephone
lines. With a complete encode/decode system, communication is full duplex; i.e.
two high quality audio channels can be sent in both directions at the same time.
Time code can also be sent down an audio channel. An additional auxiliary channel
can be used for signaling and data at 1200 baud. Where bi-directional two-channel
audio and synchronization is required, additional equipment can be used to send
and receive time code via the auxiliary data channel.
Audio inputs to the Dolby Fax are both analog and digital. The signal is converted
to PCM in the Dolby Model DP503 encoder. Three different audio coding
methods—Dolby AC-2 (the de facto standard in film and music) as well as the
Dolby Digital (AC-3) and MPEG Layer II— audio coding is used to reduce the bit
rate of the digital audio to a single bit stream of 56-384 kbps, incorporating 1 or 2
audio channels and the auxiliary data. This bit stream is sent to the ISDN terminal
adapter/inverse multiplexer unit, which interfaces with the public telephone
network, executes dialing, and handles differing call configurations between
countries. If needed, the digital data stream is divided up (inverse multiplexed) and
sent over several phone calls and is reassembled correctly at the destination. The
Dolby Model DP524 decoder converts the bit stream to audio and subsequently
feeds both analog and digital outputs.
2A.2 Algorithms and Applications
The Dolby Fax system has built its reputation as being the de facto standard for
high quality audio using Dolby AC-2. With this reputation, a worldwide network
has been created which includes recording studios, film dubbing stages, record
labels, video post houses and home studios.
One of the most important new features of the Dolby Fax system is its ability to
connect to MPEG codecs, providing compatibility with users outside of the Dolby
Fax network of studios. Now with this addition, you have access to locations
whose work typically includes voice-over and jingle work.
To remain compatible with the existing network of Dolby Fax locations using
Dolby AC-2, two BRI lines—each with two 64 kb B channels—are needed for two
channels of "CD quality" 20 kHz audio. Dolby Fax with AC-2 can also operate at
384 kbps in situations where low coding delay is required. In this format, 3 BRI
lines are required at each location
MPEG locations, using codecs from companies such as Telos, Musicam USA
(formerly CCS) and RE typically operate at 56/112 kbps or 64/128 kbps for voice
over sessions and broadcast remotes. Switched 56 is used for 56/112 type calls,
while ISDN is used for 64/128 sessions. Calls at 56/64 kbps provide a 7-10 kHz
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