Dolby Laboratories DP572 - Manual (Page 17)

Dolby E Multichannel Distribution System User Manual Introduction
1-5
1.2 System Considerations
1.2.1 Digital I/O
The digital input and output connections on the DP571 encoder and DP572 decoder may
seem unfamiliar to the first time user of a digital audio device. The Audio Engineering
Society (AES) created a standard electrical interface for digital audio with a balanced
XLR connector and 110
Ω impedance called AES3-1995. This interface was later
expanded to include an unbalanced BNC connector, 1 V peak-to-peak signal level, and
75
Ω impedance; the new standard is known as both AES3-ID-1995 and SMPTE-
276M. The DP571 encoder and DP572 decoder use this new standard for both digital
audio and Dolby E electrical interface connections.
Note: These signals must be properly terminated with a 75 Ω impedance at one point
only. We recommend a standard video terminator.
1.2.2 Video Reference
One of Dolby E’s most significant features is that the audio and video frame rates are
identical. The DP571 encoder and DP572 decoder both require a reference video signal
to lock the audio and video at frame boundaries. Standard black burst or color bars can
be used as reference video if they are locked to the plant reference. A passive loop-
through connection on the encoder and decoder can properly terminate the reference
signal or distribute it to multiple devices.
Note: This reference signal must be terminated with a 75 Ω impedance on the last device
in the signal chain.
1.2.3 Signal Characteristics
Although the Dolby E stream is carried on a standard AES pair, its format is different than
that of a conventional digital audio signal. The PCM audio that normally comprises a digital
audio signal is encoded, along with metadata and time code, into the Dolby E bitstream.
The resulting data no longer directly represents the audio. Since a standard Dolby E signal
can appear to the unwitting observer as a clipped or distorted digital audio signal, this
section explains what to expect when measuring or monitoring a Dolby E signal.
For example, a VTR’s meters indicate a digital full-scale signal when recording or play-
ing a Dolby E stream and the clip indicators may flash intermittently. Dolby E uses the
entire audio payload of an AES pair, which creates the appearance of full-scale audio on
the VTR. The Dolby E signal sounds like full-scale noise when monitored through speakers
or headphones; this is useful to quickly verify the VTR is receiving a Dolby E signal.
To simplify working with these new signals and to allow for more detailed analysis,
Dolby Laboratories has recently introduced the DM100 Portable Audio Monitor. This
hand-held unit displays metadata parameters (see Section 3.2.4) and decodes Dolby E,
Dolby Digital, and PCM bitstreams. The decoded audio can be monitored through an
internal speaker or through user-supplied headphones.