Dolby Laboratories DP571, DP572 1.2 System Considerations

Dolby E Multichannel Distribution System User Manual Introduction

1-5

1.2 System Considerations

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 110Ω 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 75Ω impedance at one point

only. We recommend a standard video terminator.

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 75Ω impedance on the last device

in the signal chain.

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.