Specifications

Standards and Practices for Authoring Dolby

Digital and Dolby E Bitstreams

How much space does an *.ac3 file consume on my hard drive?

It is simple to calculate the needed disk space for a *.ac3 file. First, determine the

number of seconds in the program. In this example, a 90-minute program and a Dolby

Digital data rate of 448 kbps is used:

5,400 sec. × 448,000 bits per sec. [Dolby Digital data rate] = 2.4192 gigabits

2.4192 gigabits / 8 [for byte conversion] = 302.4 megabytes

I’m not using Dolby Recorder. Can I save my Dolby Digital data stream

to a Digital Audio Tape (DAT)?

While both Dolby Digital and Dolby E data streams can be recorded and routed much

like regular PCM digital audio, the encoded signal is data and specific precautions must

be taken to ensure that it survives any archival process. Any reclocking or resampling

of the data stream destroys it and renders it irrecoverable. However, both Dolby Digital

and Dolby E data fit nicely on the digital audio tracks of many digital video recorders.

Additionally, Dolby Digital can be recorded onto the audio tracks of many digital

multitracks. Most DAT machines do not have the capability of recording data rather than

audio, although some professional units offer this feature. Generally, a DAT is not a good

medium for the archiving of Dolby Digital data streams. Whenever data is recorded onto

a DAT, the possibility of undecoded data being played out of the unit at maximum

volume (0 dB full scale), and possibly damaging speakers and ears, exists if the DAT

machine mistakenly interprets the data as audio or in any way corrupts the data stream.

Can I record Dolby E onto a DAT?

For Dolby E to maintain coherence with video frame boundaries, and thus be able to

be routed and switched much like video signals, it is usually necessary for the

archival medium to be referenced to video.

However, with the introduction of the DP583 Dolby Frame Synchronizer, content

producers now have the ability to archive Dolby E data streams to non-video based

media. The DP583 provides the ability to re-clock the Dolby E stream to a house

video reference, as well as reclocking Dolby Digital and baseband PCM audio to a

house sample clock reference. As with other “audio only” mediums, care must be

taken so that the DAT used to archive the Dolby E data stream is not mistakenly

played back in an audio system.

With the delays involved in using Dolby E, how should I compensate to

maintain lip-sync on playback of a DTV program?

Dolby E delays the audio a single video frame based on the video reference for each

encode or decode cycle. These delays are necessary to process the audio into the data

stream and vice versa. There are two methods to compensate for these delays: