Dolby Laboratories DP564 - Manual (Page 95)

DP564 Multichannel Audio Decoder
B-1
Appendix B
Latency Values
The latency of the DP564 is defined as the difference in time between the beginning
of a valid Dolby
®
Digital frame entering the DP564 and the output of the first PCM
sample represented by that frame.
Note: At any sample rate, a Dolby Digital frame represents 1,536 audio samples.
The DP564 accepts data over both professional and consumer industry standardized
interfaces. The professional interface is specified by SMPTE
2 . The consumer
interface is specified by IEC 61937 3 . Each interface specification includes a
specification for the latency of the decoder with respect to a reference point in the
data stream. The DP564 may add a small additional delay over that specified in the
SMPTE or IEC standards, if extra processing is performed.
B.1 SMPTE Specification (Professional AES3)
The SMPTE specification describes placement of the Dolby Digital frame on the
AES3 interface such that a decoder with the specified latency produces audio that is
timed correctly. The specified decoder latency is exactly one Dolby Digital frame
period, with respect to the reference point. The reference point is the first bit of the
Dolby Digital frame occurring on the interface. The SMPTE specification allows
equipment that outputs Dolby Digital streams on the AES3 interface to know when,
with respect to timecode or associated video, to output each Dolby Digital frame of
data in order to allow a standard decoder to reproduce audio at the proper presentation
time. The specification also allows equipment that receives the Dolby Digital data to
know when the audio in each frame should be reproduced.
Another way to maintain synchronization is by using time stamps. The SMPTE
specification provides for a time stamp to be associated with an audio frame of data.
The time stamp can indicate the timecode value that applies to an identified sample
within a Dolby Digital frame. The time stamp can also indicate an absolute time
offset of the stream, so that a Dolby Digital encoder, for example, can identify its own
latency to another piece of equipment, for example an MPEG multiplexer.
2 SMPTE 340M Format for Non-PCM Audio and Data in AES3 - ATSC A/52 (AC-3) Data Type
3 IEC 61937 Interface for non-linear PCM encoded audio bistreams applying IEC 60958