Linear Acoustic LA-5269, User Manual, Page: 18 / 42

Linear Acoustic LA-5269 Transcoder User Guide Applications
14
3.2 Latency
Audio coding takes time. Luckily, so does video, and in all modern multiplexers there is far
more video delay than audio delay and so adjustable compensation is added to the audio
inputs. When installing an external audio encoder such as the LA-5269, the latency im-
posed by the audio coding process can be “tuned out” of the MPEG multiplexer by re-
moving some of the compensating delay. This adjustment is sometimes called “External
Coding Delay” or simply “External Delay” and when set to the latency of the external en-
coder, this value is actually
removed from the audio path thereby lining up audio and video
properly in time.
The simple block diagram below illustrates the internal signal flow inside a typical MPEG
encoder and multiplexer.
Figure 3-2
A/V Sync Timing
This is an elegant method for maintaining proper lip sync. External delays could be em-
ployed, but it seems somewhat counterintuitive to introduce additional delay when delay
can actually be removed to achieve the same result.
MUX
MPEG-2/4
Video Encode
(1-4 seconds delay)
2-channel Internal
AC-3 Encoder
187 msec Delay
Fixed Delay=
MPEG-2
minus AC-3
Adjustable Delay=
MPEG-2 minus
ext Dly
HD
Video
Stereo
PCM
Audio
AC-3
Transport
Stream
Data
(PMT, PSIP, etc..)
LA-5269:
137msec (DD/DD+)
236/257msec (DE to
DD/DD+ NTSC/PAL)
0-Time
Plus Total
Latency
Timing
5.1 Ch
PCM
Audio
0-Time
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