Digital Signal Processing Reference
In-Depth Information
without the PCR information continued in them also being changed. There
is frequently a PCR jitter of up to ± 30 µs even though only ± 500 ns is al-
lowed. This can be handled by many set-top boxes but not by all. The
PCR information is transmitted in the adaptation field of a transport stream
packet belonging to the corresponding program. The precise information
about the type of TS packets in which this is done can be found in the
corresponding program map table (PMT). The PMT contains the so-called
PCR_PID which, however, corresponds to the video PID of the respective
program in most cases. After program clock synchronization has been
achieved, the video and audio coding steps are then executed in lock with
the system time clock (STC).
PTS of video PES
Video PES
PES
header
Video lip sync to audio
Audio PES
PTS of audio PES
DTS of video PES
Video PES
PES
header
Fig. 3.19.
PTS and DTS
However, another problem now presents itself. Video and audio must be
decoded and reproduced with lip synchronization. In order to be able to
achieve “lip sync”, i.e. synchronization between video and audio, addi-
tional timing information is keyed into the headers of the video and audio
PESs. This timing information is derived from the system time clock
(STC, 42 bits). Using the 33 most significant bits (MSB) of the STC, these
Search WWH ::
Custom Search