[vdr] questions about FF card demuxing, and the ECD chip...

whoMAN whoman at gmail.com
Thu Feb 24 19:54:39 CET 2005


I have a few questions regarding how FF cards acheive correct A/V sync
during live tv.
I know there have been discussions about this in the past, but it
seems they never ended up going anywhere...
Please excuse my ignorance with any or all of this...

It appears that under certain circumstances [NTSC content?  or perhaps
provider specific?], there is an A/V SYNC problem with playback [not
live tv] of a decrypted stream [using my CI module, utilizing the CAM
interface layer]...all FTA channels have perfect A/V SYNC.
This ONLY happens on FULL FEATURED cards, like my NEXUS-S.  With my
NOVA-S, I do not experience the same problems.  After some
investigation, I have found that the SOFTWARE front ends keep A/V sync
by using the AUDIO PTS as the master, and adjusting the VIDEO
FRAMERATE to keep sync.

The only possibility I can think of is that perhaps during the
decryption process, the framerate of the video stream becomes
variable...thus requiring SYNC to be kept by the method mentioned
above [adjusting framerate based on AUDIO PTS].  Since FF cards use
their own muxer, I was thinking perhaps this muxer uses some OTHER
method to keep sync...  The only way THAT could be possible is if the
ECD chip is passing information along to the muxer, which doesn't
happen at all when playing back a recorded stream....since you've
already recorded the incorrect framerate, and there is no ECD activity
during playback [since decryption was already acheived during initial
recording]

Questions:
1.  What method does the FF muxer use to ensure proper A/V SYNC?
2.  Can the FF muxer be modified at all?  Perhaps, through patching
the firmware?
3.  Is this a valid assumption:  The ECD chip provides SYNC info to
the muxer for decrypted streams during LIVE TV


I know that my questions are based on a series of assumptions that
could possibly be completely incorrect, so I apologize if that is the
case.

Thanks,
whoMAN



More information about the vdr mailing list