[mythtv] [DVB] Confused about TS/PS and HW/SW mode? Read this...

[Terry Barnaby]

Jesper Sörensen wrote:
> A quick recap for those of you who haven't followed the lists for the 
> last months:
> 
> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
> TS/PS:
> TS, ("http://www.linuxtv.org/wiki/index.php/MPEG2_Transport_Stream"), is 
> the original/raw format used in the DVB data stream. When Myth records 
> in TS mode it filters out unwanted pids (other channels & unwanted 
> streams) and does some minor rewriting of the PAT/PMT etc, but basically 
> it saves the data in its original format.
> 
> In PS mode, Myth converts the incoming TS data into PS 
> ("http://www.linuxtv.org/wiki/index.php/MPEG2_Program_Stream") format 
> before writing it to disk. PS format has slightly less overhead than TS 
> so it takes a few percent less space on disk, but it doesn't cope with 
> errors as well as TS, and since the TS->PS transform in Myth is pretty 
> "quick & dirty" it's likely to contain bugs and it won't handle the 
> "advanced" features planned for Myth (teletext, subtitles, MPEG-4 
> audio/video etc...)
> 
> Unfortunately TS recording still has bugs though (or rather the 
> *playback* of TS), e.g. the loss of audio in live TV when changing 
> between two channels that use the same audio pids. DTK and Taylor are 
> working on a fix but it's not finished yet.
> 
> Once TS recording works 100%, PS is planned to go away.
> 
> So, TS is preferred if it works for you (or you can live with the Live 
> TV audio bug).
> 
> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
> HW mode:
> The "HW mode" switch makes Myth treat the DVB card as if it was a FF 
> card ("http://www.linuxtv.org/wiki/index.php/Full-featured"). It doesn't 
> actually have to be a FF card, it's just about which settings to use 
> when setting up the demux and stuff.
> 
> Normally we always want to use the card in sw (budget) mode, because hw 
> mode cannot handle more than one audio stream (or video for that matter 
> but myth doesn't either at the moment), and the hw decoder can't be used 
> by Myth anyway.
> 
> There are also some budget card drivers that don't work in hw mode, but 
> most seem to silently accept it which means you get sw mode anyway, but 
> with the limitations of hw mode. On the other hand, some drivers (e.g. 
> the DEC2000-t) doesn't work in sw mode so it has to use hw mode.
> 
> Basically, sw mode is preferred but if that doesn't work, try hw mode. 
> Might also be a good idea to create a bug report for the DVB driver, 
> because you probably don't want to use hw mode forever because of its 
> limitations.
> 
> IIRC, Myth < 0.16 always used HW mode, 0.17 always used SW mode and 
> since 0.18 there is a switch.
> 
> There are also some people having problems with multiple audio streams. 
> As far as I know, this is caused by a XvMC bug, being unable to handle 
> multiple streams properly. Try helping the XvMC devs to fix it, disable 
> XvMC, or try enabling hw mode as a workaround (which will limit 
> recordings to one stream). If you have this bug and are NOT using XvMC, 
> please let me know.
> 
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Yes, I am one having problems with the multiple audio streams, and I concur
that this only occurs on my MythTV box with XvMC VLD HW MPEG decoding
enabled. If I use Xv output with software decoding all is fine.

This appeared after the DVB code was changed to allow multiple Audio streams,
the setting of "HW mode" works around the bug in Avcodec or XvMC that this
DVB change triggers.

However, my plays with trying to track this down suggest it may be more of a
generic AVSync problem within libavcodec or NuppelPlayer that rears its head
due to different buffer usage or timings. Thus other users, not using XvMC
may have problems with systems of different performance or AVSync timming
methods ...

XvMC is currently being integrated with the standard Xv code. I have tried
the patch that implements this change. It does not seem to affect the problem ...


Terry