[linux-dvb] Re: Recording using a TwinHan VisionDVT terrestial card

[Måns Rullgård <mru@pronto.tv>]

Tomas <psycho@pobox.com> writes:

> I want to to record using my TwinHan VisionDVT terrestial card.

[...]

> When using mplayer on a fast Athlon XP machine the CPU
> utilization stays below 1% according to top, and the file size is
> somewhere aroung 2GB/hour, which is fine, as I want to put the
> card into an older and much slower machine which also has other
> tasks such as acting as an ssh and www server, and thus don't
> want to waste CPU cycles on anything but that which is needed to
> dump the stream to disk (well, I guess A/V sync is worth spending
> some CPU time on). Any processing (with the possible exception of
> A/V sync) is done on the transport stream file when the recording
> finishes, not while recording.

The card outputs a perfectly synchronized MPEG transport stream
grabbed directly off the air.  No post-processing is required unless
you want to transcode it.

> Also, is there some way of recording two channels simultaneously
> using only one card? Is such a thing dependent on the card or the
> drivers? I just get error messages from mplayer when I start up a
> second recording, or try to view a second channel while
> recording:
> 	DVB CONFIGURATION IS EMPTY, exit
> 	Failed to open dvb://CHANNEL2
> The TwinHan web page for the card, located at
> 	http://www.twinhan.com/visiontv-2_2.htm ,
> mentions "Viewing and Recording Different DTV Channels
> Simultaneously" which to me sounds like it should be possible to
> record two channels simultaneously, as it seems the card should
> be able to deliver (at least) two channel streams simultaneously.
>
> From what I can understand of the Metzler brothers' "The Linux
> DVB API" paper, recording simultaneoulsy from two different
> channels should be possible if the separation of transport
> streams is done in software and not hardware, and one manages to
> grab data from the frontend instead of the demux. Am I wrong
> about this?

You can record several channels broadcasted on the same frequency.
Most DVB broadcasts bundle several channels into one MPEG transport
stream.  If the driver/card combination allows you to retrieve the
entire stream into userspace recording all the channels is possible.
Note that only one scrambled channel can be descrambled using normal
CI modules.

Recording multiple channels is possible with the Twinhan binary
drivers, but they don't use the same API as the open source drivers.
If you would want to use this driver you could ask them nicely for a
2.6 version, they should have one.

> Finally (yes!), a question about image quality: when I view live,
> or play back a recording, there seems to be some interlacing
> problem or similar. Fast vertical movements reveal horizontal
> lines, most obvious when a TV program is using a blue screen
> background for adding graphics, such as a weather report, but
> disturbing also when watching regular programs.

That is because the broadcasted stream is interlaced to play back
nicely on a regular TV.  Try activating some deinterlacing in your
playback software.

-- 
Måns Rullgård
mru@pronto.tv