[linux-dvb] Re: dec2000-t playback

[Alex Woods <linux-dvb@giblets.org>]

> I'm still looking for a way to playback video from my harddisk through
> the dec2000-t to my tv. So I studied the dvb-api, your driver's source
> and some mpeg documentation and now i think i basically know what has to
> be done. But this is the first time I'm working on a kernel module and
> still I'm not really sure and need some more hints to really start with
> it. so...
> - the most important thing is to implement the demux write() function of
> the api for the dec box, correct?

No idea to be perfectly honest.  I've started working on setting up audio and 
video devices for the dec for playback of video, but I've been 
procrastinating of late.  I'm not sure where demux write fits in to things.

> - as we get pes streams from the box when recording, i guess the mpeg
> decoder can only handle pes and we have to transform the transport
> stream that is given to write() so that it can process it.

The dec sends and receives video streams in pva format 
(http://www.technotrend.de/download/av_format_v1.pdf).  Sadly this means 
munging the data to play nice with the dvb api.  The video device accepts PS 
so I've been looking at converting that to pva.

> - we need to know the way the pes packets are transmitted to the dec, so
> I used usbsnoop with the windows app. Seems like i have to read some
> documentation about usb and after that find out how pes pacckets are
> transmitted via usb. Or can you give me some advice?

I can do pretty well here, I can tell you the exact format - it's the exact 
same as the incoming packets ;)

0xaa 0xaa 0xaa 0x00
pva packet
a two byte counter
a two bytes xor checksum

For some reason each byte pair in this packet is reversed.

> - most likely we have to tell the box that it has to receive data from
> the usb-bus instead of the antenna/frontend before starting the playback

Yes.  Not too sure if there is a command to tell it to, or if switching the 
usb alternative interface is enough on its own.

> - perhaps i can find out how when i understand the usbsnoop log.

Probably - the stream data packets tend to hide all the interesting bits so 
you'll want to do some parsing ;)

> - I'm sure there are various things to update in ttusb_dec.c afterwards
> but the only obvious thing i found is registering a pointer to the new
> write-function in ttusb_dec_init_dvb(...) as dec->demux.write_to_decoder.

There are a few other things to be done.  The dec can only operate in send or 
receive mode so users to be locked out of trying to do both at once.

> Am I right that far?

You seem to be on roughly the right track.

> wanted to write that last mail to alex, but took the wrong address...

I think the list is a good place to discuss this, since there are people with 
a much better idea of how the dvb api should be implemented than myself.

Cheers,
Alex


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