[mythtv-users] DVB and DirecTV

Zachary S. Bedell zaclist at adirondack.net
Mon Jul 7 15:06:18 EDT 2003


 
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> I infer from that DirecTV actually *is* DVB-S and that if you can get 
> the decryption ("Common Interface" stuff) working, you can use DVB-S 
> cards with DirecTV, removing any need for the settop boxes (incl. the 
> decoding, analog S-Video and encoding involved) and instead getting a 
> raw MPEG2 stream from the satellite. This confirms what at least one 
> person suggested so far.


There's so much about the DTV service that's *SO* close to DVB that it almost *hurts*!  But unfortunately I don't think you'll get a DVB card receiving DTV unless you can rewrite the card's firmware to deal with the differences.  There's a bit of decent information on this page:
http://www.coolstf.com/mpeg/

According to their description, DTV video is basically DVB, but the audio is way off as is most of the metadata in the stream like program guide & that sort of stuff.  One of the most obvious differences between the two is that DTV packets are 127 bytes while DVB's are 188.  Also, some of the initialization info for the Reed-Solomon error correction is different from the DVB standard.  

Even with those differences, the signal is still QPSK modulated at the same range of frequencies, so there's no reason DVB hardware couldn't tune to the DTV signal and "receive" the information.  Some of the audio/video encoding is a little odd in DTV's signal (as I'm sure the authors of the various DirecTivo video extraction software can attest to!), but since Myth's goal is just to grab the raw data & shove it out to disk, I think ultimately libavcodec could probably handle decoding DTV's odd MPEG data with perhaps only some minor edits.  I know folks like JDiner on Deal Database's forums have already gone through all the work to figure out the differences there.

I wonder if anyone like Hauppauge might be willing to release any of the programming info for their cards?  I know they could never officially support a DTV conversion effort, but it would certainly make them a nice little grey market for their stuff if anyone could figure it out.

 
> Unfortunately, their statement also suggests that it's impossible to 
> commercially (legally) get CI modules to decode the stream, 
> like you can for other providers, e.g. the subscription-based TV network 
> Premiere in Germany. 

No way in heaven or heck you'll get any kind of support from DTV on that...  They been going ape-s..t about people managing to get video out of DirecTivo's in digital form.  I'm sure this would really get them going.

> I hear there are "software-based" decoders, essentially 
> crackers, but they may be illegal to use, publish or provide 
> help with. It most likely is illegal (by now, with the new EU copyright 
> laws :-( ) for *me* to provide you any help or hints with that, so please don't 


And with the "Good Old" US-of-A's DMCA bull.. er.. I mean "bill", the thoughtpolice might just knock on my door for even thinking about thinking about this kind of thing...  In any case, I've seen software decoders that work for Nagravision (DishNetworks) and a few other various systems, but I don't think anything like that is even *possible* for DirecTV.  NDS VideoGuard uses 64 bit keys for the video that rotate every 8 seconds.  Even if you had a couple of Cray's in your basement, I don't think there's anyway to get DTV's signal without a valid DTV smartcard.  It might be possible to hook up an ISO-7816 smart card reader to your Myth box and........  but we're definitely going places the DMCA doesn't want us to go with that.  (Isn't it wonderful to live in a free country where you can get locked up for even talking about decrypting a signal, even if you're already paying DTV $90 a month for it?)


The other idea I had involved finding a way to sample the raw MPEG data out of a real DTV receiver.  You'd need to be a legal subscriber & all that good stuff, of course.  I looked at specs for the ST-55xx series of chips used in DirecTV's receivers, and I'm almost *positive* you can get those chips to give our the raw, decrypted MPEG data.  I know for a fact that's what DirecTivo's do.  But alas, most of the ST-55xx spec sheets and programming info went sailing right over my head.  I'd be happy to hand out PDF's with the programming info on that chip if anyone's game.  The specs can be a little tough to come by, unfortunately.

Best regards,
Zac Bedell

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