[mythtv-users] Crazy DTV Idea

Tarek Lubani tarek at tarek.2y.net
Sat May 10 14:47:54 EDT 2003


I think this is a great idea, and have often wondered about it myself..
If it is possible, I suspect that you will find the discussions in other
forums, not the myth forums. Specifically, the DSS Underground
(www.dssunderground.com) might deal with this sort of thing.. We can
pretty much dismiss the idea of capturing the stream and trying some
variety of software-side decryption/decoding, since it seems to be
beyond our grasp.. but somehow intercepting the data before it gets
decoded? Why not?

The big question then becomes not whether or not it's *possible*, but
whether there is enough electronics knowledge working on the idea to
make it work..

I suggest hitting up the DSS forum. That's probably the highest
concentration of hacker-type DSS-related knowledge that you'll find
online..

tarek : )

> Alright...  Lemme preface this by saying this idea is probably completely
> nuts, but....
> 
> I'm wondering what the chances of somehow pulling MPEG2 data directly out of
> a standard DTV decoder box?  I'm sure it's technically possible, and I'm
> wondering if any other Myth users might be interested in further exploring
> the idea with me?
> 
> If we could figure out how to pull the MPEG2 data out of a DTV IRD and then
> find a way to port that data into a PC somehow, I think it would be possible
> to build a multi-tuner Myth box with a fairly slow microprocessor.
> Considering all the hard work of encoding the video is already done for us
> at DTV's broadcast center, the Myth box would only need enough power to save
> the MPEG2 streams to disk and to decode one MPEG2 stream in software (or
> maybe even in hardware if we could get appropriate Linux-friendly gear).
> 
> 
> Here's what I'm thinking:
> 
> I've got sitting here a DirecTivo (series 1, Hughes) and a plain-old RCA 420
> stand alone receiver.  They both contain the exact same STI5505 chip to
> process and decode the DTV signal (Tivo has two of them, of course).  Now I
> know the DTivo gets MPEG2 data its STI5505's -- that's just the way the
> thing works.  Tivo grabs the MPEG2 data from the 5505's, runs it thru the
> TiVo ASIC to process it a bit & scramble it up (to make extraction harder...
> errrr...  impossible...  yeah...  completely impossible...), and then dumps
> it to hard disk.  A separate bit of soft-/hardware wizardry later grabs the
> stream from disk, sends it back thru the ASIC to decode the signal again,
> then sends it to an IBM CS22 MPEG decoder & out to the TV.  That decoder
> chip and the fact that the MPEG2 data comes in pre-encoded are of course how
> Tivo can get away with only a 40MHz processor and still manage to record two
> signals at once.
> 
> 
> So...  If Tivo's 5505's can output MPEG2 instead of decoding it to be sent
> to the NTSC encoders, then I'm certain the RCA420's 5505 chip must be
> capable of the exact same thing, perhaps with a minor firmware modification.
> I know the 420's firmware can be modified through the decoder's JTAG port if
> necessary.
> 
> What I'd really like to see is a pinout and data sheet w/ register listings
> for the various peripheral devices for the STI5505 chip.  I've managed to
> find a generic data sheet and instruction set listing for STI/Thompson's
> ST20 microprocessors upon which the 5505 is based, but I've yet to find
> anything specific.
> 
> I know the 5505 is used in all sorts of media hardware -- DVD players,
> various set top boxes, and I think even some home theater receivers.  Since
> this *isn't* one of DTV's in-house proprietary deals, I've got to imagine
> tech details must be available somewhere.  Anybody got 'em?  I mean...
> *Apex* builds DVD players with this chip, so we're not talking about state
> secrets, ya know? ;-)
> 
> 
> Then the next problem:  How to get the data into a PC assuming we can get it
> out of the IRD in the first place...
> 
> I'm thinking USB 2.0 could do the trick.  USB2's max bandwidth is 480Mbps.
> Most DTV streams run about 4-6Mbits, and I think the max theoretical is
> 9Mbps per channel.  Even if we assume 10Mbits per channel, USB2 should be
> able to handle 48 streams at once.  I'm sure other limits would be reached
> long before that (IDE bandwidth no doubt being among the first).
> 
> I haven't had much (or any...) experience with USB interfacing, but I'd
> imagine an interface that could take one or more serial bit streams and send
> them into the PC shouldn't be too awfully difficult.  A little custom driver
> work to grab the data & dump it into the ring buffer or a recording should
> be fairly trivial at that point.  Controlling the IRD to change channels
> over the low speed data port might even be part of the USB interface
> hardware.
> 
> USB 2.0 is on most modern motherboards at this point and PCI cards can be
> had for $50 or less.  Basic DTV IRD's can be had for $50 or less (here's one
> on ebay currently going for a whole $1.00
> http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3022782510).  Maybe it
> would take another $50 or so to throw together some USB glue to hook
> everything up.  Or maybe there's even something pre-existing that could do
> the trick?  Seems like it could be an incredibly GoodThing(tm) for Myth.
> 
> 
> So...  Any takers for this interesting mental exercise?  Feel free to tell
> me (via private email or otherwise) to lay off the crack, but I think this
> could be a do-able idea if only the STI5505 specs were to magically fall
> into someone's lap.
> 
> 
> Best regards,
> Zac
> 
> 
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