Re: [Discuss-gnuradio] Cheap ADC at 35MHz

[Steve Schear]

At 11:08 PM 4/12/2002 +0100, you wrote:
> Hi all, new reader here ..
>
>
> I've looked through the archives of this list and elsewhere,
> and can't find any references to the Bt848; so if this is
> an FAQ I apologise.
>
> The 848 (and similar 878 etc) are cheap, popular framegrabbers.
> In order to do this they capture composite video at 8x the
> colourburst frequency and decode it digitally. It's also
> possible to capture chroma (from s-video) in a second ADC.
>
> In addition to the TV-specific signal path, they also have
> a dumb capture mode for use in single-frame grabbing and
> video blanking interval data capture. This is therefore an
> 8-bit 35Mhz PCI capture card for about 30 quid (or a lot
> less, at boot sale prices : this is mass-market stuff).
> It's easy to change the crystal if you want 20Mhz (and
> I suspect 40Mhz would work, too).
>
> There are disadvantages : 8 bit is woefully low resolution,
> and they have dc-restoration circuits that really demand
> a line-synchronised dc clamp (injecting a 15kHz spike into
> the signal), but the price is hard to beat. They also have
> I2C interfaces and often tuners. The later Bt878 has audio
> digitising, too. Is this worth pursuing as a low-end
> input card ?
>
> Just to show that a signal need not be video-like to
> operate with these boards, here's a hex dump of /dev/vbi.
> The phono input is fed from a sig gen, and it's in PAL mode
> so the period of the signal you can see is about 7 samples
> at 35.46Ms/s or 5MHz. Actually, it was 5.5MHz but 7 samples
> is rather roughly counted !

[dump deleted]

> The need to fix the DC restoration circuit is fairly obvious from
> this data (loss of lower part of signal) and since the vbi device
> only captures data during the vertical interval it's necessary to
> change the bttv driver to get full capture mode. It does give an
> indication that it could work, though.

This is a great idea.

Eric put me on to the concept and a few days ago.  I posted an inquiry to 
the LinuxTV list and a responder told me that a RAW mode for these ICs 
existed. Gerd Knorr, who maintains the most popular bt848/878 (now 
cn848/878) driver, said there is a _video_ signal (before PAL/NTSC decoding 
and stuff takes place). Some people use that to crack encrypted TV and do 
the PAL/NTSC decoding later on in software.

I've been looking at the chip data sheets 
http://www.conexant.com/products/prodsubcat.jsp?id=92 but wasn't sure how 
to gain access to this raw feed.  It looks like you've already taken the 
first steps.

So, the next questions are which boards are best and whether they can be to 
reworked to remove the undesired spectral components.  According to one of 
the more complete user review sites, 
http://www.vcdhelp.com/capturecards.php, two of the most popular frame 
capture boards with Linux support are Hauppauge WinTV GO 
http://www.hauppauge.com/html/products.htm and the Pinnacle DC10 Plus
http://www.pinnaclesys.com/support/display.asp?FileID=577&ProductID=307  Un 
fortunately I have neither, though I had been thinking of getting some sort 
of DVR capability soon.

steve


BTW, it likely that the simple frame capture boards we want will soon be 
discontinued in favor of more sophisticated versions with build in MPEG 
compression (which do not seem to have a raw mode feature).