Hi Hew How Chee,
I did some experiment on the CX tv card some time
back. Looks like a continuous capture of 28MHz 8 bit
data from the video ADC is possible, at least it seems
like that.
This sounds very promising.
I tried capturing a sine wave and also raw
composite TV signal. However I did not proceed further
because the data looks noisy, even with no input. Not
sure whether is it cause by the PCB layout.
I know the bt878 changes the ADC reflevel down when it sees no or only small
signals. This way it tries to keep a constant signal level.
(A kind of AGC), The cx23888 does probably a similar thing.
When you input no signal, the ADC goes automatically to its highest sensitivity
so you get a very high noise level.
This should go away if you input a big enough signal.
Normal videosignal is 1volt peak to peak (video black is 0 V, video white is
0.7 V, sync tip is -0.3 V)
The quality of the signal will of-course not be the same as with an usrp but with my bt878 card I had low enough noise to receive broadcast FM,
narrowband fm (police) and narrow-band am (air-traffic)
As frontend I used the IF of the tuner which was on the capture card (added an
IF output myself)
The sine
wave signal came from Palm since I dont have a sig
gen. As for the video signal, it came from videocam.
You could also try to use the soundcard output of your pc as a signal generator.
(using gnuradio or audacity)
The videosignal is AC-coupled followed by a DC-restoration circuit (using sync
and blacklevel for this)
Lowest frequency it should at least let through is 50 Hz (probably lower)
If you capture the signal from the videocam it is also possible to see if you
have continuous capturing.
If the videosignal and all sync information is in the output then you really
have continuous capturing.
I deleted the capture data when I reformat the hard
disk. If anyone is interested to look at it. I can
recapture the data this weekend, hopefully I can dig
out the source code.
I would be very interested in the source-code and the capture data.
How did you put the card in continuous capture mode?
And do you remember if it missed no samples (look at more data then one field
(20 msec))
Greetings,
Martin
Regards,
HowChee
--- Martin Dvh <[EMAIL PROTECTED]> wrote:
Cory Papenfuss wrote:
Just wondering if there's been any more
progress on that front. A
recall awhile back some promising information on
continuous raw captures
on CX88-based cards. It would definately be an
inexpensive way to play.
I haven't been working on this lately.
I still don't own a capture card with this chip.
I did hack the cx3288 driver and others tried this
out.
Enabling the RAW_ALL bit didn't result in a
continuous capture (yet).
Since there are a lot of (undocumented) registers of
which the values are very interrelated hacking a
driver when others have to test every
change goes very slow.
Of course, buying one of these for myself would
increase the development speed but since the outcome
might very possibly be that it cannot be
done I did'nt do that.
If anybody who has a capture card with this chip
wants to go ahead and try hacking the driver further
I would be more then willing to help.
(My code so-far is at
http://www.olifantasia.com/pub/projects/gnuradio/mdvh/cx2388x/
http://www.olifantasia.com/pub/projects/gnuradio/mdvh/bt8x8_driver/
http://www.olifantasia.com/pub/projects/gnuradio/mdvh/examples/
http://www.olifantasia.com/pub/projects/gnuradio/doc/specsheets/
)
Also look in the gnuradio mailing list archives for
[Discuss-gnuradio] using ADC of a TV-card with
CX2388x chip or digital TV-reception chips/cards
[Discuss-gnuradio] ATTN Martin Dvh: cx8800 driver
Since recently, I own an usrp.
I am still interested in using a hacked tv-card
since my experiments with the hacked bttv card
delivered 60 Msamples/sec to main memory without
problem.
With the usrp I am limited to the usb2 bandwidth
(which restricts me to 4 Msamples/sec on my pc which
has a cheap usb2 pci add-on card).
greetings,
Martin
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