Hi How Chee,
I have found a raw data file captured from the card in
one my backups. You can have a look a the raw data
file
at http://www.geocities.com/how_chee/index.htm
Well, this indeed looks like continuous.
Seems like you captured a PAL signal with a 27 Mhz ADC clock
I will upload the code soon. Pretty amatuerish code, I
should say. Just wanted to see whether the continuous
capture mode work or not.
On the IF output from the tuner that you used, did you
amplify the signal ?. The specs for tuner IF output
that I came across is only in the mV region. Will need
amplifying before it can be feed to the ADC input of
the CX chip, which has a range compatible with video
composite signal.
I should have amplified it but didn't.
I used a little hack to get a higher signal level.
I used the signal from the input of the IF-saw filter in stead of from the
output.
The signal level here is much higher then the IF after this filter (the filter
has a high loss)
Higher means a few dozen mV.
I used two peaces of coax to export the balanced signal out of the tuner box.
At the end of the two coaxes I put a little home-made transformer to change
from balanced to single ended.
This fed into video-input (starting with the ac-decouple capacitor)
After the ac-decouple capacitor I also added a resistor to +5V and a resistor to GND to try to pull the signal to composite videolevel in stead
of symmetric around GND.
If I didn't use these resistors I only got the positive halves of every
(non-video) signal.
With the resistors I got all samples but since the bt878 keeps trying to restore the DC-level at the points in time it thinks there is a sync
pulse, these put some negative spikes in my signal.
The signal I got is still much too low for a normal composite videolevel.
But the bt878 ADC adapts automatically to lower signal levels and adapts its
reflevel to its minimum (=highest gain)
This means I get a noisy, low-signal level datastream.
But it was enough to demodulate fm or video.
As a note. Because I export the IF before the IF-filter I have NO filtering so my input bandwidth is about 60 MHz and I also get the mirrors of
every signal. 60 Mhz is higher then my sample-rate so I also get a lot of aliases.
This was also just a proof of concept.
The good way to do this would mean using a differential amplifier to amplify
the signal to composite level.
(And use the IF filter or build your own if-filter)
Maybe even add virtual sync pulses to keep the capture chip happy (dc-restore)
and later remove them again in software.
You were saying some time back that bt878 can't
capture continuous data, did you manage to get
continuous mode on the bt878?
No not yet. Still using the vbi-frame mode which is allmost continuous (missing
a few lines in every field)
Your capture looks much more like I would want it to be.
I think I am going to look for a nice cx2388x capture card.
What card are you using ?
What tuner is on it? (Is it easy to hack?)
Greetings,
Martin
Regards,
HowChee
--- Martin Dvh <[EMAIL PROTECTED]> wrote:
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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