Re: [Discuss-gnuradio] Status of BTTV/CX88 capture

[Martin Dvh]

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 <address@hidden> 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 <address@hidden>
> >
> > 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
> > > >
> > > >
> > > > _______________________________________________
> > > > Discuss-gnuradio mailing list
> > > > address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>
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