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Re: [Discuss-gnuradio] using ADC of a TV-card with CX2388x chip or digit

From: Hew How Chee
Subject: Re: [Discuss-gnuradio] using ADC of a TV-card with CX2388x chip or digital TV-reception chips/cards
Date: Mon, 24 Jan 2005 07:23:02 -0800 (PST)

--- Martin Dvh <address@hidden> wrote:

> I am using a bt878 card for sampling right now.
> There are two ways to use it.
> Using the audio ADC you can sample up to 860 kS/s in
> 8 bit (or 16 bit at lower rates). (with a modified
> linux driver, the original driver alows 
> up to about 200 kS/s)
> This works quite well although the sample rate is
> too low for any wideband applications

Months back, I did try using btaudio driver to get
back samples from the audio ADC on the Bt878
(Pixelview PlayTV Pro TV card). Didn't work at first.
Struggle for a few weeks. All I get are saturated
samples. Wondering what could be wrong. Finally,
bought myself a new TV card and walla, the audio ADC
works. Noticed that there is distortion when volume is
adjusted higher. Probably due to over amplification of
the signal. Did not have a scope to check out.

I used Audacity to record those samples but are
annoyed by the extra harmonics generated during no
input. The input is from the onboard Philip FI1213
compatible tuner composite audio ouput. You can see
the 19 kHz pilot  stereo signal, the L-R signal and
the RDS data. Abandoned the idea of trying to decode
the stereo and RDS data since the SNR is just about
6-10 dB?? above noise floor. Not sure whether this is
good enough or not.

At higher sampling rate, I think I notice more
harmonics/noise. I wondering now whether the audio ADC
can achieve more than 200k sampling rate. Can't
remember what the datasheet says but am thinking
either it is like that or it might be noise due to
poor PCB layout.

For those interested, a link posted months back


> The newer conexant CX2388x chips have similar
> descriptions in the datasheets for the
> capture-modes.
> I found however a the following register description
> which might mean what we want it to mean:
> CX23883_Datasheet.pdf:
> Capture Control Register 24 h310180
> Bits   Type   Default Name Description
> [6]    RW      0          CAP_RAW_ALL A value of 1
> enables continuous raw data mode capture.
> This might mean that we can capture without any gaps
> with this chip.
> As I don't own a TV-card with this capture chip
> (yet) I cannot say what this mode really does but I
> am willing to try to modify the driver and 
> experiment.
> I anybody has a TV-card with this chip please let me
> know.
> Maybe we could cooperate to try to find out.

I exchanged the new TV card with Pixelview PlayTV
Ultra which has a CX23881 chip on it. If the CX23883
does support raw video ADC, this will really be great.
If it is pin compatible with CX23881, I  might even be
tempted to replace the chip. Hopefully the video ADC
samples much higher than the tuner IF which is about
30 something MHz if I remember correctly.

How Chee

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