Re: [Discuss-gnuradio] NTSC Audio Demodulation

[Thom Rainwater]

> Date: Mon, 13 Jun 2005 12:39:51 -0700
> From: Eric Blossom <address@hidden>
> Subject: Re: [Discuss-gnuradio] NTSC Audio Demodulation
> To: Thom Rainwater <address@hidden>
> Cc: address@hidden
> Message-ID: <address@hidden>
> Content-Type: text/plain; charset=us-ascii
> 
> On Mon, Jun 13, 2005 at 01:08:32PM -0400, Thom Rainwater wrote:
> > I am using the below code to demodulate the Audio Component of the
> > NTSC signal. The math seems to be correct, but when I run the code I
> > do not get what I expect. This may be because I am using a FM antenna
> > to capture the NTSC signal. Is it possible that the FM antenna is the
> > problem and not the code? I don't have access to a TV antenna at
> > present so am wondering if this could solve the problem. Does anybody
> > see any glaring problems with the code, or could someone who has
> > access to TV antenna run the code and let me know if it works. The
> > input that the code uses is the video carrier (e.g. 67.25 == video
> > carrier, 71.75 == audio carrier==> user inputs 67.25)
> >
> > Thanks
> >
> > Thom Rainwater
> 
> Hi Thom,
> 
> I just modified the version of tvrx_wfm_rcv_gui.py that's in CVS so
> that it no longer limits that range of frequencies that it will tune.
> 
> You should be able to use it as is.
> 
> For example, to listen to the audio from broadcast channel 4 do this:
> 
>   $ tvrx_wfm_rcv_gui.py -f 71.75
> 
> FYI, gnuradio-core/doc/others/tv-channel-frequencies contains a table
> of the center frequencies of the North American broadcast TV channels.
> 
> I tested this using an FM dipole (with 300 ohm to 75 ohm adapter)
> plugged into the F-connector on the TVRX board.  It worked great.
> 
> Eric
> 
> 
> 
> 
> ------------------------------

Hi Eric,

I was actually trying to get the fm audio component of the NTSC signal
as part of a larger TV demodulation scheme. I was trying to tune the
USRP to the video carrier, for example 67.25 MHz for channel 4, and
then manipulate the signal within the python code to isolate the audio
carrier and components.

tvrx_wfm_rcv_gui.py, however, tunes directly to the audio station. I
don't want to do this becasue then when I get to the video
demodulation aspect, I won't have the whole signal. I don't think that
this was clear in my original posting, but I am actually trying to get
both the video and audio, I'm just working on the audio right now and
having problems getting it. Any other ideas?