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Re: [Discuss-gnuradio] Basic Tx to demodulated RCA audio?

From: Clark Pope-TWC
Subject: Re: [Discuss-gnuradio] Basic Tx to demodulated RCA audio?
Date: Fri, 19 Aug 2005 05:09:42 -0400

I don't think you'll have any problems with the 50 to 75 Ohm conversion.
I've done similar things with other hardware, but not the USRP.

Regarding playback, you may want to consider doing all the processing in the
FPGA. To get the composite NTSC signal, for example, you should be able to
just add a CORDIC to the FPGA and route the amplitude data to the TX board.
The resulting analog signal should plug right into to any tv with a
composite input. The result will be in color too.

To get simultaneous audio you'll need a DDC and another CORDIC for the FM
demod, but it should fit. Or it may be easier just to pipe the audio out the
PCs sound card.

----- Original Message ----- 
From: "Chris Pezzee" <address@hidden>
To: <address@hidden>
Sent: Thursday, August 18, 2005 4:37 PM
Subject: [Discuss-gnuradio] Basic Tx to demodulated RCA audio?

> I have my USRP capturing video/audio from the TVRX daughter card, then
> demodulating the audio to baseband and playing to sound card and
> dumping to disk.  I can play back the captured audio to the soundcard.
> Yay!
> I want to send the captured audio out through the Basic Tx, but after
> reading the group archives, I think there may be a problem.
> Should I be concerned about impedance mismatching between the Tx
> board and an RCA-type audio connection (50ohm out to 75ohm input?)
> (I am hesistant to hook up the USRP TX_A output directly to the RCA input
> in case there is an overheat/short circuit/bad mojo issue.)
> Has anyone done this successfully?
> Can I do this directly or will I need an external buffer/amplifier to
> drive these signals?
> I also read about a 'hole' in the Basic Tx frequency output, but I don't
> know if this will be a problem for me.  If I want to output 20-20kHz
> signals, will they be filtered at the TX_A output?
> If this is not a problem, I should be able to  produce a stereo output by
> using TX_A and TX_B for Left and Right, correct?
> Eventually, I hope to capture/create NTSC/PAL signals and replay them
> through an external modulator for television equipment testing.
> Thanks.
> BTW, I mounted my USRP in a Velleman G758 case, but the internal mounting
> hardware wouldn't quite fit the USRP, so I had a little drill work to do.
> It works ok, and now I don't have to worry about kids and cat effects.
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