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Re: [Discuss-gnuradio] usrp_tv_rcv issues

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] usrp_tv_rcv issues
Date: Tue, 02 May 2006 12:47:20 +0200
User-agent: Debian Thunderbird 1.0.2 (X11/20051002)

Erik Tollerud wrote:
> I'm trying to make use of this new example program to try to tune to
> some local TV stations, but I'm not having much luck... the program
> appears to work fine, and I if I tune to a channel, the fft clearly
> shows the carrier (1.75 MHz below the channel center) sticking way
> above the noise floor, but when I run usrp_tv_rcv, I see nothing other
> than static... Is this example fairly noise-sensitive or the like?  My
> antenna could be better, so the signal isn't that great...
What do you use as frequency and decimation.
Make sure you give the frequency in Hz, not in MHz.
so -f 519.25e6, not -f 519.25
(I could add support code for MHz but haven't done that yet)

If you use a high decimation factor (like 64) you need to give a frequency 
close to the picture carrier otherwise it will fall outside the
bandwidth of th etv_receiver.

An example of an 8 Mhz UHF PAL channel in europe:
using tvrx in RXA.

channel start  = 518.0 MHz
picture carrier= 519.25 MHz
channel center = 522.0 Mhz
channel end    = 526.0 Mhz

-d 64 ==> decimation=64 ==> bandwidth=1 Mhz
So you need to tune within 500e3 Hz of the picture carrier.
The normal thing to do here is use the picture carrier itsself.

-p ==> this is a PAL tv channel (use -n for NTSC)
-R A ==> use tvrx in slot A

You would use
./usrp_tv_rcv.py -d 64 -f 519.25e6 -R A -p

Play with gain slider to determine optimal gain.

If you use decimation 8 then you would normally use the centre of the channel 
because you have enough bandwidth to get the whole channel.
(Note the picture carrier is not at the centre of the channel)

Live display, with decimation 8 will only be possible on very fast machines.
Otherwise you will get underruns of the usrp will will result in a totaly 
garbled display.
(Which will still look different from noise)
For my athlon 1 GHz machine the lowest decimation I can use without underruns 
is 64.

I hope this helps,


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