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Re: [Discuss-gnuradio] Question about sampling colored image with TVRX

From: Peter Monta
Subject: Re: [Discuss-gnuradio] Question about sampling colored image with TVRX
Date: Fri, 22 Aug 2008 01:07:15 -0700
User-agent: Thunderbird (X11/20080501)

Hi Nan,

1. For TVRX board, as we know, the output signal is a 6MHz chuck. But in which band is this 6MHz bandwidth located? For example, if the carrier frequency is 477.25MHz, is the output signal from 477.25-1.5MHz to 477.25+4.5MHz, or from 477.25-3MHz to 477.25+3MHz, or in other band?

From 476 to 482 MHz for this channel. In general, if the video carrier is at f, the NTSC
signal will span the 6 MHz interval [f-1.25,f+4.75].

2. I analyzed the frequency response of the sampled signal, and found there are two peaks in spectrum. A higher peak is located in DC frequency. Another one, which is a little bit lower, is located at roughly f = 2.5MHz. It is different from what I expected. My original thought is that the second peak should be in f=3.58MHz which is the frequency of the subcarrier carrying color information. Could someone help to explain this?

The largest peak will be the video carrier; there should also be a peak at +4.5 MHz about 15 dB below the video, which is the audio subcarrier. Yes, you should also see a broad feature around +3.58 MHz, but chroma uses (almost) suppressed carrier, so it won't be as distinct. If you're really seeing something at +2.5, it could be local interference or IMD in the front end. Might want to play with the gains for best
dynamic range.

For best flatness you'll want to set your frequency so as to center the NTSC signal as a whole in the USRP passband; once you have the digital complex baseband signal, you can translate the video back to centered at DC for carrier acquisition and tracking,
VSB filtering, demodulation, and so on.


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