# Question about sampling colored image with TVRX

Hi,

I tried to sample NTSC signal with decimation rate = 8 (8Msps) using
TVRX
board. The 8Mspc sample rate is supposed to covered the whole range of
6MHz
TV channel bandwidth. Thus, the obtained signal should contain all TV
information including luminance and color. I have two related questions:

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?
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?

Thanks,
Nan

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].

1. 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.

Cheers,
Peter