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

[Nan Zhang]

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