Hi Martin, internally, the RTL dongles are fast enough to capture full DVB-T (not -T2) channels, and demodulate, and decode them, and deliver the video stream to the host. However, RTL-SDR can't use that mode - it uses a "bypass the whole Digital TV specific stuff" mode and directly passes IQ samples through USB.
In that mode, it simply can't do more than 2 or 3 MS/s (can't remember), which isn't enough to cover 6 MHz - so everyone's right, you can basically receive the AM black/white info at a partial bandwidth of the ca 5 MHz of the luma signal, but you won't get any color information that way, or audio with the same receiver as you do video. Cheers, Marcus On Fri, 2018-08-24 at 12:22 -0500, Martin McCormick wrote: > First, I will talk about the things I know for sure. The > NTSC analog system as well as Pal systems in a lot of the rest of > the world had a lot in common with eachother. Both systems > transmitted an AM video signal in Vestigial single sideband mode > such that the carrier frequency was always about 1.25 MHZ above > the start of a channel. NTSC systems in the Americas also > transmitted an audio carrier in FM which was always 4.9 MHZ above > the video carrier. Pal systems used exactly the same type of > transmissions except that the 625-line video at 25 frames per > second made a slightly wider spectrum such that the audio and > video carriers were separated by 5.x MHZ, making each Pal channel > 7 or 8 MHZ wide. > > As others have suggested, you could probably get a > monochrome fuzzy image if you can get your sound card to sample > fast enough. You can also decode the mono sound by setting your > RTL receiver to behave just like a FM broadcast receiver but set > the frequency to whatever the video carrier frequency is plus 4.5 > MHZ. if the video carrier is 55.250 MHZ, the audio will be at > 59.75 MHZ. The deviation is 75 KHZ unlike FM radio which is 150 > KHZ. > > That would be a good simple test to see if you are > receiving the channel at all. > > I am guessing that since the RTL chips were designed for > the European television market for cable and over-the-air > broadcasts, they can be sampled extremely fast since the digital > channels still take up the same bandwidth as their analog > ancestors. > > Martin McCormick WB5AGZ > > Anders Hammarquist <[email protected]> writes: > > In a message of Fri, 24 Aug 2018 10:27:40 +0200, "Ralph A. Schmid, > > dk5ras" writes: > > > > Hi Andres, > > > > > > > > just had a short look: doesn't NTSC use a nearly 6 MHz > > > > bandwidth? > > > > > > > > Best regards, > > > > Marcus > > > > > > Yes, no way with the RTL to catch NTSC, it does in SDR mode only > > > 2.smth > > > > MHz bandwidth. > > > > Actually, you should be able to get a picture. The horizontal > > resolution > > will be > > about half of what it would be for the full bandwidth, and no > > colour (as > > the colour > > subcarrier at 3.58 MHz is outside the pass band). You want the pass > > band > > of the reciever > > from just below the video carrier and as high as it will go. > > > > /Anders > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
