Re: [Discuss-gnuradio] MAKING A NTSC TV RECEIVER

[CEL]

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 <address@hidden> 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
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio