70 cm Amateur TV reception

Copyerselveson

I recently bought one of those Realtek DVB-T dongles and have had a lot of fun using the SDR# software listening to 2m and 70cm ham radio.

So much so I have sent off for a course to get the GB Foundation licence!

SDR# is showing me a big carrier on 437 mhz. I suspect it's a 70cm ATV station. What software could I use to decode 70cm DVB ATV?

GekkePrutser

Copyerselveson wrote: »

I recently bought one of those Realtek DVB-T dongles and have had a lot of fun using the SDR# software listening to 2m and 70cm ham radio.

So much so I have sent off for a course to get the GB Foundation licence!

SDR# is showing me a big carrier on 437 mhz. I suspect it's a 70cm ATV station. What software could I use to decode 70cm DVB ATV?

I'd say that if it really was Digital ATV, you could use the supplied software and drivers of the stick to receive it. Since that's its original purpose.

I don't think ATV over digital (DVB-T) is an allowed mode on 70cm though. The band plan mentions only analog ATV (and discourages use of this on 70cm in favor of the microwave bands). I'm not sure if the standard software with these sticks supports receiving analog TV. I've never used the disc that came with them, I always just use the osmocom drivers to use them as a general SDR (like yourself, I suspect).

But it could indeed be TV, the 435-438 segment seems to be set aside for ATV and satellite service in the band plan.

martinsvi

a typical UHF Digital TV transmission should take up about 6 to 10 MHz of bandwidth, depending on compression, modulation and so on. If it's something that sits well within 1MHz, I doubt it's any type of tv

Is the signal constantly there? Check with a different computer, it might be nothing more than an interference from your computer or other device near by

watty

It's possible to do Digital TV in 1MHz, or even in theory in 500KHz or less. It would not contravene licence. Using DVB-C the bandwidth is just a bit more than symbol rate I've done 2MHz channel HD and SD DVB-C tests with MPEG4 using 1.7M symbol rate 10% FEC and 64QAM

But it's unlikely to be ATV of any kind.

405 Line Analogue will fit in 3.5MHz and Vintage people do have 220 Line (the last mechanical TV using mirror based displays and scanners) which will fit 1MHz easily. But while both of those exist today, I doubt anyone in Ireland is transmitting them.

Antenna

watty wrote: »

405 Line Analogue will fit in 3.5MHz

A Black-and-white 625 analogue signal, with VHS-like horizontal definition, can exists in a 3.5MHz bandwidth, such a signal could be tuned in on any Irish domestic TV with full cable TV bands tuning. A domestic grid type UHF TV aerial would have some gain on 70cms and if in approx the right direction (and same polarisation) as the amateur station - it would pull in a picture if the signal was strong enough.
There could be no audio on the TV though, that would have to be transmitted seperately, as normal TV audio is 6MHz above video carrier.

The 70cms band would be approximately (with fine tuning upwards if needed) "S37" in Hyperband
http://en.wikipedia.org/wiki/European_cable_television_frequencies

watty

As you mention you'd need a custom sound carrier spacing though, the sound on "professional" systems does use a separate carrier/Modulator and not "subcarrier" method. Perhaps 3.57MHz and then you can easily add NTSC ceramic filters to an intercarrier type sound decoder. Except TVs & VHS tuners stopped using 6MHz Intercarrier and separate IFs instead for FM and Nicam.

I think unlikely anyone is doing this on 70cm band.

Antenna

martinsvi wrote: »

Is the signal constantly there? Check with a different computer, it might be nothing more than an interference from your computer or other device near by

or maybe it is self-generated by the dongle itself ?

Anyone using wideband scanners in the past would be used to this. Some people call them 'birdies' (false carriers generated by the receiver itself).
http://www.qrz.com/page/hamspeak.html

The other day for example I tried out an old AOR scanner (AR2000) and that has a strong phantom carrier on 435.2000 MHz in the 70cms band. It remains there if the aerial is removed.

watty

Yes, good idea. Replace aerial with 47, 50, 75 or 82 Ohm resistor or a short and see what changes.