Page 1 of 2

2 output channels on DATV repeaters

Posted: Sun Jun 28, 2020 10:16 am
by g7ntg
How about DATV repeaters transmitting two simultaneous signals say, one at 1000s/r so that it can be received on a set top box and the other at 333 or 125 s/r for the fringe receivers like myself? The carriers can be very close together - I have done this on 23cms terrestrial TV and also via Es'Hail with very good results. I didn't see any application at the time - I just wanted to try it!

I used two portsdown transmitters fed into a 23cms splitter combiner via variable attenuators into one power amplifier.

I will do some more experiments here between myself here in Kettering and Arthur G4CPE in Luton next week.

This would hopefully make a big increase in DATV repeater useage and encourage more people into DATV

Re: 2 output channels on DATV repeaters

Posted: Sun Jun 28, 2020 8:50 pm
by G4EWJ
DVB-S2 Multistream can do this on the one carrier.

DVB-S2 transmits data in frames of 64800 or 16200 bits. Each frame can have a different FEC and modulation type. There is a header on each frame that tells the receiver the frame parameters.

You could transmit a high quality video channel by sending it on frames modulated at 8PSK, FEC 3/4. On the same carrier, you can also transmit a low quality channel, perhaps a test card with info, or a lower resolution version of the high quality channel. This could be sent on frames modulated at QPSK FEC 1/4, so it would be receivable at a signal strength 10dB lower than you would need to receive the high quality channel.

Both DVB-S and DVB-S2 can transmit mutliple channels on the same carrier, but with DVB-S, the carrier has one FEC and QPSK is the only modulation type.

The NIM needs to be told which stream to allow through, so I don't think that MiniTioune or Longmynd can receive Mutistream correctly at the moment.

The frame header consists of 90 symbols, modulated in a sort of QPSK. 64 symbols of the header are used to indicate long or short frame, FEC, modulation type and the presence of pilot tones. 7 bits modulated onto 64 symbols is about as resilient to errors as FEC 1/4. This is how the receiving software can tell what the frame parameters are, even though the signal is not strong enough for the frame itself to be decoded.

I'm experimenting with Multistream transmission across the shack for the Ryde project at the moment.


Re: 2 output channels on DATV repeaters

Posted: Sun Jun 28, 2020 9:30 pm
by g0mjw
That's really interesting - but can the current hardware transmit it?


Re: 2 output channels on DATV repeaters

Posted: Mon Jun 29, 2020 12:22 am
by G4EWJ
The hardware should be fine. It would need new software.

It could be done on a Pluto by feeding it a separate transport stream for each multistream. The software would build separate frames for each input stream and when it had a full frame, encapsulate it for the required FEC, add the header, split it into symbols for the required modulation type and pass it to the Nyquist filter software. I've written software to do that, although the multistream part hasn't been tested live yet.

It would need careful calculation of the data rates for each incoming stream to achieve the required output symbol rate.

Another useful DVB-S2 function is null packet deletion. Null packets are not transmitted, but a counter is set on other packets so that the NIM can re-insert the null packets into the output stream. The null packets that were not transmitted can be replaced by packets containing some other data.


Re: 2 output channels on DATV repeaters

Posted: Mon Jun 29, 2020 11:02 am
by pabr
FYI leandvb (work branch) has basic support for multistream (or at least ACM/VCM). You can select the desired FEC with --modcods and --framesizes options on the command-line. I'd be happy to check interoperability against IQ recordings and discuss how to make the functionality more usable.

For the TX side, I have been told that some commercial modulators receive a single TS and route packets into suitable frames according to a PID->FEC mapping table. This sounds simpler than merging independent transport streams if the purpose is to transmit several quality levels of a single video source.


Re: 2 output channels on DATV repeaters

Posted: Mon Jun 29, 2020 12:34 pm
by G4EWJ
On receive, I'm not sure what would happen if you let all multistreams through the NIM, as long as all the video and audio PIDs were unique. Then it would not need any change in the receiver software. Something to try. I'll have a think about how to generate an IQ output.

For tx, PID to MODCOD (FEC+modulation type) mapping would be a simple way of doing it, although there must be some method of handling the control PIDs below 32 that you wouldn't be able to avoid duplicating.


Re: 2 output channels on DATV repeaters

Posted: Tue Jun 30, 2020 6:52 am
by g7ntg
my solution was very simple - I transmitted 1000s/r at 1255MHZ and 125s/r at 1256MHz simultaneously through the same PA
each signal was generated by a separate Portsdown transmitter each feeding a splitter combiner via a variable attenuator.
The output of the combiner was fed into my W6PQL 170 watt linear and thus to my antenna with both carriers at the same amplitude.
receive was easy - a minitiouner could easily separate the adjacent signals.
In effect the transmission used only a slightly wider bandwidth.
I will repeat the experiment locally tonight at about 19.00 and report back.

The whole aim of this is to get repeater range extended to more reeceivers as DATV interest seems to be picking up

Re: 2 output channels on DATV repeaters

Posted: Tue Jun 30, 2020 7:06 am
by g7ntg
for newcomers to ATV the receiver should be simple i.e. a minitiouner plus antenna plus tuned preamp
the argument is that a set top box can be used, which is true , but you still need an antenna and a preamp.
For our local net , I think I have built a dozen preamps and a similar number of 23cms Yagis which were originally on loan to try with a set-top-box and, once success of RX was acheived they were purchased from me (antennas were given - I like making yagis) the next step was to do the same with a minitiouner and subsequently a portsdown and PA.
I got a lot of people started with no repeater access round here at all.
If they could receive a repeater, there would be a huge increase in interest!

Re: 2 output channels on DATV repeaters

Posted: Thu Jul 02, 2020 9:35 am
by g7ntg
Just done some local tests with great results - just waiting to try some longer distances

Re: 2 output channels on DATV repeaters

Posted: Fri Jul 03, 2020 3:30 pm
by g7ntg
Just done some longer distance testing with Arthur G4CPE and Steve M0SKM between Kettering and Luton with 2 channels 1MHz apart , one on 437MHz 1000s/r together with 436MHz at 333 s/r both transmitted through the same PA with very good results and no degradation of either signal.
Therefore it should work with repeater outputs and I would suggest two symbol rates; 1000 for 99.99% of set top box receivers and 125 s/r to vastly increase the coverage area albeit with lower quality.