Comparing Video Modes on 1249MHz

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g8vpg
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Joined: Fri Oct 11, 2013 9:35 am
Location: Bristol

Comparing Video Modes on 1249MHz

Postby g8vpg » Fri Feb 10, 2017 4:25 pm

Here in GB3ZZ-land, we have been doing some very interesting tests that some of you may be interested in. I thought it would be interesting to try some of the new modes which v1.23 of the DATV Express software makes available. We bought a Revez HDS610 satellite receiver off of Ebay. This is a DVB-S/DVB-S2 receiver that does QPSK, 8PSK, 16PSK & 32PSK. It will also receive down to 1ms/s. It will switch automatically between DVB-S & DVB-S2, but will not switch codecs automatically - you have to tune a separate channel for each. It also has a lock lamp.
We temporarily added the receiver to GB3ZZ, by daisy-chaining it off the loop through from the standard Comag receiver we use. We did some tests to see the access threshold for lower symbol rates using DVB-S2;
The weather was cold, 4deg.C, dry, calm, no leaves on the trees, ie good local conditions.
The path G8VPG to GB3ZZ is 14km, not line of site, transmit aerial a single 28 element Wimo, Alford slot at the repeater.
Using the standard mode, 1249MHz, 4ms/s, DVB-S, QPSK, MPEG2, FEC1/2, the threshold to just access and hold the repeater was 3W transmitter power.
Using 1249MHz, 1ms/s, DVB-S2, QPSK, H264, FEC3/4, I was able to access a steady signal with just 0.25W! Picture quality is still good. This is an improvement of 10.8dB. Several thoughts occur to me;
1) If all the newcomers building Portsdowns stick to low symbol rates on DVB-S2, they can save a lot of money on linear amplifiers.
2) If we use low symbol rates on DVB-S2 for Es'Hail Sat2, we should be able to get in with lower power and have more simultaneous channels.
3) Could the DTX-1 be made to send H264 DVB-S2? Perhaps daisy chaining another DVB-S2 receiver on repeater inputs would allow lower power/longer range inputs?
This was just a manned trial at GB3ZZ and everything is now back to normal, but it was certainly an eye opener for me to access a 23cm repeater from 14km away on 250mW!
73 Shaun G8VPG.

G8GKQ
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Re: Comparing Video Modes on 1249MHz

Postby G8GKQ » Fri Feb 10, 2017 10:50 pm

Hi Shaun

Very interesting. The reduction in SR from 4M to 1M should give you 6 dB. Changing FEC from 1/2 to 3/4 loses a dB or so, and going to DVB-S2 gains about 3dB, so you've done well there.

Unfortunately Portsdown on its own won't transmit DVB-S2, just H264 on DVB-S. Connected to a DATV Express, it currently has a similar capability, but there may be the possibility of DVB-S2 (from the DATV Express) in the future.

DVB-S2 QPSK looks to be the most efficient easily available mode for Es'hailSat 2, with a compromise between symbol rate and available power. That is not to say that DVB-S RBTV from Portsdown should not be used - it does look to be viable and has the potential to significantly reduce the equipment entry level requirements and make the satellite accessible to more stations (over 50 Portsdown builders already). I'm certainly going to try it!

Dave, G8GKQ

g0mjw
Posts: 331
Joined: Sat Sep 20, 2014 9:15 am

Re: Comparing Video Modes on 1249MHz

Postby g0mjw » Sat Feb 25, 2017 12:19 pm

Hi all

Following the tip I ordered one of these (Ebay item no 192058199583 ) Freesat V7 COMBO HD DVB-S2/T2 TV Boxes for and it was £25.64 delivered.

Its quite a neat little box and looking inside, there isn't much in there, just a small PCB with tiny NIMs and a SOC with a heatsink, but it looked to be well made. It works as intended as a Satellite or terrestrial receiver, it's quite good actually. It does all the usual stuff, motor control and so on. Bonus is you can record the entire TS onto a USB stick. With a WiFi dongle I had it playing youtube videos.

For DATV use, these boxes can be run off a 12V @2A supply. They have HDMI, RF and composite video outputs. There is also an RS232 port on the back of mine - not as advertised, no idea if its useful or not.

In a quick test this afternoon its working on 1249 MHz across my bench @ 1Ms/s DVB-S2, 3/4 rate 8-PSK H264. Nothing scientific, its just what I had my DATV express set to. However, this allows for reasonable video in a narrow bandwidth. It takes a while to work out the interface but once you have relatively easy to put in a DATV transponder. I used an LNB frequency of 10GHz to make the maths easy. After that its just a matter of selecting a channel. You can export settings to USB so it might be possible to edit these and bypass the interface - and maybe even enter settings you are not supposed to.

I will see if I can get it to go lower in sample rate and find out what it can do on the DVB-T modes.

Mike

g8vpg
Posts: 154
Joined: Fri Oct 11, 2013 9:35 am
Location: Bristol

Re: Comparing Video Modes on 1249MHz

Postby g8vpg » Sun Feb 26, 2017 9:01 am

Mike,
I am glad you found the Freesat v7 to work well. To those of us that started dabbling with satellite receivers in the 1980's, it is something of a miracle what is available today for so little money! This one is working quite well. The DVB-T input will only go down to 6MHz, and the input is the "RF in" Belling Lee socket, not the one marked aerial, which threw me at first! It mixes DVB-T channels in with the satellite ones and it only does DVB-T2, ie the HD channels, not the SD channels which are the majority.
My experience is that most of the current breed of FTA satellite receivers will go down to 1ms/s, even when the spec. states just 2ms/s. However, they will not go lower, even if they accept a lower input, which most are error trapped to reject. The Revez HDS610, which we are using at GB3ZZ, will also do 32PSK, although most are limited to 8 or 16PSK.
One thing I have found with the Freesat v7 is that if it loses the signal, perhaps because you turn off the TX, it doesn't find it again until you manually re-select the channel. I have never encountered this before and it would make it totally unsuitable for repeater use. Also, it seems to do a scan of all its satellites when it is switched on, another unique feature. Another very minor point is that it is not supplied with batteries for the remote control; again, I have never encountered that before! Let us know if you find any hidden features Mike.
73 Shaun G8VPG.

g0mjw
Posts: 331
Joined: Sat Sep 20, 2014 9:15 am

Re: Comparing Video Modes on 1249MHz

Postby g0mjw » Sun Feb 26, 2017 10:59 am

Shaun,

I noticed that curious behaviour in testing yesterday afternoon. There seems to be quite an active forum so maybe worth posting it as a bug. The firmware hex code is available should anyone feel the desire to modify it. I think its probably best to just have the one entry in the satellite database and delete the rest. The GUI allows you to set lower symbol rates, but they do not work. So far I have not found any hidden features but have not yet tried the serial port. It might be worth trying to SSH into the box. Unfortunately the Revex 610 isn't a current model so you can't get them any more. I did look.

I started in satellites in the late 1980s, building my own tuner - it was more expensive even though it was all FM back then. LNBs were quite expensive and much less sensitive, typically with a 2dB noise figure.

Mike


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