BATC Forum

Hi.

I have been trying for some time now (unsuccessfully) to send a picture to my local repeater, GB3GV.

Being a bit of a masochist, I decided that I didn't want to go the usual route of DTX1 to achieve this so set about acquiring suitable ex-broadcast kit. After a few false starts, I ended-up with a Tandberg E5611 MPEG-2 encoder and an SED Startcast L-band DVB/S modulator.

Thanks to this site, I discovered the trick for changing the default PIDs configured in the Tandberg and can get a good stable picture if I connect the modulator directly to my Technomate DVB-S receiver. I have also verified that the transport stream is valid with an ASI analyzer.

I then went ahead and bought a 0.2mw->3w linear from Bert PE1RKI (great build quality btw and a very easy transaction). I hooked everything-up and set the modulator output power to give me about 3w out of the amplifier.

When looking on the batc.tv stream for GB3GV, I saw my picture appear for a fraction of a second and then disappear. GV then reset and went back to standby before waking-up again to show my picture for a fraction of a second again.

Obviously there are numerous settings on both the encoder and modulator so I wondered if it is likely to be an invalid setting causing this? If anybody has any ideas, they will be gratefully received!

I did wonder how critical the symbol rate is as to get 4M/S transmit symbol rate from the modulator, I needed to set the stream rate out of the encoder to around 3.6M/S, this was trial-and-error although the closest I could actually get was 4.000014 M/S. My DVB-S receiver seems quite happy to receive it though and I believe that GB3GV also uses a Technomate?

73 Phil

Hi Phil.

As you probably know I have passed on the keeping of GB3GV to Nigel at the LRG.

Looks like you are doing a good job with the commercial modulator, I don't know what the tolerances are on Symbol Rate.

Couple of things, others have mentioned 437MHz input not holding when GV is asleep, and I wondered if there is a problem with switching,
voltage levels dropping when in that mode, I have had fairly long Transmissions during the day.

I will try a sleep mode test when back at home QTH, the /A location can drop out due to some of path almost at ground level.

There are days when the usual 433MHz etc. etc. primary and other users knock out the 437 input, despite lots of filtering, cavities etc.

One test would be to get the LRG Team to bypass the A/V switch and feed it straight into the DTX1 for you and others to also test?

Maybe your kit is fine and dandy

73

Peter

Thanks for your reply Peter,

I spoke to Nigel about this very problem a couple of weeks ago as it is exactly what Mike G4SJX was seeing on the 1249Mhz input (I think he has spoken to you about it?) Mike can successfully use the 437Mhz input so that does indicate that maybe there is a desensing issue on 1249Mhz?

I plan to give Nigel a call over the next couple of days and might get him to try what you suggest next time he is on site. This doesn't seem to effect everyone though so maybe I need more power?

73 Phil

Hi Phil.

I've done various tests on 23cms Analog Digital and 437Mhz using DigiLite on 23cms and Datv Express on 437Mhz.

Been running very low power in the mW range, and also beaming away from GV, 23cms is fine at present on Analog and Digital.

The only problem with dropout is using a MP4 file that has been converted from Avi to MP4 or MP4 from odd sources, then getting drop out on 23cms Digital, camera and .jpg test card images from Sandisk Photo viewer are fine, MP4s from ESA Space all ok.

There are tons of video players and file types out there, so what will work for one might not work for another, all the problems are video sources on 23cms Digital on the Rx at GV, but that is just what I'm getting.

The thing is my tests are very low power and clear line of site, if trees and building Etc taken into account from several miles away then the problems start?

One thing I noticed that 437Mhz worked fine at very low power, but to get a constant clean signal back had to start at 500mW to the 3ell beam at my 2 miles or so away.

And yes there were odd occasions when 437Mhz didn't work, probably down to the usual 70cms problems, other modes on close frequencies.

One thing no difference found during tests before and after sleep mode, but will monitor this when in the shack.

Just my take on things over a few days of testing, you may find different, will update if I have made any wrong claims Etc

73

Peter

Hi Phil,

As you directly feed the modulator by the encoder through ASI, Transport stream bitrate is very important : on such modulator there could be some "switch off alarm" if the ppm is not OK, or it could add packets which disturb the PCR Clock of the transport stream.
Some receiver don't care about the PCR (like your set to pbox) , some are very sensitive (particulary professional ones).

Suggest that you put the right Transport stream rate out of your encoder :
TSRate= SR*188*2*FEC/204 so at 4MSymbol/s : 3,68627451 Mbits on Transport stream

73's Evariste F5OEO