GB3FW TX FEC Preferences

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Location: Fort Widley, Portsdown Hill, Portsmouth PO6 3LS
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G8PUO
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GB3FW TX FEC Preferences

Post by G8PUO » Sun Jan 30, 2022 6:30 pm

Hi,

Now FW has gained its NoV (which is excellent news), I am in the process of setting up the hardware and this question relates to the TX.

Can see from the below other repeaters on 3GHz, some varying FEC/Ms setups and wonder what the generally preferred 'standard' is?

JV
3.404GHz - 2MS/s - FEC 2/3
CT
3404MHz : DVB-S2 1M/S 2/3FEC
KM
3.406GHz 2Ms, DVB-S2 - 7/8 FEC - h264 - 1080i
HV
3408 MHz using DVB-S 2 Msymbols 2/3 FEC

The TX base is a Lime SDR and driven from the Portsdown 4, all analogue inputs via a MUX. Any thoughts are always welcomed.

Thanks!
Dave
G8PUO http://g8puo.com
HAMKit http://hamkit.co.uk
RILGES Repeater Group http://rilges.org.uk
SCART Television Group http://scartgroup.co.uk

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G3GJA
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Re: GB3FW TX FEC Preferences

Post by G3GJA » Sun Jan 30, 2022 10:42 pm

Hi Dave

Congratulations on getting an NoV for GB3FW.

The recommendation is for 1MS/s on all bands, but this may reduce to 333kS/s for 23cm should compatibility with Galileo be a problem. The reduced bandwidth significantly improves coverage compared with the higher data rates. The downside is that some satellite set-top boxes won't work, but that should be a minor consideration with Minitioune and Ryde receivers in common use.

Encoding with H265 is preferred, but H264 is perfectly acceptable. Modulation type QPSK is preferred, but I've got good results with 8PSK too.

I am a little surprised that, apart from KM, the 9cm repeaters on your list are wasting so much data capacity with 2/3 FEC. I don't believe it's really necessary to use so much error correction. The 7/8 that KM uses makes much more sense. Perhaps the FEC should be set suit local conditions; if there's multipath on receive there's a chance it might show up on transmit, so there may be a need to use more than other repeaters can use successfully.

Clive G3GJA / G8EQZ

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G8PUO
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Re: GB3FW TX FEC Preferences

Post by G8PUO » Mon Jan 31, 2022 2:46 pm

Hi Clive,

Appreciate you taking time to provide some thoughts.

Feel 23cm has value at being 2, 1Ms, or 333kS/s. Left JT at 2Ms purely for the ease of Sat box users (reasons as you describe) . Some thing to consider though for the future.

FW is another matter as 'listers' need to use an LNB anyway for 9cm, so something like a Titanium and Minitioune/Ryde receiver (which is what we have already), makes sense to drop to 1Ms. Thus feel this is the right route. Thanks!

H265 appears the sensible route, if available and understand it is with the Lime SDR / Portsdown 4 combo, so no brainer - unless I am missing something.

Regarding FEC, still learning and so your thoughts are interesting and feel sure could open up a discussion topic on its own. Would have thought the keepers considered the reasoning between using 2/3 or 7/8. Will maybe setup 2/3 on the bench and we can always change remotely at a later date.

Thanks again.

Dave G8PUO
G8PUO http://g8puo.com
HAMKit http://hamkit.co.uk
RILGES Repeater Group http://rilges.org.uk
SCART Television Group http://scartgroup.co.uk

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G8PUO
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Re: GB3FW TX FEC Preferences

Post by G8PUO » Mon Jan 31, 2022 2:55 pm

As an aside, I see talk-through audio can be shown as being on right and some repeaters use the left channel.

Would the ideal convention be that the main repeater audio would be on the right and talk through on left. Would this be correct?
G8PUO http://g8puo.com
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RILGES Repeater Group http://rilges.org.uk
SCART Television Group http://scartgroup.co.uk

G8YTZ
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Re: GB3FW TX FEC Preferences

Post by G8YTZ » Wed Jun 14, 2023 3:40 pm

I'm of the view that 2MS/s is a good number, there is plenty of band space after all, at JV I've settled on an FEC of 4/5,this provides a data rate on 3.1Mb/s, but the key to a good coverage is also in the transmitted MER. I'm using an SR-Systems HD Mod that produces final frequency directly (without up converter from 40-1000MHz. I then send this into a Kuhne up-converter, that has very low phase noise and this feeds a Stealth Microwave amplifier and PE1RKI slotted wave-guide antenna. The antenna feeder is only 4m long, as the TX is high on the wall in the elevator room and the antenna directly above, but good return loss is also critical for a good transmit MER. Antenna reflections (standing waves) will impact the actual transmit MER. My transmit MER measured with a 30dB coupler is over 45dB and the shoulders so tiny that I cannot measure. EIRP is 150W at GB3JV and the station can be received on a hand-held LNB 15 miles (LoS) away). The best Rx arrangement is the Norsat LNB (ex-VSAT from eBay Indian shipping salvage yard) with a PE1RKI built launcher and an ex-Wi-Max flat plate antenna.

The HD Mod has an on-board H.264 encoder, but also a 2nd TS input and we'll be upgrading to H.265 when the new coder is available.

For repeater monitoring reception I can recommend the GT Media V9 Prime receiver, now at £25 on Ali Express inc. VAT and postage, or £55 on amazon if you want a warranty.

For 70cm Rx I can recommend the Wimo Big Wheel Antenna, works so well! Filtering on the Rx is important I use an Inter-digital filter (PE1RKI) Low noise preamp (PE1RKI) and a custom 70cm SAW filter before the Minitiouner (most important!)

The control system I described in CQ-TV last year, it has an all-digital signal path, management is using a Chinese ModBUS TCP RTU and terminal server, both from Ali Express (USR-IOT Factory Shop). PSUs from Meanwell and an APC UPS for the whole thing.


Justin.
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