Long and short frames, can someone please explain them?

Digital ATV - The latest generation, cutting edge ATV - Please discuss it all here.
Forum rules
This forum is run by the BATC (British Amateur Television Club), it is service made freely available to all interested parties, please do not abuse this privilege.

Thank you
Post Reply
Basil
Posts: 287
Joined: Tue Sep 10, 2013 7:28 pm

Long and short frames, can someone please explain them?

Post by Basil » Mon Feb 12, 2024 9:14 pm

I watched a gentleman experimenting with long and short frames on the WB satellite channel yesterday. Their setting is not something I have fiddled with as I do not know what changing them is supposed to achieve, nor how to decide which is better or more appropriate.

Can someone very kindly tell me a little more about this setting as Google has not really assisted me?

Thanks Chris 2E0ILY

g0mjw
Posts: 2265
Joined: Sat Sep 20, 2014 9:15 am

Re: Long and short frames, can someone please explain them?

Post by g0mjw » Tue Feb 13, 2024 9:03 am

DVB-S2 used LDPC codes for forward error correction (FEC) The DVB-S2 standard supports two different frame lengths, short frames of 16200bits and the normal 'long 'frames of 64800 bits. So a short frame is 1/4 of the size of a long frame. The normal frame supports all the FEC rates, the short frame all except 9/10.

So, for example in a standard (long) frame at say 1/2 rate FEC, 32400 bits are the information and 32400 bits are encoded into a 64800 bit code word. In a short frame at the same rate, 8100 bits of information are encoded into a 16200 bit codeword. Longer codewords yield better performance, within limits. However, the complexity of the decoder is greater for longer frames because it is working on more bits and the latency is higher as it takes 4 times as long to send each frame. There is a frame overhead, so shorter frames mean more frames and therefore more overhead.

For our purposes this in interesting. At high symbol rates the latency isn't much of an issue. For low SRs it is. a 64k frame takes around a second at 66ks. However, we are also trying to work close to the limits and longer frames give slightly better performance.

Perhaps more critically at low symbol rates, the extra overhead of the short frame steals bandwidth from the data and at 66ks that might mean a 20% penalty, which if operating at 1/2 FEC QPSK doesn't leave a lot for the video.

So, in a contest, just the numbers, you might want to use short frames and 1/4 rate FEC, assuming the encoder and decoder are up to it, at as low a symbol rate as practical. But bear in mind the latency will be higher which could be an issue if the signal is varying rapidly.

See https://en.wikipedia.org/wiki/DVB-S2
https://en.wikipedia.org/wiki/Low-densi ... check_code

Mike

Basil
Posts: 287
Joined: Tue Sep 10, 2013 7:28 pm

Re: Long and short frames, can someone please explain them?

Post by Basil » Tue Feb 13, 2024 12:25 pm

Excellent Mike, excellent! A great resume and very helpful, thank you very much.

Post Reply

Return to “DATV - Digital ATV”