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Hello, I've a technical question.
What is the exact meaning of FEC in a sat stream?
Greets...
What is the exact meaning of FEC in a sat stream?
Greets...
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Re: FEC on Sat-Stream
That’s on the one side very simple and on the other side a very complicated theme.
FEC means “Forward error correction”.
It’s a part of coding theory. It’s used when you transmit digital streams (like on sat, digital telephoning etc.).
When you transmit you will have more or less transmission errors. There are different kinds of such errors which must be “repaired” on mathematical methods. These methods need parameters which are added to the normal stream as correction datas.
For example if you have FEC 3/4 then 75 Percent are normal datas and 25 percent are correction datas. This reduces (as one of a view parameters) your transmitted datas, but it raises the data-security. -
Re: FEC on Sat-Stream
OK, makes sense.
What do you mean with: different kind of errors?Comment
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Re: FEC on Sat-Stream
There are two typical and completly different types of errors in a sat stream:
1. Bits are wrong
2. Bits are missing
Both types of errors must be "repaired".Comment
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Re: FEC on Sat-Stream
It might sound strange, but it is more easy for the correction algos to correct missing bits than wrong bits.Comment
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Re: FEC on Sat-Stream
Why is a missing bit easier to repair and how is this possible? I have no idea how this could work?Comment
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Re: FEC on Sat-Stream
It is easier because you know that it is missing (based on checksum).
There are two main correction algorythms in a sat stream. One is based on a variable amount of correction in the FEC, e. g. 1/3 or 1/4 of the complete stream and the other is an intern algorythm (Reed Solomon). This intern Algo is the same as it is used on CDs.
This basic correction algorythm is based on polynom and monom mathematics. A bit complicated to explain…
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Re: FEC on Sat-Stream
I dont like too much mathematics...
Is it possible to see if this algorythms works in my box or the stream?Comment
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Re: FEC on Sat-Stream
Yes, on some receivers you have a correction rate. On the good old DBox2 e.g. you can see a BER-Rate. This should be zero (optimal). If it is too high, even the repair-algos can't correct the stream and your Picture gets artefacts (blocks, dropouts etc.) or the picture freezes. Sometimes the mathematic-routines work so "heavy" that the box-system crashes and you have to reboot your box.Comment
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Re: FEC on Sat-Stream
We're going a little OT...
But these things deserve perhaps a thread, even if they're more strategical for terrestrial broadcast than satellite.
I'd like only to say (because is connected to previous discussion) that as a matter of fact you can measure BER before and after correction (before and after Viterbi).
Of course for judging signal quality is important before, without the work of correction...Comment
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Re: FEC on Sat-Stream
Ohh, I think error correction is in topic.
The viterbi-algorythm (based on Andrew J. Viterbi in 1967 for "convolutional codes over noisy digital communication links") is elementary for the data-amount of a sat transponder characterized by the FEC.
High FEC (1/2 or 2/3) means more stream protection than 7/8, but the "usedatas" are getting lower. Security has its price.Comment
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Re: FEC on Sat-Stream
I'm beginning to understand - a bit...
Has every transponder or sat his special correction algorythm or all the same?Comment
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Re: FEC on Sat-Stream
All use the same procedures, the FEC - viterbi (extern) and Reed Solomon (intern with the Relation 188/204 as a second "FEC" for DVB-S signals).
This is a part of the receiver OS.
If not - no signal...
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Re: FEC on Sat-Stream
@Fireflyer: you look very interested, but perhaps some friend think that's philosophy...
That's not: an example:
SK* IT uses a peculiar combination of FEC and symbol rate (5/6 and 29900) to stuff more channels inside transponders.
But this means more critical signal and if you have a little dish, when it rains hard you will see nothing more...
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