Discussed were some important aspects of DTV reception, along with some clear misunderstandings, and points that could confuse a reader. It was buried away under a general post, so I brought it out here where it may be more useful.
The post is here, under "Get The Best Reception Regional Nsw, Which transmitter and which antenna".
"of less than 8e-4"
bellotv gave a pretty detailed set of examples of why the above comment could be confusing.
There is another aspect to this which bellotv also highlighted a good few years ago in another post. This relates to using a figure that is the cut-off for what error figure an installer can give you.
"If they cannot do this, get someone else"
It isn't always a situation where the best can be given.
For some jobs (I'd imagine in particular remote country ones like belotv seems to have) you may not be able to achieve, as an installer, what you'd like in an ideal situation. But the customer may be happy with what you can give them.
Or the cost to deliver better and better signal error margin becomes increasingly high, and commercial reality and job cost take over as the main issue.
Values of errors I have quoted are prior to any error correction ie Channel BER or aBER or Pre-Viterbi. The value quoted is from Australian Standard 1367.
MER is the error ratio after all error correction and is not quoted as a ratio but in dB instead.
So BER will give the true level of errors. Post Viterbi BER is the errors left after the first attempt at error correction using the Viterbi Error Corrector. The signal is now Reed Solomon error corrected. The MER shows how well the error correction system can remove the errors.
So the antenna installation should be adjusted to minimise the channel BER. Once this has been achieved the MER should be >25 dB
MER and BER are both key measurements to be taken.
In the DVB-T system, their measurement take-off point is essentially the same.
Modulation Error Ratio (MER) is not measured "after all error correction".
And it doesn't ... shows how well the error correction system can remove the errors.
This shows a fundemental misunderstanding of how the DVB-T system operates. And what some of the key measurements are to try and make sure there is a reliable digital signal there.
Discussions of MER in this area of Australian TV reception (what the forum is about) would lead to QAM constellation diagrams and symbol representation. And there is plenty of good reading available on that.
In simple terms though, real world signal impairments in the transmission and reception of the broadcast signal mean that the symbol landing points (when thought of in terms of a constellation diagram) aren't as precise as we'd like. A certain 'fuzziness' occurs.
MER is a measure of how much difference there is.
The vector difference between the ideal target symbol vector and the transmitted symbol vector. Expressed in dB.
It is measured after the digital signal is recovered from the analogue transmission. And before it is passed on to other components of the receiver that handle the Forward Error Correction (FEC), specifically the Viterbi and Reed Solomon error correctors.
EDIT: Change to title to better reflect post purpose
Edited by charlesc, 03 May 2010 - 09:07 PM.