Signal Quality & 2D

[geoffreys]

Still trying to receive 2D here in Eastern Cyprus (southern sector, but only 5 mins by car from Famagusta if we cross the 'green line').
Going back to basics it is clear that the critical factor is getting the Signal Quality up to 50% or more in order to be able to receive 2D; Signal strength does not seem so critical.
What exactly is Sig. Quality a measurement of?
Geoff

 

[rolfw]

I don't know how the digibox measures it, but I guess that it is a signal to noise ratio and not a bit error rate measurement.

 

[Leee]

This is what I have noted in order to look at strength + quality on 2D transponders....I assume you're looking at strength on those transponders and not the default one?

BBC - Manual tuning/viewing signal and quality strength
You will have to go to the installer menu Services 4, 0, 1 Select (exactly in that order), then go to Manual Installation.

Frequency 10773 Horizontal, FEC = 5/6 and Symbol Rate=22.0

Then go to the last line then Select, you should then get the signal meters showing you the strength of the 2D channel.

 

[Zorba]

I don't know how the digibox measures it, but I guess that it is a signal to noise ratio and not a bit error rate measurement.

I think you are right,Because when i measure with a spectrum analyser i can clearly see the 2D signals(horizontals at least) but the spikes are obviously shorter than the 2B and eurobird.Therefore the signal is present,thats why the signal strength shows something.But when you switch on your digibox you get "no signal" on bbc because the short spikes mean low signal to noise ratio,which has to be over a certain figure for the picture to appear.So when you get one "notch" on your signal quality,the picture is on the treshold and probably pixellating,like sky movies in the afternoon on a 2.4m dish in larnaca.
The other factor is that the symbol and fec rates(22000-5/6)on all channels on 2D are different from 2B etc (27500-2/3) which is like a higher threshold for the digibox to deal with.
Thats only my theory though.
Sorry,Geoffreys,but it looks like its a 3.8/4.2 for you.
Or move to Paphos(nice this time of year)

 

[Old Satellite]

Just to high light the FEC ( Forward error correction) is very important when you are at, or close to the systems capibility.

The FEC (Forward Error Correction) indicates how many Bytes are used for the actual signal, and how many for correction of errors.

A FEC of 1/2 means that 1 Byte out of 2 is used for error correction, while a ratio of 7/8 means 7 Bytes are used for the actual signal, and only one for error correction.

A FEC of 1/2 gives as perfect as possible reception, since every Byte containing actual signal is controlled by another Byte checking it.

When a provider chooses a FEC of 7/8 it means he is not wasting any bandwidth at the cost of delivering a signal.

The lower amount of error correction means that more sophisticated equipment at the receiving end (for example a more stable and sensitive LNB, or higher reserves with the dish) are needed compared to the same transmission using a FEC of 1/2.

So transmission data rate = customer information rate x 1/ (FEC rate).

FEC rate is typically in the range 1/2 to 7/8 so the transmission data rate is always significantly more than the customer information rate.

This is the actual formula:

SR = Symbol Rate
DR = Data Rate = the customer information rate.
CRv = Viterbi forward error correction (FEC) Code Rate. Eg. 1/2, 2/3, 3/4, 5/6, 7/8
CRrs = Reed Soloman forward error correction (FEC) Code Rate. Eg. 188/204
m = modulation factor (transmission rate bits per symbol). BPSK=1, QPSK=2, 8PSK=3 etc

Formula: SR = DR / (m x CRv x CRrs)
DVB-S Carrier bandwidth
The bandwidth of the carrier at the -3.8 dB points is approx the same as the symbol rate.
The bandwidth of the carrier at the -12 dB points is approx 1.28 times the symbol rate.
The occupied bandwidth, i.e. spacing between carriers is approx 1.35 times the symbol rate.

For example: Symbol rate=27.5 Msym/s. Bandwidth = -1 dB 20.9 MHz, -2 dB 24.2 MHz, -3 dB 26.25 MHz, -3.8 dB 27.5 MHz, -4 dB 27.7 MHz, -6 dB 30.3 MHz, -12 dB 35 MHz.

in Brief

If the BBC were to move their 2D channels to a FEC of 1/2 then a lot more out of footprint customers would be much happier and could make do with smaller dishes as well.


Kind Regards

Old Satellite

 

[geoffreys]

Just to high light the FEC ( Forward error correction) is very important when you are at, or close to the systems capibility.

The FEC (Forward Error Correction) indicates how many Bytes are used for the actual signal, and how many for correction of errors.

A FEC of 1/2 means that 1 Byte out of 2 is used for error correction, while a ratio of 7/8 means 7 Bytes are used for the actual signal, and only one for error correction.

A FEC of 1/2 gives as perfect as possible reception, since every Byte containing actual signal is controlled by another Byte checking it.

When a provider chooses a FEC of 7/8 it means he is not wasting any bandwidth at the cost of delivering a signal.

The lower amount of error correction means that more sophisticated equipment at the receiving end (for example a more stable and sensitive LNB, or higher reserves with the dish) are needed compared to the same transmission using a FEC of 1/2.

So transmission data rate = customer information rate x 1/ (FEC rate).

FEC rate is typically in the range 1/2 to 7/8 so the transmission data rate is always significantly more than the customer information rate.

This is the actual formula:

SR = Symbol Rate
DR = Data Rate = the customer information rate.
CRv = Viterbi forward error correction (FEC) Code Rate. Eg. 1/2, 2/3, 3/4, 5/6, 7/8
CRrs = Reed Soloman forward error correction (FEC) Code Rate. Eg. 188/204
m = modulation factor (transmission rate bits per symbol). BPSK=1, QPSK=2, 8PSK=3 etc

Formula: SR = DR / (m x CRv x CRrs)
DVB-S Carrier bandwidth
The bandwidth of the carrier at the -3.8 dB points is approx the same as the symbol rate.
The bandwidth of the carrier at the -12 dB points is approx 1.28 times the symbol rate.
The occupied bandwidth, i.e. spacing between carriers is approx 1.35 times the symbol rate.

For example: Symbol rate=27.5 Msym/s. Bandwidth = -1 dB 20.9 MHz, -2 dB 24.2 MHz, -3 dB 26.25 MHz, -3.8 dB 27.5 MHz, -4 dB 27.7 MHz, -6 dB 30.3 MHz, -12 dB 35 MHz.

in Brief

If the BBC were to move their 2D channels to a FEC of 1/2 then a lot more out of footprint customers would be much happier and could make do with smaller dishes as well.


Kind Regards

Old Satellite

Many thanks for that info, most useful. I find it most annoying to know that a (2D) signal is actually there at my receiver's aerial socket but the technology used prevents me seeing the picture - unlike the analogue scenario where I would see it but with a snowy picture.
Geoff
Cyprus