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Baffled by science...

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Katie Bush
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I don't pretend to know, or understand, digital television, but I've often been wondering about what all this means..

Can anyone explain what all those fields mean, and what that means in practical tems?

Marion

 
Posted : 07/01/2013 10:22 pm
Anonymous
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To be pedantic I think 8K is 6700 carriers. Thus each is modulated 6700 slower and uses about 1/6700th of the 8MHz channel (after guardbands)

FFT = Fast Fourier Transform. 8K means I think the number of FFT run in parallel DVB-T2 allows 32K mode There is a 2K mode too

FEC = Forward Error Correction. The data is processed and increased in amount. Thus some of the signal can be lost due to the redundancy in the transmission.
1/2 means LOADs
7/8 means very little, so massive aerial/dish or more power or less range or tiny speed per bandwidth.
3/4 and 5/6 common on Satellite.

 
Posted : 07/01/2013 10:35 pm
Katie Bush
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Owe - Kaye....

So, what do the initials FFT, and QAM, stand for? I mean FFT could mean "Flawed and Faulty Transmission" :O QAM could mean "Quaint Amplitude Modulation" :=D :O

Obviously not, but the initials are meaning less to me unless I know what they stand for, and then what that means in a practical sense?

Marion

>> EDIT <<
Ah, Michael, you've got back with part of what I was looking for in this post - Crossed in post :omg:

Marion

>> ADDTIONALLY EDITTED EDIT << :O

I should have mentioned, this was a screenshot of my Icecrypt T5000 DVB-T receiver's "Programme information" panel.. It seems a bit pointless providing all that info, if the person reading it has no idea of what it conveys, and there must a lot more people out there who have even less of idea than I.

Marion

 
Posted : 07/01/2013 10:38 pm
Anonymous
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You have AM Level changes
The mimimal FM deviation is Phase Modulation less than 180 degrees.

So imagine you can have various steps amplitudes and phases. The more steps the smaller the "hamming distance" and the less noise immunity, but you can send more data at each point in time.

QPSK is Quad Phase Shift Keying. 2 bits at a time can be sent.
QAM is Quadrature Amplitude Modulation, changing phase and Amplitude

Morse is OOK = On Off keying, only two levels of amplitude, none or all. Also called ASK = Amplitude Shift Keying.

16 QAM is 16 possible combinations of Phase and Amplitude. 4 bits of data can be sent for each "symbol".
256 QAM is used on cable, allows 8 bits to be sent at once, so less noise immune.

The bandwidth is related to the rate you change the symbol you send (mix of phase and level). The Symbol Rate or SR
Satellite can use 45M symbols per second which takes nearly 50MHz.

 
Posted : 07/01/2013 10:53 pm
Anonymous
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FSK is Frequency Shift Keying as used in Radio Teleprinters and 300 Baud Modems. Horribly inefficient,

14.4K 28K, 33k and 56K modems are a mix of data compression and QAM to squeeze as much out of 3KHz bandwidth line as possible. Using FSK the limit would be about 750 bps to 1200 bps.

So "Analogue" dialup is in one sense Digital and Digital TV is one sense Analogue, as the digital data is packed into quantised Analogue patterns the receiver looks for.

You can see that distortion or overloading can be as fatal to a "Digital" coded signal as noise in the channel as symbol patterns are interpreted as the wrong ones. This is why pre-amplifiers, modulators, band shifters, power amplifiers all have to be MUCH better quality for Digital or you are reducing the margin of error in decoding and thus noise has more effect or even with no noise it doesn't work!

 
Posted : 07/01/2013 11:05 pm
Anonymous
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DTT uses COFDM, (Coded Orthagonal Frequency Division Multiplexing), i.e. the 6,700 carriers = 8 K FFT. This means multipath and single or narrow frequency interference has less effect. But the Amplifiers and Mixers have to be better than Satellite and Cable which use single carriers per channel.

There are capacity limits for any given medium, depending on power, aerial/dish size and bandwidth used
http://www.techtir.ie/comms/signal-loss-limits

There a physical & mathematical limits that can be exceeded. Most of what is claimed for 4G is fantasy
http://www.techtir.ie/comms/signal-loss-limits

 
Posted : 07/01/2013 11:09 pm
Terrykc
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Marion, having gone to all the trouble of displaying all that information it is a pity that SNR and BER aren't included ...

The receiver probably doesn't have the capability to measure the SNR (Signal to Noise Ratio) but should be able to calculate the BER (Bit Error Rate) which is a more meaningful figure than that Signal Quality bar graph.

The Signal Intensity bar graph is a waste of space as it doesn't tell you what 100% equates to! A calculated figure would be of much more use - particularly in identifying situations where the level is excessive. This is a common problem in strong signal areas with the power increase at DSO, causing receiver overloading and consequent poor performance but the level graph shows 100%, leading the user to believe that it is the receiver rather than the aerial system that is at fault.

Writing that has made me think of something - all those displays in all sort of shops, from pound shops upwards, of coax flyleads, outlet boxes, two-way splitters, coax plugs and couplers, etc. Has anybody ever seen an attenuator on one of these displays ...?

I wonder how many people buy 'booster' amplifiers in such situations and are baffled when their problems get worse ...?

When all else fails, read the instructions

 
Posted : 08/01/2013 11:00 am
sideband
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I wonder how many people buy 'booster' amplifiers in such situations and are baffled when their problems get worse ...?

Don't know about now but it was common in the early days. Back in 1999, I was involved in sorting out local issues with digital reception on the (then) new Philips OnDigital boxes. One customer had gone to great expense with an aerial 'expert' and splashed out on a specialy designed 'digital aerial' only to find that at best, he could receive 6 channels and they were the weaker ones..... The rest were blocking, freezing or just not available. He had a 12 element aerial and was only about six miles from Crystal Palace. Of course it was the Philips box at fault......!

When I checked the signal level, it was so far over the top I couldn't understand how so much signal was finding its way down the aerial. He then mentioned a mast-head amplifier......! We went up to the aerial in the loft and disconnected the amplifier which brought the signals down to a more reasonable level but still too much. We then went back down to the set and I fitted a 12dB attenuator which then brought in all channels on digital and also gave noise-free reception of analogue. I handed him back the mast-head amplifier and told him to go and get his money back!

Rich

 
Posted : 08/01/2013 11:53 am
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