UK DTT: Change to 16 QAM today

DMC

Hi all,

On Digital Spy, you may have seen that the transmitters broadcasting DTT are in the process of being changed to 16QAM, which is aimed at strengthening the signal on the Digital Terrestrial network. At present MUX1 is getting the kick, which, in some cases is boosting the signal by upto 20%.

So can our resident DTT expert, the artist formerly known as Mad let us know what is occurring from Preseli?

DMC.

Winging my way down to that box now...I may have to bring it next door to see tho, as my brother, not a great tv watcher hasn't gotten round to doing his aerial yet:(
Will post tonight or tomorrow whats happening:)
mm

Well I had to pop next door:D
Unfortunately, Presely is so poor today, that analog channels are quite snowy and this is a good reception area.
With a storm system coming in tomorrow, I don't reckon there will be an improvement before mid week.
Result : Red dot:eek: nothing at all not even channel five.

Btw is that signal improvement going to be visible on my box, as it is an old Itv digital box as opposed to the newer fta boxes??
perhaps ijc can give a clue on that as he is using one?
mm

DMC

You may need to download the channel lineup again, man, and check only the Mux where BBC 1 and BBC 2 are.

Yeah, I only thought of that when I got back.
Will wait now untill the storm has passed.
On a positive note i've persuaded my brother to put up twin widebands, and considering the location thats going to be very interesting.
mm

ijc

Hi Man (so you're not mad anymore then?)
Sorry for the lack of response so far, I haven't been using the net much for the last week. (I'm getting really sick of waiting for fairly priced broadband to be available in Ireland - but that's a rant for a different forum, I know). Anyway, I last tested the FTA box last weekend . The weather was pretty bad then too, and although the analog channels weren't too bad, I got nothing from the digital receiver all weekend.

There are two bar indicators for reception on my Grundig box, one for signal strength, and one for bit error. The signal strength bar has 10 blocks I think, and goes from red to yellow to green after about 5 or 6 blocks. The best I saw it at all weekend was two red blocks. The bit error (I would have thought that zero would be good to indicate no errors, but positive numbers above 4 seem to be enough to have a consistent picture when the signal strength is at about 6 green blocks) was 2 or 3 maybe. I couldn't even get channel 5, which does seem to be on a multiplex that is the easiest to pick up. Someone else in this forum noticed that the lower the channel number, the better the signal strength seems to be for that mux.

Well, I haven't tried it since then, but next time I get a signal I'll try retuning it. Is it the case now that the Presley transmitter has been upgraded? One other question - sorry to ask what must be something pretty basic, but I don't know what QAM is. Can someone explain what that is?
Cheers,
Ian.

DMC

Originally posted by ijc
One other question - sorry to ask what must be something pretty basic, but I don't know what QAM is. Can someone explain what that is?

Here we go. (DMC puts on Open University bald head, horn-rimmed glasses and beard......)

QAM is a technique of encoding the digital signal, at its basic form. ITV Digital used 64 QAM, which encodes 6 bits of digital data per piece of picture (and there are 64 different states for each per piece of picture). With 16 QAM, it encodes just 4 bits of digital data per per piece of picture (down to 16 different states). As 16 QAM uses far fewer states per piece of picture they are far more different from each other - and so easier to differentiate in a poor quality signal area - meaning fewer errors on screen, so the quality increases.

However the loss of 1/3 of the data (down from 6 bits to 4) carried per piece of picture means that the total amount of data carried in each multiplex is reduced from 24Mbs to 18Mbs (1/3rd). This means that only 4 video streams, or 4 channels can realistically be carried per stream, rather than the current 5 or 6, which IMO, was the flaw of ITV Digital. Too many channels per MUX. Now, with the reduced number of channels, the picture quality (if all MUX's move to 16 QAM) will improve dramatically.

Playaway will be along shortly.....

watty

You probabily don't want the more technical explaination about QAM.

Of course the other options to increase S/N and reduce suseptibility to Interference is:
To increase Transmitter power.
Amount of FEC (Forward Error Correction).
Decrease bit rate but keep bandwidth & power the same.
Increase bandwidth at same data rate, and power /bw density.

Reducing QAM doesn't actually help S/N as much as reducing suseptibility to Interference. Which isn't the same thing as noise.


Increasing transmitter power or Receiver Aerial size to capture more signal are the only things that actually reduce non-Interference noise in the system.

Personally I'd prefer 4 times the transmitter power, becuase for Irish DX reception the channel white noise is a bigger problem than local interference (Assuming you have big mast, high gain aerial array well away from ground, *VERY LOW NOISE* mast head amp and Satellite grade double screened cable).

The QAM reduction will help people most in urban / city areas with poorly shielded coax and poor orignal UHF analog aerial installs, maybe in attic.

If I'm watching 2nd Digibox in Kitchen via RF to TV in lounge and a motorcycle goes past we see the interference on the Analog feed that is only 8ft long! It would freeze a DTT signal.

The Satellite feeds arn't affected because:
1) Although the lower channels are same frequencies on Cable as DTT, the signal level is maybe 100 times higher amplified by LNB.
2) They use hi-grade double screened cable buried in pipe in graden from shed roof to house.
3) The Satellite signals feeding the LNB are 20 times higher in frequency (less interference) and the dish picks up 400 times less ground interference than an Aerial or a Dish would at DTT frequencies.

This is why ROI DMMDS is a better system than UK DTT. The higher frequency and LNB on the mini-mesh MMDS dish ensure greater interfernce immunity.

So bottom line is I expect QAM change to be not very significant to "good" Irish installations for UK DTT.. I could be wrong

watty

It would also be more interesting to live on top of "Keepers Hill" rather than low lying hump west of Limerick (Though I used to get Spanish TV on VHF analog), for any UK DTT experiment.

DMC

watty, the bones of it is, its picture quality that improves with the change in QAM, not the actual strength of the reception. Thats still down to the ERP of the transmitter. I was putting it in lay man terms from a lay man. KISS

Where are you located, watty?

Still, I'll be trying a friends DTT box from 3 or 4 different locations at the weekend in North Meath/South Cavan, and see if it makes a difference.

watty

Patrickswell, Limerick.
Originally Co. Antrim 20 years ago.

In my usual rambling way I was suggesting that picture quality is improved the most for those with intermittant quality due to traffic interference. Which will be dramatic.

I suspect improvement of range (basic S/N limited), i.e. Irish DX, Picture Quality will be less dramatic or little as the BER threshold for white noise is not a whole lot different on 64QAM and 16QAM, though it is higher for 16 QAM. Id have to get big thick book of shelf and hurt head with sums to work out the BER S/N threshold change.

Are they changing FEC?
My book on digital comms doesn't quantify pros and cons of changing FEC rates to changing QAM levels.

watty

Also I have ATV licence. Tests are 1295MHz FM PAL video, and FM sound on 6.0 and 6.5MHz sub carriers using 500mW on 21 Element yagi aimed at Limerick City.

No special Licence required for reception. You can connect a 1295MHz aerial via an isolating capacitor direct to an Analog Satellite Receiver to receive, or better, via an LNA on the aerial (a bit like a mast head pre-amp), which can be powered by the H/V signal on the Satellite Receiver coax (Though some LNA don't like 18V, the lower 12/14V setting polarity is OK).

At present the mast is down but if weather good test transmissions will recommence this weekend.

I hope to also investigate 2.4GHz and 10GHz and even possibly QAM based digital MPEG transmission encoded by a DSP chip.

NorthDown

Up here in NorthDown I'm happily getting DTT from all Muxs - including C but I have a group a and b aerials diplexed for BBC Scot / NI for it. Two other boxes here and one in Moira can;t load Mux C from Divis. Moira can get channel names but no good signal, here they don;t even register on an outdoor aerial.

If only Cambret Hill would go DTT across in Scotland for digital BBC Scot (and the joys of one hour of TeleG a day!!! :-) )