Picture breakup on Freeview reception

[George L]

I have a problem with my Freeview box which I hope some of you guys can help with. I’ve looked through a number of threads on the forum but none seem to explain my particular problem. I’ve included as much info as possible, based on the questions that were asked on the other threads, and hopefully this will help you guys diagnose the problem

Background:
I’ve got 3 TV’s in my house and I get good reception of all 5 main terrestrial channels – including Ch5. I’ve had a Humax F2 FOX T digital TV box for nearly a year (it only feeds the main TV in my lounge) but not used it much. Following the recent purchase of a new widescreen CRT TV I now use it regularly.

The problem:
There is sometimes some picture break-up, usually across the centre in the lower third of the picture, occasionally the picture freezes for a second or two, and there are “cracks” and other noises on the sound when this happens. There are periods when this problem is frequent, then periods where everything is fine. It may be my imagination, but the problem seems to be worse on ITV channels.

I have checked the signal monitoring menu on the box when there have been problems. The signal strength shows 70% (so the signal is not too strong?) and the signal quality shows 100%. The signal quality dips below 70% for a second or less when the interference is happening

Also, I have a couple of general problems with the Humax box. Almost every few days it loses the detailed program information (the display that comes up when you press the “i” button twice) and I have to keep rebooting it to get this info text back. The box also occasionally hangs up and will not respond to any commands and has to be rebooted. This second problem seems to happen when I have been switching channels and calling up program information a lot

My set-up:
My aerial is an “Antiference Extragain UHF Model XG8A” which is “tuned” to channels 21-37 (Red cap) and is Digital Compatible. It is mounted as high up in my loft as possible and has been set with a compass & map to point at the Crystal Palace transmitter which is approx 25 miles from my house. All digital broadcasts from CP are on channels C22 to C34 and all analogue broadcasts from CP are on channels C23 to C37, so my aerial ought to be ideal

I have a Triax Masthead Amplifier (model TMA 24W 20W-U) right next to the aerial and this has a Triax dedicated power supply (IFP 551) sited right next to it. I also have an FM aerial which is connected through the same amplifier (which has a dedicated input for this). The co-ax cable from the loft to the wall plate in my lounge is long but is one continuous run and does not come close to any mains cables other than the power supply to the amplifier. I have a second (“Maxview” variable gain) signal booster in the lounge to help send TV & FM signals around the house.

These amps were set up originally to get the best picture (on analogue channels) with the minimum gain to keep noise levels to a minimum. The co-ax feed from the signal booster goes into the digibox first, then the VCR, then the TV. All the co-ax connections are made with good quality cable which has copper braid and copper foil screening. All the co-ax leads were made by me and have been kept as short as possible.

The digibox is connected directly to the TV via a Scart lead using an Ext socket on the TV recommended for this purpose. All Scart leads used in my set-up are good quality with individually screened wires, and they are all only 1m long.

The 1 year warranty on the Humax box runs out on 1st July, so if there is any possibility that the problem is in the box I would like to take action soon.

Any ideas on what might be causing my problem would be greatly appreciated :0)

George.

[Papua]

You seem to have covered most things, but a few thoughts, is your masthead amplifier delivering too strong a signal to the box and overloading the tuner in the box?

Also a compass is not normally the best way of setting an aerial. Adjust the aerial whilst someone watches the signal strength on the tv, you may have to move it slowly depending on the speed of update of the strength by the digibox. You need to find the centre point for the maximum strength. One way of getting the best signal is to remove the aerial amplifier before performing the above. Again try the box without the aerial amplifier in circuit and see if this helps.

[SamRadford]

You haven't mentioned the gain of the masthead amp, nor the cable length from it to the Freeview receiver. This information is critical. CT100 cable (probably near enough to what you've described) loses about 0.18 dB per metre at 860 MHz. So, for example, 33 metres of cable would lose roughly 6dB of signal.

In this scenario you'd need a 6dB masthead amplifier. If you had, say, a 12 db amp then you've got 6db too much gain and you'd need a 6db attenuator at the aerial input of the Freeview receiver to compensate. Get the idea?

This presupposes that your aerial gain is correct. If it's 3db too high then you'd need a 9dB attenuator. If it's 3dB too little then you'd need a better aerial.

However I'd like to say that the usual reason for intermittent problems like yours is that the aerial is just too close to the house wiring. You can prove this by switching lights on and off upstairs. Fluorescent lights are particularly spectacular at interrupting viewing!

[MikeK]

It may be my imagination, but the problem seems to be worse on ITV channels.


Probably not your imagination - the ITV multiplex uses a transmission system called QAM64 which allows more data to be squeezed onto a multiplex, than the QAM16 used by the BBC (IIRC it's 24Mbps for QAM64 and 18Mbps for QAM16). The tradeoff though is that it's not as robust a system, leaving many people with problems on those multiplexes (IIRC it's 2 multplexes using QAM64), while they have no problems on the others, which use QAM16 (again IIRC, that's the other 4 multiplexes).

ITV seem to have taken the view that people won't mind the reception problems, as long as they get an extra shopping channel or two to watch