DVB-T signal problem

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JefWright

85 posts

Master Geek

#87775 6-Aug-2011 20:05

Hi.

Location:
Installation in Totara Vale (between Glenfield and Sunnynook on Auckland's North Shore). No line-of-sight to Sky tower, Pine hill or Waiatarua. Neighbour's UHF aerials are split between Pine Hill & Waiatarua.

Hardware:
Single 91 element UHF aerial from Clayworth Electronics with F connector mounted 3m above tile roof on barge board hockey stick mast. 12 metre run of double screen RG6 directly to RF port on Magic TV MTV3600 PVR. RCA cables to an elderly Loewe CRT TV.

Signal diagnostic:
PVR channel diagnostic shows very similar signal levels for Pine Hill or Waiatarua (when aerial re-oriented). 538MHz 79%, 570MHz 78% and 666MHz 80% signal - all 100% quality.

Symptoms:
Most of the time all freeview channels have perfect sound & picture but sometimes the audio has a short "drop-out" and the video a burst of pixelation with some picture elements being disrupted completely for several seconds. When several of these events happen in rapid succession for a prolonged period of time it becomes impossible to follow the program. Sometimes a recorded program will show this damage and be unable to play beyond that point (continually skipping back).

The problem is very intermittent. That's good but it makes debugging very difficult!

The severity and frequency of this problem didn't seem to change when I changed from Pine Hill to Waiatarua. This seems to be a distinctly "digital" problem so I'm not suspecting my TV as a cause.

So far:
Tech support for the MTV3600 PVR says this signal strength should be easily adequate so I'm putting my signal quality problems down to intermittent interference from an external source. I guess this could be in-band or out-of-band spillover.

I have obtained a Kingray MDU20U (20dB UHF-only masthead distribution amplifier with 520MHz High Pass filter). I wanted to check this out before climbing on the roof so I installed it at the PVR instead of on the mast. I was surprised to see the PVR signal strength only increase by (perhaps) 1% with the amplifier in place. Turning off the amplifier power cut signal completely so it is working; it just doesn't seem to be amplifying much. It doesn't seem to me that adding this device is likely to change my problem much.

Next:
I could obtain another aerial but I don't seem to have many meaningful alternative options.
I could move my aerial nearer to my TV.
I could replace the (perhaps 5 year old) 12m run of twin screen RG6 with new quad screen RG6.
I could beg, buy, borrow or steal another STB or digital TV and make sure the problem isn't inside my PVR.

Am I missing anything obvious?

B1GGLZ

1961 posts

Uber Geek

#502844 6-Aug-2011 22:19

You seem to have done everything right so far.
It sounds like either a PVR fault or some external interference which is going to be very hard to track down.
Initially I would suggest trying another STB or a Freeview capable TV.
I'm in an area with 100 + 100 signal (Sony ex720) and still get the rarel momentary pixellation from Waiatarua but not very often anad not like your symptons. I suspect it's in the transmission although I have trees very close to the antenna.
With the figures you quote I doubt the amp is necessary.
Does it do it in calm conditions or when wet and/or windy?
Any obstructions near the aerial?

JefWright

85 posts

Master Geek

#502873 7-Aug-2011 01:59

B1GGLZ:
Does it do it in calm conditions or when wet and/or windy?
Any obstructions near the aerial?

Thanks for the advice. There's a good chance I'll upgrade the TV soon so that will give me a second Freeview receiver to compare.

Am I better to use the RF feed-through on the PVR or add a splitter? I've heard adverse comment about the quality of signal on feed-through connections but I can't see how dropping 4dB on both feeds with a passive splitter is going to help much either.

I initially wondered if my interference was caused by distant lightning (stormy conditions). That was before I removed my VHF aerial & diplexer. I had a much weaker signal then than now and the receiver may have had quite a bit of out of band signal to reject. I am sure I also see the problem in calm, dry conditions now.

I've got a neighbour's pohutukawa near my sight line to Pine Hill and the Sunset Road ridge at some distance beyond that. The tree won't be blocking signal per se but it may provide some multi-path (can trees do that?) within the aerial's primary lobe. I'm on the eastern flank of the Target road ridge which blocks my line of sight to Waiatarua and a neighbour's 2-story house with a corrugated iron roof could easily be providing multi-path in that direction.

Cheers,
Jef.

andrewcnz

961 posts

Ultimate Geek

#502880 7-Aug-2011 07:56

With the MTV3600 PVR i would use a splitter rather than it's RF feed-through.

B1GGLZ

1961 posts

Uber Geek

#502884 7-Aug-2011 08:38

OK
Sounds like nothing in the way of nearby obstructions likely to be causing the problem. That leaves just a weak tuner unit in the PVR (STB's and PVR's not noted for their high quality components) or I was wondering about electrical switching interference. Some heat pumps, refrigerators etc, can cause problems with rf signals. Touch lamps are a no-no and some energy efficient bulbs can be nasty. The well knon branded TVs and Recorders with Freeview seem to have much better and more sensitive tuners in them.
I would try the rf feedthrough first before a splitter although both will introduce some loss in the signal path.
I would seriously consider upgrading the TV and perhaps go for a Freeview Certified recorder ready for next year when Analogue disappears.
I recently upgraded to a Sony ex720 and Panasonic BW880 Blu Ray Freeview Recorder and am rapt with both. Good discount too at JB. I see Samsung have also released a 3D Blu Ray recorder now and Panasonic have a new 3D model just out and prices are still dropping.

JefWright

85 posts

Master Geek

#502952 7-Aug-2011 11:56

B1GGLZ: OK
That leaves just a weak tuner unit in the PVR (STB's and PVR's not noted for their high quality components). The well known branded TVs and Recorders with Freeview seem to have much better and more sensitive tuners in them.
I would try the rf feedthrough first before a splitter although both will introduce some loss in the signal path.

andrewcnz: With the MTV3600 PVR i would use a splitter rather than it's RF feed-through.

What I take from this advice is that products in general from more established manufacturers (Sony, Panasonic, Samsung) tend to have better RF performance than products from relatively newer entrants to the consumer RF business.

B1GGLZ: OK
I was wondering about electrical switching interference. Some heat pumps, refrigerators etc, can cause problems with rf signals. Touch lamps are a no-no and some energy efficient bulbs can be nasty.

Thanks for this tip. I've methodically been through most of my electrical equipment but not the energy efficient bulbs. I can get onto that today.

JefWright

85 posts

Master Geek

#506953 15-Aug-2011 18:40

OK.

My energy-efficient bulbs seem to be vindicated. I appear to be on top of the problem (without really proving its source).

It seems like I was getting some out-of-band (most likely VHF) signal break-through. Since I had it on hand, I installed a Kingray Masthead Distribution Amplifier (MDA20U) with a built in High Pass, 520MHz filter. The PVR diagnostic didn't show an appreciable increase in signal strength or quality but I'm not getting the interference any more.

Thanks for all the advice.