Faulty Preamp PSU Caused Humax Reception Problems on Some Muxes

Newcoppiceman said:
Selecting the same service (Talking Pictures TV) on the TV (ie changing input from HDMI1 to Antenna) produces problem-free pictures and sound.
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Comparing the TV with the HDR-FOX in terms of ability to receive a poor signal is pointless, because we don't have any specifications for the HDR-FOX and I doubt you have any for your TV.

However: What have you done to minimise interference between the outgoing HDMI and the incoming signal? Does the PQ improve if you touch the aerial plug?

Try using the V-FORMAT button to switch HDMI output to 576i and see if that changes anything. If it does, you might want to improve the separation of the HDMI and aerial leads, and/or use higher quality screened cables. Changing your customary format from 1080p to 1080i might also solve the issue without apparent degradation of the picture.
 
Black Hole said:
Comparing the TV with the HDR-FOX in terms of ability to receive a poor signal is pointless

Does the PQ improve if you touch the aerial plug?

Try using the V-FORMAT button to switch HDMI output to 576i and see if that changes anything. If it does, you might want to improve the separation of the HDMI and aerial leads, and/or use higher quality screened cables. Changing your customary format from 1080p to 1080i might also solve the issue without apparent degradation of the picture.
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That the HDR-Fox T2 had been working fine for years and that two TVs on the same RF feed seemed untroubled suggested (but didn't prove) that the problem lay with the PVR; the alternative, ie that the signal had degraded sufficiently to seriously trouble the PVR but neither TV, seemed less likely - although of course possible. I'm not aware that the PVR is particularly fussy about the signal (in a way that the TVs obviously aren't - the lounge TV, an LG32LE5300, is of a similar vintage to the PVR; we're changing it soon).

Some interesting ideas there around possible HDMI:downlead interference. However...

We may have to change the title of this thread as the spare PVR is behaving the same, which suggests a problem with the RF feed after all. The Quality bars on the Manual Search screens for each mux (normally 100%) were variously 20/30/40% other than the T2 HD mux which was at 100%. Curiously, the Strength bars were all around their normal readings - 70%, other than the weak mux on UHF35 which was at about 50%.

The aerial is 25+ years old and I suspect has deteriorated to the extent that with the onset of autumn rain is getting inside the housing where the downlead attaches to the balun/dipole. It could of course be the preamp or its PSU that's at fault so I will firstly check that +12V is reaching the preamp, then get a new DVB-T/T2 meter (likely a Satlink) to measure levels - unless someone near Woking can lend me one!
 
Did you check the transmitter information for Crystal Palace which currently says "Pixelation or flickering on some or all channels"?
 
MartinLiddle said:
Did you check the transmitter information for Crystal Palace which currently says "Pixelation or flickering on some or all channels"?
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No. That's probably weather-related co-channel interference, which is what I put the problem down to when it first occurred on the morning of 9 October - has been problematic since then which can't surely be down to CCI. In 10+ years we've only had one previous instance of CCI which I logged thus: High pressure weather 21 Sep 2020 caused obliteration of channels 23, 25 and 28 with strength/quality readings of 60 or 70/10 while other channels unaffected.
 
Newcoppiceman said:
Trouble is, there's about 20dB difference in received levels between the weakest and strongest muxes, so I figured I'd risk overdriving the Humax rather than allow the weakest mux to drop below the "digital window" (45-70dBuV) which is what would happen with a 10dB preamp. Over the years that's proved to have been the right call, with rock-steady reception on all multiplexes (which is the object of the exercise, after all).

Yes 7dB distribution losses: eg PSB2: 58+25-76=7dB
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There is no point amplifying more than the downstream losses. If the signal isn't there off the aerial, then you aren't going to get it back by amplification.
It's the C/N ratio that's more important than the actual signal level. Amps just boost the input noise as much as the input signal, as well as adding their own, hence worsening the C/N ratio overall. I'm not entirely sure I need to tell you this though.
Given the above picture, a bypass-the-amp test would seem to be a worthwhile thing to do, just to get a feel for some other numbers and what happens.
Newcoppiceman said:
The aerial is 25+ years old and I suspect has deteriorated
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This is what happened to mine after 15 (I still keep meaning to post the pictures). I suspect your excess gain may be clouding your judgement, but I would think this is the root of the problem. You don't of course know how close your TVs are to the threshold of not working without some testing.
 
prpr said:
There is no point amplifying more than the downstream losses. If the signal isn't there off the aerial, then you aren't going to get it back by amplification.
It's the C/N ratio that's more important than the actual signal level. Amps just boost the input noise as much as the input signal, as well as adding their own, hence worsening the C/N ratio overall. I'm not entirely sure I need to tell you this though.
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All the above rings true from the extensive research I undertook in the mid-2000s when struggling to get the (then) weak muxes which were side-by-side with the analogue channels. I consulted CAI, DTG and manufacturers' publications and borrowed a Promax from work several times to measure levels. I still have the tables of readings, plots, etc.

Maybe I focussed too much on that "digital window" of 45-70dBuV which was oft-cited as the range within which signals should be presented to set-top boxes; you're implying that with UHF35 down at 40dBuV (with a 10dB preamp) the Humax would still work reliably - and maybe you're right as the C/N will be better. I need to check my old files but I think the current setup is a legacy from those mixed-working days as I don't think I made any changes post analogue switch-off.
prpr said:
Given the above picture, a bypass-the-amp test would seem to be a worthwhile thing to do, just to get a feel for some other numbers and what happens.
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Easier said than done as the preamp connections are clamp-and-screw type, unfortunately.
prpr said:
This is what happened to mine after 15 (I still keep meaning to post the pictures). I suspect your excess gain may be clouding your judgement, but I would think this is the root of the problem. You don't of course know how close your TVs are to the threshold of not working without some testing.
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Once I've fixed the current problem I'll see how the weakest mux behaves with a 12dB in-line attenuator at the input to the Humax which should closely mimic the effect of using a 10dB preamp, albeit without improving the current C/N.
 
Newcoppiceman said:
Maybe I focussed too much
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MartinLiddle said:
It comes from a table headed "Planned engineering works".
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I spent a lot of time and effort trying to fix my problem, but since the engineering work finished (which was several weeks) the problem has gone away.

That said, I don't know if our TV was affected too as we don't watch live TV as a rule.

Whatever, I'd be wary of doing too much to your system while there is a known issue at the transmitter end.
 
MartinLiddle said:
It comes from a table headed "Planned engineering works".
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MikeSh said:
I spent a lot of time and effort trying to fix my problem, but since the engineering work finished (which was several weeks) the problem has gone away.

Whatever, I'd be wary of doing too much to your system while there is a known issue at the transmitter end.
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Might have been too quick to accuse the RF feed after replacing the PVR yesterday morning if the pic in my post #20, taken at 05:51, was taken while ERP or radiation pattern was affected by those planned engineering works at CP. Heavy rain last night, but no break-up on replacement PVR and Talking Pictures TV ok this morning. That said, there were a few early mornings when TPTV was ok on the original PVR (there was also break-up some early-evenings). Will investigate if there are some major works at CP (which would have started 9 October).

Replacement (in-service) PVR is on TV's HDMI1 and original PVR (now with 1.03.12 custom firmware) on HDMI2 for easy comparison over the next week or two.

I've also ordered a https://www.emitor.se/index_htm_files/Digiair_Pro_T_T2_C_ver1_ENG.pdf - not cheap but I got a very good price when I sold my previous meter ...to someone in Australia.
 
Black Hole said:
Ooh, shiny!!

Sledgehammer to crack a nut? Not criticising, I like shiny things, but do you think you'll get the value out of it??
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The previous Maxpeak TAM DVB-T meter (didn't decode -T2) had a lot of use around digital switchover, not much thereafter it's true to say. So yes, a bit of an indulgence, but invaluable when trying to troubleshoot reception issues and a reassuring presence in the household. (I just think of all the money I haven't spent on cars over the years!)

On p4 of the manual link I provided there's a figure showing a reading for C/N underlining that this is a (the?) key metric.
 
Not being an RF engineer I wasn't making the distinction between signal and carrier.

For non-RF engineers, I think the easiest way to understand it is that RF generally has a signal modulated onto a carrier frequency, so although there might be a S/N ratio post-demodulation, pre-demodulation it's a C/N ratio.
 
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