UHF interference

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Simba27

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#1
I've recently added a freeview box and a DVD/HDD Recorder to my already complicated setup and my picture quality has degraded. :( I'm looking for some suggestions on troubleshooting it.

Now I have a chain of devices on my UHF cable: My incoming TV aerial cable goes into my freeview box but that only has a pass-thru, so the output of that goes to a SCART-to-UHF modulator driven by the SCART of the freeview box. The output from that modulator goes into a second modulator, driven by the SCART of my sat receiver. The output of the second modulator goes to my VCR. The output from the VCR goes into my DVD/HDD Recorder - but that is also a pass-thru so I've added a third modulator driven by the SCART of the DVD/HDD Recorder. This then goes to a six way booster/splitter and is sent to all the different rooms of the house.

Aerial --> FV --> FV Mod --> Sat Mod --> VCR --> DHR --> DHR Mod --> Splitter --> TVs

(Item's in bold are the new ones)

With this setup I can watch any device anywhere in the house. I have made sure that the modulators for the freeview and DVD/HDD recorders are after their respective pass-thru inputs, so no device is feeding back into itself. I don't know why, but I had a feeling that might be a bad thing.

The ordinary TV channels are fine, but the modulated channels show interference - typically thin evenly spaced horizontal lines scrolling up the screen or just general fuzziness. I'm thinking that maybe this is coming from the power-supply transformers in the mods - but I don't have any real electronics knowledge to back that up. They do hum a bit. Also, I'm getting quite a shock every time I inadvertantly touch the centre pin of the UHF connectors. :eek: Can't be good.

My plan so far is to replace the overly long and thin UHF leads with satellite cable cut to the right lengths to try and cut down on external interference, but that won't help if the interference is being caused by the devices themselves.

I've also been trying different frequencies on the mods because some are clearer than others.

Anybody got any ideas?
 
rolfw

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#2
An easy test to see what the problem is, is to pull out the aerial lead from the first device, if all of your pictures clear up, then you need to look harder for clear RF channels for the devices, or perhaps insert a band pass filter to clear up spurious off air signals.
 
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handyman 44

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#3
hi. i here the word spuious quite a bit what do it mean. ie rolfw said insert a band pass filter to clear up spurious off air signals. what are spurious off air signals and what probs do they cause .
 
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#4
handyman 44 said:
hi. i here the word spuious quite a bit what do it mean. ie rolfw said insert a band pass filter to clear up spurious off air signals. what are spurious off air signals and what probs do they cause .
Interference on your TV picture
 
rolfw

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#5
LOL, they are the ones you don't intend to receive, so normally out of band and yes, they cause interference. If you amplify a tv signal, then it will also amplify the fringe reception channels, sometimes to such a degree that they are almost watchable.
 
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spiney

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#6
1) yes, you must use sat cable, standard uhf isn't sufficiently screened.

2) that's 5 UHF amplifiers in the "chain", rather a lot. there's probably problems on all channels but you don't see them!

3) spurious products arise from non-linearity, ie amplifiers will produce out-of-band harmonics due to distortion, this effect is cumulative with increasing numbers of chained amplifiers.

4) as Rolf says, bypass each device in turn, to see if that's the problem.

5) You can try hunting for "better" UHF channles.

6) If the problem won't go away, then before feeding into dstribution amp, replace the cascaded amps with a multi-way UHF signal splitter used in reverse, eg: http://cpc.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=AP00307&N=411 .

(make sure it's wideband for UHF frequencies, not just sat ones!).

(this should reduce the general "patterning").

7) If problem partly disappears on disconnecting roof aerial, then you could try a UHF bandpass filter, eg:

http://cpc.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=AP01220&N=411 .

PS, for somebody in Uxbridge, the Grandata shop opp Wembley Stadium sells these over the shop counter, cheaper than cpc! There's also Henrys Radio, Edgeware Road, near the Bakerloo line underground station.
 
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Simba27

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#7
Thanks for the replies. I just tried pulling the first aerial cable and it did help quite a bit, but not completely.

Having played with it a little more, I think that some of the distortion is coming from the loop output on the freeview box.

I think I understand what you are saying about the UHF splitter in reverse. See my attached diagram. Is that what you mean?

I have one of these lying around it's a "Viewsonics Platinum Plus VSP2WS-P 3-way splitter 5MHz-1GHz". It has a connector labelled "in", a connector labelled "-3.5dB" and two connectors labelled "-7dB". Is it suitable for the task?
 

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spiney

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#8
to simba 27 ....

1) yes, that type of splitter is suitable, being a 3-way splitter with power pass on 1 port (but that's irrelevant here!).

However, you'll have to replace the UHF push connectors with f types. Before doing so, since removing the aerial makes some difference, you might want to first try a UHF bandpass filter between aerial and f/v box. If that "does the trick", then you wouldn't have to do anything else!

2) Do not split the aerial output prior to Freeview receiver input, also there's no need to include the freeview passthrough output, unless you specially want to get the analogue tv channels as well (but for what reason ?).

3) I'd try:

f/v mod - splitter output port 1.

dvd mod - port 2.

sat mod - port 3.

splitter input - video aerial socket.

video output - multiway distribution amplifier.

4) If you still want the analogue tv channels - in addition to 4 modulators - then this should cut down the patterning but you'll have to be careful about which UHF channels to re-use! You can re-use the freeview channels, but stay away from the analogue ones, and space the re-used channels at least 2 channel numbers apart (since domestic modulators are double sideband!).
 
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Simba27

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#9
Thanks Spiney.

I need the analogue channels too - otherwise everyone in the house can only watch the single digital channel that the freeview box is decoding.

Some of the interference seems to be coming from the passthrough of the freeview box. So I'm planning to split before it and feed the clean aerial signal into the reversed 3-way splitter rather than using the passthrough.

The DVD is a HDD/DVD recorder. I want it to be the last item before the distribution amp so that it can record from TV, FV, SAT and VCR.

I'll be sure to keep the channels well apart as you've suggested. Will let you know how I get on.

Edit: Maybe I should have posted this in the Multi-receiver/TV section. I originally thought that the noise was due to the transformers in the modulators and that the solution would involve capacitors and a soldering iron.
 
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Simba27

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#10
Well. I connected everything up like in my diagram, with the exception of the bandpass filter. I know I've got one somewhere but I can't find it at present - will have to buy a new one.

The result is *much* improved. There is still some slight distortion but all channels are watchable. Channel 5 is worse than the others.

Thanks everyone.
 
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spiney

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#11
Good!

I'd still recommend adding a UHF tv bandass filter, only small cost, should improve things even further!

Note you can add remote control from elsewhere in house using the tv link system, grandata sell a complete kit. Go www.grandata.co.uk , catalogue, audio video accessories, it's 1st item that page. Then get as many slx links as you want, and original remotes will work from anywhere in house.

Note, for this to work, the distribution amplifier must be a Sky link passthrough type, if yours isn't then you'll need to replace it with an SLX type, also from grandata.

(see: www.tvlink.co.uk/tvlinkplus.htm ).
 
jeallen01

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#12
Look here
http://www.maxview.ltd.uk/

under Trade Products -> Signal Filters

Quite "sharp" responses:
Used one of the MHF2137 to cut out what I think was Tetra interference from just below 470MHz, and it worked well - others might work to cure different problems.

Note: These are outdoor masthead filters, but mine is fitted to the back of the HiFi & TV rack, at the end of the downlead from the loft distribution amp.

Maxview have (or at least had) no problem selling direct to private buyers.

John A
 
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#13
If I were the OP, I would try to remove as many RF cables as possible, since only the Freeview box, VCR (and maybe the TV) require them , and connect the rest via other methods - SCARTS and S-VHS cables as required.

An attenuator on the incoming aerial lead may be required, especially if theres a masthead amp fitted to boost signals, and keep the chain of signal as far away from the remaining equipment as possible.

Physically, space all boxes away from each other using corks to prevent internal interference from the PSUs from giving harmonics, and a good mains supply extension with suppressors (anti spike) built in to help cut down on any ground interference.
 
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Simba27

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#14
Channel Hopper said:
If I were the OP, I would try to remove as many RF cables as possible, since only the Freeview box, VCR (and maybe the TV) require them , and connect the rest via other methods - SCARTS and S-VHS cables as required.

An attenuator on the incoming aerial lead may be required, especially if theres a masthead amp fitted to boost signals, and keep the chain of signal as far away from the remaining equipment as possible.

Physically, space all boxes away from each other using corks to prevent internal interference from the PSUs from giving harmonics, and a good mains supply extension with suppressors (anti spike) built in to help cut down on any ground interference.
But the whole point of the exercise is to get all the devices using RF so I can distribute them around the house easily. I added a bandpass filter, but that made the picture worse, so I took it out again.

I think that most of the interference was being introduced by the freeview box and then amplified by the all the modulators in series. Rewiring as suggested above (thanks again) solved that. Actually, it works really well (so I've stopped messing with it for now). My remaining issue (grainy picture on Ch5) is down to signal strength I think. Probably I am losing at little at each connection.

I plan to remake the RF cables with some good quality plugs. I useds plugs from B&Q which are so cheap and nasty that the little screw which clamps the centre wire strips its thread as soon as you try to tighten it. Plus, the head of the screw shorts against the body of the plug unless you put tape over it. B&Q charge over a pound for this crap.

I'll do what you suggest with regard to the mains suppressor. If better plugs improve the signal strength I'll have another go with the filter.
 
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#15
It'd be preferable to do a good job on the RF plugs, i.e solder the inner lead to make sure of a good connection -

_www.megalithia.com/elect/bellinglee/

Llew
 
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spiney

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#16
Replacing the plug isn't going to make much difference!

If you suspect the Freeview box itself is producing interference, then try a UHF bandpass filter coming out of the UHF output of that!

UHF filters with/without Tetra bandstop are quite cheap, in London easily available quite a few places, eg Grandata shop opp Wembley Stadium (at "Popin center"), and Henry's Radio Edgeware road, near tube station.
 
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Simba27

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#17
spiney said:
Replacing the plug isn't going to make much difference!

If you suspect the Freeview box itself is producing interference, then try a UHF bandpass filter coming out of the UHF output of that!

UHF filters with/without Tetra bandstop are quite cheap, in London easily available quite a few places, eg Grandata shop opp Wembley Stadium (at "Popin center"), and Henry's Radio Edgeware road, near tube station.
I'm no longer using the UHF loop through of the freeview box so it's no longer an issue. (see above diagram)
 
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#18
Simba27 said:
But the whole point of the exercise is to get all the devices using RF so I can distribute them around the house easily.
The video recorder would be used to distribute any of the SCART wired items round the house, Freeview and the analogue signals would of course pass round naturally.
 
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