UHF interference on old TV

[Hampus]

Hi,

I have recently moved and now have cable tv instead of my own antenna. I have a vintage Philips colour tv that I use as my bedroom tv. When I had my own antenna I used a digital box. The catv system distributes both analogue and digital channels and therefore I don't need the DTV converter anymore. Since the tv don't have enough of preset buttons for all the channels I have here I connected a VCR as a tuner. This can output RF on the UHF band (21-69) and I use the VCR remote to change programs instead of using the TVs tuner.

The problem is that whatever RF output channel I choose there is a little bit of snow in the picture. I see it when watching a TV broadcast, and if I watch a tape there is also a bit of snow in the picture when the aerial cable is connected. If I disconnect this the snow dissappears. The VCR's on-screen-menus is also affected this way. Somehow there is something in the antenna signal that causes interference with the VCR modulator. I tried another VCR but that made no change. I bought a cheap VHF modulator to use between the VCR and the TV and when using this the snow is all gone

This cable tv system is combined with internet, there is a coaxial connector for internet access on the antenna jack. I don't use this service, but I wonder if there still is some form of signalling there that interferes with UHF signals?

Any thoughts?

Regards, Hampus

[Heatercathodeshort]

You may be receiving a strong digital signal that is swamping your analogue output from the VCR. This will be seen as a huge signal by the TV/VCR and the AGC system will wind down the gain producing a grainy picture. Try tuning the VCR's output to an unused channel free of digital signals. John.

[Restoration73]

What were analogue TV channels 61-68 should be clear if there's no 4G transmission in
your area.

[Andy Green]

This sometimes happens with a satellite and terrestrial combiner system, the stronger signal from the lnb at 1.07 GHz and up will leak enough into the AGC loop causing gain reduction. You need a band pass filter on the video output tuned to channel 36, (or whatever the video outputs. You may be able to build one using two quarter wave shorted coaxial stubs spaced a quarter wave apart. cheap but may require some experimentation

[BGmidsUK]

It is very likely the cable network is using most of the channels for digital TV and broadband. Using VHF obviously provides an easy solution but there are filters available which should help with those channels now occupied by 4G. Notch filters were also available (in the UK) for the analogue Channel 5 transmissions to allow fixed-channel VCRs to be used.

I have been having similar issues with my setup even though it's not connected to a cable feed or antenna. Digital signals are evidently leaking in somewhere, probably via the numerous cables linking the devices in to the combiners/diplexer/amplifier. The top of the UHF band appears to be full of 4G data transmissions judging by the patterning on analogue pictures on those channels.

Band III doesn't have this problem despite DAB occupying the top end but Band I is full of mush. I'd be interested to know what band/channel you are using with your modulator.


Brian

[colourking]

Hampus,
If you have no cable TV stations being received above Ch 60 -

get a 4G filter (commonly available in Maplin etc in the UK)
and put that on the RF input of the VCR.
Set your VCR modulator to Ch65, I would expect now noise free pictures.

(the 4G filter will also filter out non-4G unwanted noise/signals in the frequency range it is designed to filter out)

[Hybrid tellies]

Our main TV is a 16 year old Philips CRT set and I have noticed that the picture was slightly grainy and has been for a while now. The set is connected to a Humax set top box via scart. Whilst I was trying to connect up an Xbox for my son I unplugged and plugged back in the scart lead to the back of the TV which has cleared the grainy effect and made the picture much sharper.

[colourking]

The SCART socket of the TV can also OUTPUT (composite video) directly what is received on the TV's own tuner (noise if no analogue signal was being received) to feed recording devices etc, obviously you had some crosstalk to the INPUT wires of the SCART due to bad connection of Ground pins on SCART plug which reseating fixed. If you had been receiving an analogue signal (from a modulator if no over the air) just prior to switching to SCART, you would have seen a weak' 'floating' image of that superimposed on the SCART input picture. The same effect can happen with audio, with slight hiss (or audio of an analogue TV signal) in the background with a SCART input if the plug is a bit loose.

On some European analogue cable systems with types of line encryption compatible with a normal TV signal, the SCART tuner output of TVs was used to feed a cable decoder and back in again via SCART. This had the advantage of the viewer being able to use the TV's remote control to change channels.

[emeritus]

I had recently been experiencing a slightly snowy picture on the (pre-SCART) Philips colour TV we have in our kitchen. It is used with an Icecrypt digibox, and as I got the same effect when feeding the modulator output into another TV, assumed it was the digibox's modulator that was at fault. Over a few weeks the snow got progressively more pronounced (but not so bad as to be unwatchable), but then suddenly vanished a couple of weeks ago. If it returns I will try some of the suggestions mentioned in this thread.

[hamid_1]

I had exactly the same analogue cable TV service from 1997 until it ended in 2009. From 2001 onwards, I received high-speed internet through the same TV cable, using a DOCSIS cable modem.

The analogue TV channels were distributed on VHF and UHF, but since the 1970s most British TV sets did not have VHF tuners, they were UHF only. Therefore the cable company supplied a VHF to UHF converter box, like this one:
http://theoldcatvequipmentmuseum.org...DPV7214--1.JPG

This worked perfectly with old TV sets that didn't have SCART, like the ones I was using at the time. It also solved the problem of the TV not having enough preset channels.

The converter box came with a remote control, which turned an old TV into a remote-controlled one. It converted any one of the VHF cable channels to UHF channel 54 (in my area) and had an RF bypass module fitted, which mixed and passed the UHF cable channels along with the converter channel (54) but blocked the VHF channels. The cable company had chosen channel 54 for the cable converter box in order not to conflict with other UHF channels in use on the system. In case of a conflict at the user's house, the cable system operator could send a signal to the converter box instructing it to change to another RF channel. The user could not make any adjustments.

As time went by, digital cable TV and internet were offered by the cable company. These services used up some of the UHF channels. When I eventually added a digital satellite receiver to my analogue cable setup, the satellite box RF modulator conflicted with the digital cable services, resulting in a 'snowy' picture, like you have. I kind of solved the problem by choosing another UHF channel that didn't have too much 'snow'. The other way to solve the problem involved removing the RF bypass unit from the converter box. This stopped the UHF from the cable service from interfering with the satellite box RF output. But it also stopped me receiving the UHF cable TV channels.

I could have solved the problem in a number of ways. I could have used a simple A-B antenna switch to change between satellite RF and cable RF sources. I could have used a modern TV with SCART . I could have connected another RF modulator to the cable converter box (it had baseband video and audio output as well as RF), just as you could connect an external RF modulator to your VCR, or use a VCR that has a TV/VCR button on it. This stops the UHF passthrough and avoids interference when the VCR is in use.

Instead I found another cable converter that could convert both VHF and UHF to UHF - the General Instrument CFT-2100 series. This enabled me to use one remote control for all VHF and UHF channels. It was a bit tricky to set up though.

Nowadays you could find an analogue cable converter box like the CFT-2100 for practically nothing. They are worthless here, as all analogue cable TV services have ceased. But a filter, as suggested, should also work, and will be easier to use. Place it in series with the cable supply and it will block a range of UHF channels. You can then set your VCR modulator to one of the blocked UHF channels and avoid the interference from digital cable services which appears as snow on an analogue TV. Hopefully you can find a filter that doesn't block any of the analogue channels in use on your system. 4G filters sold in the UK will block UHF channels 60-69. Channel Five when they started in the UK offered filters that blocked UHF channel 35-37. So you should be able to find a filter that blocks a channel that is not in use for analogue TV on your cable system, then set your modulator to that channel.

[Andy Green]

Here's a website that explains how to make a bandpass filter with coaxial stubs. I've done this before and it works very well
http://dl4xav.sysve.de/coax.filter/coax-filter.html

[Roberto41240972]

Hi Hampus 1982
I had that problem. if your cable tv system still broadcasting Analogue signals. I would suggest fitting a Attuneatta that reduces the Cable TV signal into the set or VCR. Another thing i have to point out. Broadband via Cable tv is Analogue but the cable modem converts it to digital to pc...... won't get to technical. but that can interfere with UHF.

[Hampus]

Thanks for all the answers, I have been away from home a lot but will check further into this soon and report back!

[Hampus]

I have done some research now, not really needed since it has been working well with the external VHF modulator, but I don't like unsolved mysteries! The VCR's modulator can be set to channel 22 to 69. I guess I did not try all channels when I connected it first time, I have now tried every channel and this is the conclusion:

Channel 22-60 is unusable as long as the "aerial" is connected to the vcr. It generates snow which seems like when you have a weak signal. Channel 61-63 is completely free from this problem while 64-69 also is unusable. I have set the modulator to 62 and now it works great, no need for the external modulator anymore.

I don't have a digital box connected so I can't see if the UHF band is full of digital channels which causes this, but I suppose that is what's happening. Since the aerial signal causes snow even when playing a tape I guess it's somehow messes with the TVs/VCRs AGC-circuit, exactly the diagnosis as some of you suggested

Anyway, problem solved!

[Roberto41240972]

Well done.... I had that problem with freeview (DVBT) if video or satalitte tv box too close to the transmitting channel example UHF 22,23,25,28,29) this is My local London transmitter broadcasts on.... I had to re-tune all video recorders and my Sky box away from that band... moved them right up to 31 to 40 (nearby transmitter broadcasts on 41,43,44,46,48. and another transmitter in Oxford broadcasts on 51,53,55,58,60. the 31-40 is brillant for vcrs from the 1980's the upper bands i use vcrs and sky from the 1990's i had to fit a small variable gain reducer... so i got 5 vcrs, 2 freeview,and sky working on UHF for my older tv's without scart sockets... didnt want boxes of modulators... its working fine apart from weather conditions.... I bet you all think i am mad... i have 15 tv's wired up .... will post pics.... i never have sound on just subtitles can follow a few stations at one time.... thats the perks of being deaf and more visual aware.

I never knew why we never had VHF colour TV here, while continental Europe did.

[julie_m]

I think we wanted to make a clean break from old technology in the UK. VHF was the old, 405 line mono system; UHF was always supposed to be the new, 625 line colour system.

[Mike. Watterson]

The VHF 405 line fully occupied VHF in the UK. So 625 Line VHF wasn't an option.

In Europe many countries restarted TV after WWII (1948 to 1952) in 625 lines.

It was IMO a foolish political decision to restart TV in 1946 in 405 for maybe only 300 people (INITIALLY) in London. Many of the regions didn't get 405 TV till 625 was underway in Europe.

In the later 1960s and early 1970s etc other European countries (inc Ireland) were simply able to add colour to the existing 625 line transmission.

Belgium and France had two incompatible with each other 819 line systems. Belgians had the first multistandard sets for Belgian, French, Dutch and German TV!

So France and Belgium had to adopt 625 for Colour.

The idea of 405 Colour based on NTSC was investigated. They rightly decided it would be a mistake.

I know many collectors have a nostalgic attachment for 405 and on a 12" screen in 1936 to 1939 it really was "HD" if you compare visual resolution with a 42" HDTV at the same distance.
The USA went from 440 to 525 lines (480 visible) and never stopped TV during WWII, though they did stop TV production in 1942.
The 525 line was chosen to be similar quality to ordinary 16mm news film. There was no huge reason to change resolution for Color.

The 405 line was going to be inferior to 625 above 21" screen size. The 405 line wasn't protecting UK TV industry as much as making it improbable to export. Though Romac amazingly tried producing a TV for US market (The Motor Accessory people that made the beautiful portable B7G battery radios in 1946-1948, later produced in Austria)