This brief article is about how I recreated my three programme channels of analogue UHF, into one RF feed, that could be distributed to all my TVs in the museum. The aim of which was to provide an authentic experience on the TVs.
The idea was to mimic the channels these TVs would have been able to tune into back in the 1970s. For us in the UK, that would be just three channels. We actually ended up with five channels, the final two were channel 4, which came along in 1982 and finally channel 5 in 1997. As the vast majority of my TVs pre-date those final two, I shall not be recreating them, just the BBC1, BBC2 and ITV experience. For further authenticity, the system will further include Teletext, Test card F, both with test card music, provided via the IMOGen generator.
These three channels will be, depending on the time of day, broadcasting the following:-
- BBC1 Test Card F or Ceefax pages with Test card Music or BBC box sets/Films.
- BBC2 Colour Bar test signal, or Movies, Documentaries, Music, Films.
- ITV Colour Bar test signal or ITV box sets & Movies
BBC1, the Test card F and 15-pages of teletext, plus test card music, are provided via the IMOGen generator. The content material provided via DVD, both sources connected to a switch.
BBC2, colour bar with tone provided via a Tektronix TSG95 and programme material via a DVD, again also connected to a switch.
ITV, colour bar provided via the HDMI to composite convertor upon no signal. Programme material via DVD, signal cancels colour bar test signal.
The diagram below shows how I went about it. The main problem I had to avoid was channel interference.CrustyTV Museum Signals
I wanted the ability to monitor the three channels direct in the signals rack where the kit would be installed. This would be achieved via the use of a Decca RU.4011. The RU.4011 is an off-air receiver, rack mountable, baseband video & also audio at 600 ohms balanced. One use of these had been for re-broadcasting & wired networks systems.
In a BBC engineering article No.112 from April 1979, shows the RU.4011 mounted inside a Range Rover reception survey vehicle, along with a monitor, scope and teletext decoding margin meter.
I will be installing the RU.4011 into my signals rack, enabling me to monitor my RF distribution system receiving three UHF sources. Audio monitoring of the network is via the internal speaker and video monitoring via one of the six outputs on the back, which is connected to the rack’s 10″ SONY PVM.
However, the RU.4011 had a video fault, although it had good audio, but there wasn’t any video being output to the six 75R BNC connections to the rear. Checking the supply rails, all were correct except one, the +15V rail, it was sitting at +25V. This turned out to be a fault MC1352P chip, once replaced and a setup performed, the device functioned flawlessly.
Avoid Channel interference
Now the next hurdle, in the past, my attempt at combining three signals down one coax feed for redistribution, resulted in a fair bit of cross channel interference. The previous system was a bit of a lash-up, and it suffered with much interference, in part due to using those truly awful cheap modulators you find on eBay, not only are most dangerous, but they just constantly develop faults, and frequently fool me into believing I have other faults, well no more!
I found the Maplin VH89 PAL System/I modulator, a superb choice, also these frequently turn up on eBay. They are solidly constructed and have a decent power arrangement unlike the Chinese ones, and what’s more, thus far appear to function as intended.
BBC1 Modulator receives the output from a two-way switch, this has two sources, the IMOGen and a DVD. The switch allows me to send programme material or at the flick of a switch Test card F and 15-pages of Teletext, both with music.
BBC2 Modulator receives the output from a two-way switch, this has two sources, a Tektronix TSG95 and a DVD. The switch allows me to send programme material or at the flick of a switch Colour bar and test-tone.
ITV Modulator receives output from a DVD via an HDMI to composite convertor. Under no signal conditions (DVD not on) the convertor outputs colour bars, hence no switch required on this channel.
BBC1 & BBC2 modulators are set to CH56 & CH60, these two outputs pass through a Bell CableMedia Combiner, specifically designed for CH56-63. This was kindly given to me a few years ago by the late Terry Cassey. The output from this goes into one input on a Taylor TS2008 UHF distribution amp.
ITV modulator, is set to CH21, whose output is sent into a PROception overlap blocking filter, which as a passband for CH21-30. The output from this fed into another input on the above-mentioned Taylor TS2008 UHF distribution amp.
This is achieved via a Taylor TS2008 UHF distribution amp with a gain of 20dB on each outlet, of which there are 8. One of the eight output feeds RF into the Decca RU.4011, where the Decca’s internal tuner has the first three channels tuned to CH56, Ch60 & CH21 respectively. The Sony PVM monitor is fed direct from the 75R video output, with the audio monitored via the RU.4011 internal speaker, all to monitor the RF network signal status. Another feed goes to the museum workbench and the rest off into the museum. From there the feeds will be fed in to Tee’s that I removed from a TV shop.
Pi C.O.W. & Pi-Patgen
The Computer Originated World replaced the previous mechanical globe symbols at 7 pm on 18 February 1985 and was used until the of 16 February 1991. The globe was created by the BBC graphics and BBC computer departments. The few of these machines ended up in private ownership and the chances of obtaining one slim. I posed the question to Andy Doz, who had recently created a TV pattern generator using a Raspberry Pi, if he could recreate C.O.W. using a Pi. Long story short, yes! Andy’s project can be found here.
And here it is working in the museum, proving that additional authentic feed along with test card F, Teletext and all the other period signals.
Until recently, my Teletext TV’s and Teletext set-top boxes were provided pages via the IMOGen. This has been wonderful, and I’d not be without it, but my IMOGen has just 15 static pages to view. Wouldn’t it be wonderful to have a live Ceefax service back … well you can.
Today I built yet another Pi and set up Vbit2, a superb implementation of Peter Kwan. Once installed, you configure the service of your choice (I chose Ceefax Worldwide) and then tell it to run at boot time.
The Pi is connected to the internet and my analogue TV signals distribution system. Now all my teletext equipped TV’s in the museum have a live teletext feed, the date and clock too, just as it used to be.
At any time, you can change the way VBIT2 behaves. You can also ‘Update services’ which will check for newer collections of pages and updates to existing ones. Some are art collections, while others are collections of popular pages from the past (such as the writings of Mr Biffo and the Bamboozle quiz games). Others are snapshots of an entire service in a moment of time. Some even dynamically update from news feeds. You just ‘Select service’ to change what is being broadcast in real time.
The only thing I didn’t like was the teletext service just boots to console, so all the TV’s initially would see this, which is naff. So, once I was happy the Teletext service was working, I then installed fbi (frame buffer imageviewer), copied over a test card, added the fbi command line to the user .bashrc file, and set the system to auto login. Now when it boots, it drops straight into a test card so all the TV’s display that, and those with text abilities, can hit text to load all the TXT pages.
I’ve managed to achieve what I set out to do, namely three channels down a single lead and no co-channel interference. The key pieces enabling that being the Bell CableMedia Comb and Proception Overlap blocking filter. The Pi devices providing bonus signal sources for the C.O.W. (Computer Originated World) replica, Test Generator and Teletext service.
Note: What remains now is the installation of multiple coax runs to the various parts of the museum, allowing me to run multiple sets at a time, this is well underway.
Version 2.0 is currently under development. All the above has been deconstructed. More to come in the following days.