Rediffusion Television: Early Cable TV Delivered Like Telephone

Recently I spent an enjoyable weekend in Canterbury, staying in my friend’s flat with a superb view across the rooftops to the city’s mediaeval cathedral. Bleary-eyed and in search of a coffee on the Sunday morning, my attention was immediately drawn to one of her abode’s original built-in features. There on the wall in the corner of the room was a mysterious switch.

Housed on a standard-sized British electrical fascia was a 12-position rotary switch, marked with letters A through L. An unexpected thing to see in the 21st century and one probably unfamiliar to most people under about 40, I’d found something I’d not seen since my university days in the early 1990s: a Rediffusion selector switch.

If you have cable TV, there is probably a co-axial cable coming into your home. It is likely to carry a VHF signal, either a series of traditional analogue channels or a set of digital multiplexes. “Cable ready” analogue TVs had wideband VHF tuners to allow the channels to be viewed, and on encrypted systems there would have been a set-top box with its own analogue tuner and decoder circuitry.

Your digital cable TV set-top box will do a similar thing, giving you the channels you have subscribed to as it decodes the multiplex. At the dawn of television transmission though, none of this would have been possible. Co-axial cable was expensive and not particularly high quality, and transistorised wideband VHF tuners were still a very long way away. Engineers designing the earliest cable TV systems were left with the technology of the day derived from that of the telephone networks, and in Britain at least that manifested itself in the Rediffusion system whose relics I’d found.

Cable For The ’40s

Inside a Rediffusion cabinet in Canterbury. Adam Sampson, (CC BY 3.0).
Inside a Rediffusion cabinet in Canterbury. Adam Sampson, (CC BY 3.0).

So imagine for a minute that it’s the late 1940s, and a network of VHF TV transmitters (in the lower VHF band from about 50 to 80 MHz) is being put in place across the nation to carry the single BBC 405-line TV channel in glorious monochrome. Some areas of the country can’t pick up an acceptable signal, because of annoying geography. Entire cities will miss the chance to see important events, so they are fitted with a network of twisted pair cables in a tree topology of multiple nodes with repeater boxes on street corners.

Of course, you can’t take a bundle of twisted pairs and send baseband video down them over any significant distance, so the video had to be modulated onto something. There were tubes available that could do VHF and even UHF in the post-war period, but the twisted-pair network only had a bandwidth somewhere in the HF.

The solution was to vestigial-sideband modulate video for distribution onto an HF carrier. When a second channel arrive in the mid-1950s, different carrier frequencies and opposite sidebands were used to minimise crosstalk for adjacent pairs. For the two-channel 405-line system the frequencies were 4·95 Mhz and 8·45 MHz. Crosstalk due to the overlapping sidebands remained at a point that couldn’t be seen on the customer set as long as the cable system had no mismatches. In the home, instead of a broadcast TV set the subscriber had a Rediffusion-branded TV with a built-in demodulator.

At the close of the 1960s the country moved from the 405-line system to a 625-line PAL colour standard, and the Rediffusion systems were upgraded to cope. The selector switch I’d found in my friend’s flat dated from this era, by now the cabling to each house possessed multiple twisted pairs for an anticipated multi-channel future. Slightly different carrier frequencies were used, and since most Rediffusion customers at the time also rented a TV set from the company, the viewing equipment was also upgraded.

The Decline and Fall of the Rediffusion Empire

So through the 1970s and early 1980s there were some British cities that still had a 1950s cable TV system. By now the conditions that had given rise to it no longer existed, as the new UHF transmission network had many more channels and thus far fewer low signal areas. The multichannel future was seen as coming from satellite broadcasting instead, and after some ownership changes and mergers the company ceased trading some time at the end of the 1980s.

In most cases the network was then simply abandoned, as was the case in Canterbury, but in a few cities it remained in use for a few more years before shutting down. The Rediffusion boxes I’d seen as a 1990s student in Hull were operated by a local company and carried some of the Sky satellite channels, and because by then the antiquated technology was comparatively open compared to encrypted satellite channels there was a lively hacking scene. The former Rediffusion regional headquarters was a derelict shell on the main road out of town, and there were technical documents aplenty ripe for the taking.

Potted transformers inside a Canterbury switch box. Adam Sampson, (CC BY 3.0).
Potted transformers inside a Canterbury switch box. Adam Sampson, (CC BY 3.0).

Drawing on my memories from the time as well as referring to the HackHull site, both the “official” set-top box and the clones were simply upconverters that shifted the HF carrier from the twisted pair up into the range that a standard TV could tune into. A simple enough circuit with a single transistor could do the job, and as I remember it could even be done with a modified mechanical TV tuner. There are sites on the web that incorrectly describe the distribution as being at VHF frequencies and suggest the use of a VHF upconverter: these would work, but by an unintended means. In those devices there is an oscillator somewhere in the VHF range that would produce a harmonic that your UHF TV set could tune into, so when mixed with the HF Rediffusion signal it would still produce a picture on your TV.  Subscriber management was simply a case of disconnecting a house at the junction box, and I remember in particular that my student house was not one of the lucky ones that had been left connected by accident.

Years later, all that remains of the Rediffusion networks are a few relics. Switch boxes in older houses and apartments, a few manhole covers, and very occasionally an intact distribution hub. Cable TV in the form of co-axial cable came to most UK cities in the 1990s, but even that is being displaced by internet streaming over fibre and has instead become primarily an internet delivery medium in its own right. The idea that there were once miles of multi-way twisted pair cables snaking around the neighbourhoods simply to deliver a pair of low-ish definition black-and-white TV channels seems bizarre in 2018, but for its time it was at the absolute bleeding edge. If you happen to live in a Rediffusion town, keep an eye out. There have to be some artifacts of the network still remaining.

35 thoughts on “Rediffusion Television: Early Cable TV Delivered Like Telephone

      1. Possibly a future topic for this series, but when we built our new house out in the country in Iowa twenty years ago, we noticed that a lot of our neighbors had parabolic antennas pointing in the same direction. When we installed a new tv in our bedroom and did a scan for channels, a bunch of interesting UHF channels appeared, including one that was HBO!
        A little research on the internet revealed the service known oxymoronically as Wireless Cable…..a tower in a little town about 30 miles away was providing Cable TV over microwave (the parabolic antennas) and some UHF frequencies. Channel assignment seemed to change frequently, but at times we could get premium channels like HBO for free.

  1. Very cool. In another 20 years, they’ll be doing this kind of archaeology on the POTS network. It’s already falling into disrepair where I live, as the telco abandons it (no future and no profit in maintaining a copper network when everyone wants FTTH).

    This sounds like a very early version of DSL (though without the awesomely cool dynamically adaptive multi-carrier scheme)

  2. As a kid in the late 1950’s through 1960’s. our Lincoln council house had piped radio… the ‘receivers’ were basically a rather simple demodulator+amp with a big speaker and a lot of empty space (yep, opened it up to have a peek). The receiver was attached to a plastic box via a (then) standard 2 pin mains plug and the station was selected by a multiway switch on top. After so many years I can’t remember how many channels there were, it was moot anyway beause it was always tuned to the BBC Home service for Two Way Family Faviourites.

    1. A similar system called “draadomroep” (literally: wirebroadcast) or radio distribution was widely used in the Netherlands from the 1920’s up to 1975. It offered a selection of four(!) stations and a volume control knob. Sound quality was excellent but the limited choice of stations doomed the system.

      1. We definitely had more than four, though I cannot remember anything other than BBC Home Service and (very occasionally) BBC Light Service. I have no doubt that I tried all the positions, but more than 50 years on I really can’t remember what was there :-) Now you mention it, there was a volume knob on the side of the wooden box holding the speaker (quite large) and the remainder of the circuit (which was a tag strip mounted on the back of the speaker). Happy days.

      2. I was going to say that :)

        The system was run by the Dutch nationalized mail and telephone company PTT (now known as KPN).

        For a long time I had a Philips tube radio that has belonged to my parents and had a “radio distri” input and source selector. When the radio receiver stopped working, I connected the input to the headphone output of a small transistor radio and that way I could still record radio shows on my little cassette recorder. Good times.

        By the way I saw the actual outlet for the “draadomroep” only once. It was made out of brown bakelite and had a switch to choose from the 4 programs. Note, in the days that the network was active. There were only two national Dutch radio stations, so it was definitely prepared for the future! :)

    2. Sorry, somewhat OT but geek relevant. In the 60’s, the most awesome UK import to the USA were the progeny of Gerry/Slyvia Anderson. I remember growing up on “The Thunderbirds”, also later in the 70’s “UFO” (SHADO), SkyDiver was brilliant (engineeringly implausible, but still kick-ass!).

  3. At some point along the line one do start to wonder if they could have just switched to coax…
    Since the coax probably would be better then the twisted pair. After all, they didn’t seem to have any problem adding more twisted pairs, and fiddling around with everything else. Seems more like their failure were down to not taking the step to something less fiddly.

    Even though a twisted pair of wires and some good differential signaling can technically give 0.5 Gb/s with ease for fairly good distances (this being standard Gigabit Ethernet and yes the full 1 Gb/s is on two pairs) and even then we have 12 Gb/s per twisted pair in an HDMI cable. Although, an hdmi cable with is individually shielded twisted pairs is kinda “advanced”. (Could probably have been manufactured with ease and at a cheap price back in the 60’s… Since twisting wires and wrapping foil around them isn’t hard…)

    Though, biggest downside with all electrical networks for high bandwidths is the high signal loss for a given distance. While an optical fiber generally has far less loss over the same distance. Fibers though typically have far higher losses in connectors, not to mention other connector related issues that electrical standards doesn’t have to nearly the same degree. And this being why short distance communication (sub 100 meters) most likely will remain electrical for years to come. (Although, there is always those people that use fiber for the pure reason that it surely must be better, “it is freaking light after all!”…)

    1. “Since the coax probably would be better then the twisted pair.”
      While coax could handle higher rates than twisted pair, Cat5e TP is handling far higher data traffic than thinNet ever did. Maybe it is because of the crappy coax that was out there, and the likely occurrence of crimping a lousy coax connector. Or maybe because ThinNet wasn’t point to point, it could be mis-routed or under/over terminated, oh yeah, let’s not forget packet collisions.

  4. “The idea that there were once miles of multi-way twisted pair cables snaking around the neighborhoods simply to deliver a pair of low-ish definition black-and-white TV channels seems bizarre in 2018, but for its time it was at the absolute bleeding edge. ”

    If the internet is a series of “tubes” then everyone would be getting their media via pneumatic tube transport (PTT).

        1. I think they meant the electrical kind of tube.
          But a vacuum driven pneumatic system is also a thing that works. Downside here is that the highest differential pressure would only reach 1 atmosphere. Unless one uses both pneumatic pressure and vacuum, then with the same tube we should be able to reach even larger differential pressures. (Unless our tube has horrid tensile strength.)

      1. I always assumed Sen. Stevens was referring to pneumatic tubes. And I’m an EE!

        Funny, how the “series of tubes” has become part of our vocabulary. When The Internet is slow, or down, “the tubes are clogged”. Planned network outage is “tube cleaning”, etc. Well, maybe it’s only at my house that you’ll hear this.

  5. I used to holiday in a village in Scotland that due to geography had to have its TV provided in pretty much this way – the village was in the shadow of one of the taller mountains in the area, so while the far side of the loch a few miles away had line of sight on a transmitter mast, the village itself was in the shadow. I believe they ended up with this until the analogue signal was turned off in 2012 – no idea what they do now.

    The real fun was when Sky started service, lots of people got excited that they’d finally get decent TV.

    Turns out that the same mountain causing the problems with the terrestrial TV transmitter had an outcropping *directly* in the line of sight you needed to get transmissions from the Astra satellite. I did hear that the manager at a local activity center had told his hill team to “take a few of the lads and 5lb of gelignite and have an accident somewhere in the direction of that bloody peak”.

    BSB wasn’t any better – that part of Scotland simply couldn’t get the coverage from the tiny Squarials that the Marcopolo based system was using.

  6. A friend of mine (now dead) designed and installed this system in Burnley and Newcastle and maybe other places, I suppose it really was his life’s work.

    In 1994 I took a head end and several consumer end units and used them to demonstrate video over power lines at a conference in Paris. I still have the gear and it was a pretty impressive demonstration. At that time it was still working in Burnley though they had moved to a very small building for the head end and just used a consumer satellite dish for the source, some arrangement with BskyB and Sky.

    In Burnley they needed to get it to the next town (Padiham) about four miles away without laying cable so they took a leased line from BT and used it for HF, this was before bandwidth limiters were fitted to the telephone lines. It worked well for a long time until one day it didn’t, that was when they installed the filters at BT.

    1. I rolled out quite a lot of HDSL using BTs EPS8 and EPS9 “alarm wires” which were originally just naked pairs looped at the exchange distribution frame and connected to another set. Could run them at 2MB/s all day every day until, as you say, BT noticed.

      Specifically, they realised that the same wires they were about to start selling ADSL over could be used by other people the same way and “Conditioned” the alarm wires – done by sticking a honking great inductor in place of the DF patch cable and reducing the effective bitrate on one of those lines from “How fast can you wiggle the electrons” to no more than 1200bps, with some horrible ringing artefacts if you tried to go much beyond that.. The lines still worked for their intended purpose of allowing you to close a switch and have a circuit complete setting off a trigger on a remote alarm monitor, but no more than that.

      BT in the late 90s were a bit of a nightmare – that the sudden uptake in internet connectivty hugely increased the number of landlines, and BT being BT rather than running copper would put the infamous DACS TDMA pairgain boxes in the lines to split off another line – your nice V34 at 33600 suddenly drops to 9600 because your line’s suddenly gone from being an analogue circuit to a digital circuit quantised for minimal speech frequencies and fax machines at a “guaranteed” rate of 2400bps.

      On the other hand, they did some cool stuff – one connected city trial (In Swindon I think) had set top boxes which were basically Macintosh motherboards with some exotic modem installed (I think 10broad36 – go look it up – it was a fairly cool tech in its day)

      1. I still have one last EPS8 in my network, can’t get FTTC to the site yet (if ever)… It’s currently running at 6.5Mbps (ish) which is adequate for the requirement :-)

  7. I remember stumbling across the HackHull site ages ago and being puzzled by how it described I could open the box on the side of my house and rearrange some wires and get free TV. Opening the box just revealed the traps that were used to provide the pay channels, and I was quite bewildered until I realized the site was in England :-P (The cable provider injected a garbage signal that I believe was 4.5mhz downband from the actual TV signal, that the AFC in the TV tuner would latch onto, so if you tuned to HBO for example you’d get a “beep beep beep” over the audio and a herringbone pattern over the video signal, rather than the suppressed sync other cable systems used. One could use a TV with manual fine tuning and get a watchable signal but if you paid for the channel they just installed a notch filter that filtered the garbage signal out. I remember one of the neighbors moved and gave me her traps and I had all the premium channels until AT&T bought out TCI and they changed the system in the early 2000s.)

  8. People still refer to programmes being “on the pipe” or “on the relay”.

    They could only do it because in the densely packed terraced housing owner occupiers were rare so when they nailed the wire to your wall just under the troffin (gutter) people didn’t complain but towards the end people would not put up with it so large parts of the network were rendered useless because someone wanted to paint their front wall and just ripped it off leaving it dangling in the road.

  9. As soon as I saw Redifusion mentioned in the email I figured Jenny would be the author. They were a very British company, if you look around the sources on the net archiving this stuff you’ll find all about their dev and repair labs and other sites … I believe Edinburgh /Hullwas another ?!?! Either way it shows you what you can do with some twisted pair
    http://www.rediffusion.info/FHAusten/

  10. I grew up on a newly built council estate that had redifusion we had 4 tv channels bbc 1 bbc2 ITV and tyne tees tv . plus several national and local radio. my parents also bought me a video console from redifusion that had an adapter. a box wiht a large male din type plug and socket. this plugged in to tv and the cable from the wall switch plugged in to this. the console then plugged in to the side of this via a 1/4″ jack plug.

    many years later i moved to a house which still had the redifusion oved cable attached. the whole street did. with the system now defunk. i tapped in to the cable to use it as a long wire for short wave listening. dont know the actual length. but i used to get some dam good signals

  11. I was part of HackHull back in the 90s when I was also at University. I remember the rediffusion building (including one of the times it caught fire). I know the authors of the article on the HackHull website and can put you in contact if required.

  12. my dad was a rediffusion engineer, as a kid i got to play with all sorts of cool stuff, the first radio i built was using a signal amplifier box, it had a 64C valve, i used the plastic box upside down to mount the components and a socket to change coils for different vhf bands.

  13. Ahh…hacking cable TV: an activity dear to my heart.

    In the 80’s, we had cable using addressable Jerrold 450 converters, and one of the channels always had an annoying buzz-saw noise on it (which I recognized as data). Hmmm, I thought to myself, wonder what they’re transmitting?

    A little research revealed that the channel in question was in the middle of the FM band. A stripped down clock radio provided the means to see the data, and a 6800 D2 kit (think: KIM-1 but for the 6800) allowed me to look at the bits.
    Turned out, what was being transmitted was box addresses and channel permission bits (a “1” gave the box access to that channel, a “0” told the box not to display it) — after quite a bit of noodliing, I was able to get all the channels on my box, simply by replacing the address PROM with a circuit board with switches to set the box address to anything I wanted.

    Good times. After proving it worked, I lost interest.

    Along the way, I discovered the varactor tuner used in the box made a fairly good poor man’s spectrum analyzer, when combined with the a sawtooth output of a signal generator and an oscilloscope.

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