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.

from Blog – Hackaday

Thoughts or comments?