The A to B of Early Video Mixer Technology

By: Paul Marshall PhD G8MJW

Trevor Brown has been documenting his work in breathing new technology into a vintage Grass Valley video mixing console; you can read the entire series of articles in his free CQ-DATV electronic magazines, starting in issue 68.

This led me to think about the even older history of video mixers generally.


When it comes to audio mixers you can find lots of material about the famous REDD ‘Beatles’ desks, stories about the legendary Neves, how Sun Studios recorded Elvis with a 5 channel Presto and everything in between. When it comes to video mixing in the 1950s and the early 1960s there is almost nothing to be found. It’s as if the core technologies and techniques that we now carry out as a piece of software emerged from nowhere. The classic A/B arrangement of picture source selectors and fader (T Bar) came from somewhere, but when, and how? This article aims to start the ball rolling in finding out how this particular piece of video technology and practice evolved. There are as yet no truly definitive answers and it is a work in progress. It spans pre-war practice to the beginning of the 1960s and uses the Broadcast Engineering Conservation Group’s (BECG) 1950s Marconi BD841 Video Mixer as a reference. This unit has recently been restored and is operational, although it still requires some further work to be fully complete. If the A to Z of video mixer history is from pre-war to the current day, this really is just the A to B.

Pre-war video mixing

Looking back to the days of electro-mechanical television – the period of J L Baird and the rest of the international pioneers – there was little in the way of video mixing to be done. You were lucky to have one working camera and so mixing picture sources was hardly a priority. In the case of flying spot scanners working with a reflected scanning light picked up by photocells, there was the opportunity to mix the outputs from various sensors but this was really a form of lighting, not video mixing. The BBC with their experimental 30-line studio did venture into mixing the output of a live studio camera with that of a caption scanner but that was about as far as it went. The video signals were constrained to be within an audio channel bandwidth so provided the sources were synchronised, an approximate fade could be made using conventional equipment.

Come the mid-1930s and the dawning of the electronic era of television technology pioneered by RCA in the USA and adapted by Marconi-EMI in the UK, the need for some form of video mixing arose. The film industry was already mature and film productions were already heavily edited, giving the impression of multi-camera working even though it typically wasn’t. TV had – at least superficially – to compete with the standards of production set by their established rival. In the case of ‘direct TV’ (live studio/OB), the new all-electronic technology embraced the idea of multi-camera shooting. There was no recording or editing – not even tele-recording with film – so a live production had to use several cameras. Operationally, production staff were still feeling their way through new practices and there were really only conventions borrowed from the film and radio industries to base things on.

Mixing wide-bandwidth video in the late 1930s was no trivial matter – even with fully system synchronised cameras. It was new territory with video bandwidths reaching to 3 MHz and beyond. The video mixers of the day could not ‘cut’ (that really messed up the sync for the home-viewer) and only a slow cross-fade was practicable. This was because the video mixer was of the ‘knob a channel’ type – one for each picture source – which controlled a variable-mu channel valve amplifier sharing its anode load resistor with all the other channels. In principle this should have worked but it didn’t due to the ageing of the valves affecting the gain slope, DC transients and not terribly well controlled input sources. A fade between channels could be done safely and cleanly in about 8 seconds. Cutting was out of the question. In essence, the video mixer was still a sound mixer but with much increased bandwidth.

Post-war video mixing

In September 1939, British television faded to black and didn’t resume until 1946 using pretty much all the same pre-war technology. The USA managed to continue with television during the war and made some significant advances, including the Image Orthicon tube for a new generation of TV cameras. It’s perhaps not surprising to find that the first major post-war development that I can find in video mixing technology is from RCA in the USA.

Representative hardware of the new technique can be seen at the Early Television Foundation and Museum in Hilliard, Ohio. This is a real, physical museum where you can go and see up-close a whole range of artefacts, including working pre-war British TVs and even an operational iconoscope TV camera. The museum has on show what has to be the oldest and absolutely most completely original TV Remote Truck (Outside Broadcast Van). Originally built for KDYL of Salt Lake City in 1948, it carries three 3” Image Orthicon TV cameras and all the necessary support equipment including the video mixer. Figure 1 shows a picture of the truck and Figure 2 a close-up view of its video mixer. This is a radically different piece of equipment – it has an A/B bus and split faders as we recognise today. This is the oldest A/B video mixer that I’ve ever seen and I would be pleased to hear of anything earlier. I have no information about why the RCA engineers developed it or how they overcame the still significant technical problems. It was probably driven by the needs of production staff, but there are no references to be had yet. Looking at Figure 2, you could be forgiven for calling it a black/white mixer (the button rows are coded black and white), but that would be confusing! A/B seems like a good way to describe it but the terminology seems to come later. It has the classic split fader bar mixing between the A and B rows enabling not just a cross-fade, but two sources to appear together at full amplitude. This is useful in situations such as interviews with one person facing left and the other facing right.

Figure 1: KDYL TV Remote Truck
Picture credit: author
Figure 2: RCA Vision Mixer
Picture credit: author

Back across the Atlantic, the Marconi Company was developing a lot of new TV technologies with cameras using the new Image Orthicon tube based on RCA’s work. For video mixing, the BBC ‘knob a channel’ faders still seemed to be the technique of choice. The BD633 Video Mixer, first seen in the 1951 Catalogue and shown in Figure 3, harks back to the past – likely because of BBC preferences. The company had automatic rights to any RCA ‘transmission’ technology (that included cameras and all ancillary equipment), so the RCA ‘A/B’ technology could have been adopted without payment but ‘knob a channel’ was what the state broadcaster wanted.

Figure 3: Marconi BD633 Video Mixer
Picture credit: The Marconi Company – Product Catalogue 1952

Another two years goes by before what appears to be the first British made A/B mixer appears in the form of the Marconi 8 channel BD841 introduced in 1953. It is this model that we have in BECG’s Project Vivat and the video mixer system is now nearing completion as a fully operational example. The desk unit, shown in Figure 4, contains only the control and fader amplifiers, the electronic processing being carried out in a separate large unit, the BD813 Line Clamp Amplifier (LCA) or ‘Stabilising Amplifier’, in BBC jargon. The BD813 is a sophisticated unit in its own right in that it strips incoming syncs, clips peak white, re-blanks and adds sync back-in as well as controlling gain and black level. It can also add pedestal (USA market) and provides four outputs. Most of these functions can be controlled remotely with cable linked panels enabling control from the vision mixer desk unit. Figure 5 shows the internal construction revealing the vast quantity of valves. There’s 41 in total and a lot of work went into its restoration to operational status.

Figure 4: Marconi BD841 Video Mixer
Picture credit: author
Figure 5: Marconi BD813 LCA
Picture credit: author

By comparison, the A/B mixer desk-top control unit is quite simple, even with its preview panel and cues/communication section. Interestingly, this unit is almost certainly ex-ITV because it has a modification – a top-left and top-right cue dot insertion facility – something the commercial break free BBC never needed! The A/B banks are there, as is an overall fade-to-black control and a preview selector. There’s also a pair of ashtrays mounted in the top (now missing) – not something you see these days! None of the cutting takes place in blanking and there’s no wipes – that comes in the next series of video mixing equipment.

Another interesting feature is its green cue light system in addition the normal red ones. The green cue indicates the availability of a source and in the case of Marconi cameras of the period, this is controlled from the Camera Control Unit (CCU) in the form of a front panel switch. Operating the switch lights local lamps and also causes the appropriate video mixer channel to light-up green in the centre section. A channel push button turns to red if selected to go to air, but when faded down, turns to amber. This use of coded colour cues is an early example of new operating methodology being applied to video mixing.

The BD813/BD841 video mixer system installed in Project Vivat is running 625 lines 50 field rather than the BBC standard of the time, 405 lines, 50 field. For pragmatic reasons, the whole project (with only two exceptions) is based on 625 lines operation. The equipment was designed to work with all TV systems of the time and operation at this rather more demanding TV format is not a major issue.

Later in the decade, Pye produced a similar A/B mixer and presumably EMI did too, but the Marconi unit does appear to be first in the UK. A ‘first’ is always a difficult claim, but until something to the contrary appears, it will stand. Practice and technology in other countries is not known – what was happening in France and Germany? I do not (yet) know.

This article has only been a brief foray into what appears to be an area of TV technology history which has not been investigated and much further research will be needed to produce a more definitive and nuanced account.