My mid-80s video framestore

I was looking through some old photos the other day and was reminded of a thing I built back in the mid-80s. I had become obsessed by the idea of building a device to capture and display video images at a time when no normal computer could do it. Your standard home computer like a BBC micro, for example, could only display a small number of colours and couldn’t even display a smoothly-graduated monochrome image. Later, more sophisticated computers like the Commodore Amiga couldn’t do it without strange restrictions on which colours could be adjacent to others.

I was fully aware of the basic idea of digitising waveforms and storing the results in RAM, having played around with audio sampling prior to this. I found that it was possible to buy chips that could generate frame and line sync pulses from a composite video stream i.e. the output of a standard video recorder or camcorder, and also to split composite video into R, G and B analogue components suitable for sampling. A standard computer monitor could take in separate sync & RGB signals so it wasn’t then necessary to do the reverse and generate composite video again.

Putting it all together, I could build a device that would enable me to grab a single video frame and store it in RAM. I could then replay the frame over and over, reconstituting it via a DAC, to be fed to a standard RGB monitor. I could also stop this process, and allow a computer (a BBC Micro) to read the contents of the RAM for storage on disk. The computer could also upload stored images into the RAM for display – and this would also allow for the possibility of ‘Photoshopping’ images or synthesising them in software.

The pièce de résistance was that what fell out of this arrangement was a live digitised image on the monitor that could be frozen by pressing a button.

As I recall, the main technical hurdles were:

  1. High speed ADCs and DACs were expensive and/or outside my comfort zone. In the end, I used three 6-bit ‘flash’ ADCs, and my own home-made R-2R DACs. So I could capture and display 262,144 colours which doesn’t sound much compared to today’s standard 16 million but was adequate. In monochrome I could display a 64 grey scale image which was sufficient to be called ‘photographic’.
  2. How to lock my pixel clock to the incoming video stream. As a stopgap while I thought of something better, I made a super-simple analogue oscillator out of CMOS Schmitt triggers that could be started (as opposed to its output being ‘gated’) by setting an input logic level.
  3. RAM was pretty expensive, except for dynamic RAM – and I thought this was too complicated to contemplate. In the end I used a bunch of static RAM chips to give me a resolution of 256×256 pixels – again it doesn’t sound like much, but with the relatively fine colour graduations, it was not too bad at the time.
  4. A standard UK PAL video frame has 625 lines (although only 576 lines are visible) comprising two interlaced fields of half that number of lines. If I was aiming for a resolution of 256 pixels, I clearly could not digitise the whole frame. In the end I think I sampled and displayed just one of the fields, cropping the middle 256 lines out of the 288 visible lines by starting to digitise once a certain line count was reached after the top of the field. When displaying a sampled image, the same image was in effect displayed in each field.
  5. I needed to make double-sided PCBs for at least part of this device in order to simplify its construction. This involved extremely arduous work with acetate sheets, self-adhesive tape and transfer symbols, and a scalpel.

framestore_pcb

I eventually made it work pretty well. I started with a single channel of monochrome, and I remember that the first time I ‘froze’ a perfect monochrome image was one of those moments that I probably live for.

I didn’t progress beyond the simple analogue pixel clock – which effectively meant that I set the image’s horizontal width with a potentiometer. It seemed to work perfectly well.

Of course, as so often happens, once the initial thrill was over I didn’t use it much after that, eventually putting the thing to the back of a cupboard and never touching it again – it is still there!

Here are a few of the images I grabbed, mainly from broadcast TV or pointing a camera at magazines. As you can see, it actually worked.

(I seem to remember transferring the images from the BBC Micro via serial cable to a PC in 1998 – the datestamp on the images –  and, knowing me, probably substituted the raw image data into a 256×256 image created in Photoshop or similar so I never had to actually understand the image header. I would then have resized the images from their non-square pixels to what looked right on the PC monitor using Photoshop. There would have been no more image manipulation after that, so these are effectively the raw images).

Skruvsta

I am currently installing myself into a new room in our house extension. My KEFs will be housed there.

I did want to buy a vintage 1960s or 70s swivel armchair for listening to my stereo in the ultimate style, but they are expensive and/or shabby. I bought this one from Ikea for £75 instead. (I have no association with Ikea btw!)

Image result for skruvsta

It may seem like something hardly worth mentioning, but you don’t want a chair that has a high back because of acoustic reflections (as highlighted in the £3150 Lobster listening chair). This one is low but very comfortable, and you don’t have to fit the castors. It’s very light, and the adjustable height means that it can double up as an office chair even without the castors. Clearly it was designed for audio with its carefully shaped, acoustically-absorbent surface. Anyway, I find I can sit it in it for long periods very comfortably, and the stereo sounds pretty good without having to spend three grand…