FPGA Raises Component Video From A Sinclair ZX Spectrum

An abiding memory of the early-80s heyday of 8-bit computing for many is operating their computer from the carpet in front of the family TV. While the kids in the computer adverts had parents who bought them a portable colour telly on which to play Jet Set Willy, the average kid had used up all the Christmas present money on the computer itself. The cable would have been an RF connection to the TV antenna socket, and the picture quality? At the time we thought it was amazing because we didn’t know any different, but with the benefit of nearly 40 years’ hindsight, it was awful.

For ZX Spectrum owners in 2020 a standard modification is to bring out a composite video signal, but [c0pperdragon] has gone a step or two beyond that with a component video interface. And this isn’t a mod in which the signals are lifted from the Spectrum’s colour encoder circuitry, instead it uses an FPGA hooked directly to the ULA chip to generate the component video itself.

The Altera chip sits on a little PCB designed to occupy the footprint of the original Astec modulator, and sports a neat bundle of wires hooked up to the various Spectrum signals it needs. There are a couple of jumpers to select the output type and resolution, it supports YPbPr or RGsB outputs and both 288p and 576p. If you think perhaps it looks a little familiar, that’s because it’s the sister project of an earlier board for the Commodore 64. So if you have a Spectrum and are annoyed by UHF and PAL, perhaps it’s worth a look.

39 thoughts on “FPGA Raises Component Video From A Sinclair ZX Spectrum

    1. It was definitely more common at least in the UK, years ago in the UK a lot of people bought it for playing games.
      The TS-1000 seems to only have 1 / 2KB of Ram
      The spectrum started out with 48K of Ram, with later ones having 128Kb

        1. 48kB on ZX SPECTRUM, you are thinking about ZX80 & ZX81 predecessors of Spectrum. There where also Russian clones with 516kB RAM and now there’s a clone with 1MB ram.

          1. No – there were two initial versions of the Spectrum. I had (and still have) the 16k, but there was also (slightly later) a 48k version

      1. Actually 2KB of RAM while the original ZX81 had just 1KB. Another big difference is that both had only black & white video while the Spectrum already had color video (otherwise its name wouldn’t make sense anyway)

        1. Actually, no. The ZX80 and ZX81 both had 1KB of RAM. They could both be upgraded to 16KB with an external RAM pack – the latter, later to 64KB with an addressable 56KB. In the USA and Portugal, the ZX81 had a brother (in looks) by Timex, the Timex Sinclair 1000, which came with an internal 2KB of RAM, also expandable externally. The TS1500 had 16KB of RAM. The ZX Spectrum Issue 1 had 16KB of RAM (upgradeable) ; 48KB were introduced from the Series 2 motherboards. Then there was the Spectrum Plus, still with 48KB and an “upgraded” keyboard. Sinclair’s last commercial micro was the ZX Spectrum 128KB, nicknamed the “Toastrack”, before Amstrad took the reigns of the brand and produced further models that started to diverge. Finally, have a look around for a more recent, modern take on the family: the Sinclair ZX Spectrum Next – it’s fascinating to see how far we’ve come…

      2. small nit to pick: the original standard Spectrum had 16Kb RAM. IIRC, the 48K model was available a bit later and you could send in your 16K model for an upgrade. The 16K version did not last long because the popular games all needed 48K and was eventually withdrawn.

          1. The ZX Spectrum was released originally with 16k with the 48k model close behind. If you Google 16k ZX Spectrum the information is there in plain sight.

          2. No we’re not mistaken. The original model had 16k ram. Check out the cassette cases for games such as Manic Miner and Atic Atacit will say for 16/48k spectrum. Alternatively read about the first models of the spectrum. The zx8p and 82 had to have an extra ram pack added to the port in the back to give them 16k

          3. Hi peter, I’m afraid it is true, the ZX Spectrum was released in a 16Kb and 48Kb versions. One of my friends had the 16Kb version for Christmas & many early games (eg Arcadia or JetPac) ran in 16Kb.

          4. No i am 100% not wrong. I bought a Spectrum when both models were still available. And these facts can be easily checked. The original advertisements and commercials are available on the interwebs.

          5. No, I could only afford the 16k model when spectrum launched (long wait due to demand on preorder) later sent it back for an upgrade. I recall that 48k model was available at launch (£125 & £175 from memory). You’ll find 16k games on World of Spectrum – I think Psst and Jetpac were.

          6. My recollection is, that they were announced at the same time, but only 16K was in stock anywhere for the first 3 months. When we finally got one, it was an Issue 2 board, which still went back under warranty a couple of times until we ended up with an issue 3. There was a rumour that the entire production run of Issue 1 48ks went to servicing the warranties of the first 100 people that bought them LOL

          7. I had (in the day) and still have ALL the Spectrum models. I also have a ZX80 & a ZX81.

            The 80/81 were/are 1K with RAM Pack upgrades available (3K & 16K then 32K – I still have my Memotech Memopak 32K RAM for my ZX81) .

            The Speccy was initially 16K,then 48K,then 128K. I also have the Spectrum Next, which is 1MB base, upgradable to 2MB.

        1. 16k is correct… To start with…
          Zx81 had 1k and you bought a very,very unreliable plug in ram…. Which if jolted whilst using often stopped the zx81 working all together…
          Used a zxspectrum 16k when first released…

  1. RF was bloody terrible at the time. We had the choice of a 12″ b&w with a tuning dial, or the ‘big telly’ which bizarrely gave the wrong colours. Compare that against the microvitec cub RGB monitors on the BBCs at school, and you knew you were getting a crap deal. Still infinitely better than nothing though!

    1. A typical RGB monitor of the 80s was extremely expensive. Unfortunately for you in the UK, PAL color televisions rarely have tint knobs to adjust the colors if they’re out of tolerance, whereas in the USA with NTSC this was considered a user adjustment. There’s also a trimmer inside the Spectrum that adjusted the output for this.

      With a proper setup, RF doesn’t look bad at all. It’s mostly the nitpickers and people who don’t know what they’re doing that end up with a shit signal. Using a good quality cable to a good quality television which is tuned properly will make a world of difference. Most people are using bargain bin 50 cent Walmart special cables on black plastic crap 90s televisions with digital tuning, and of course it looks terrible.

  2. But the Spectrum did natively generate a composite signal, it’s just that it was routed through a modulator. If you cut the wire on the modulator then you get the native composite signal coming out the back of the Spectrum. I’ve done it on mine. Am I missing something here?

    1. Wasn’t it only monochrome? Though I never tried it with other than a mono monitor, because I scored a CCTV monitor for cheap and it was way crisper and more linear than most cheapy B/W portables.

  3. Good project …With that FPGA in use may of got rid of the ULA completely and gained a socket to work from. Removing that Modulator for most people would be rather a task as it’s held in good and proper by dry 40 year old solder tabs

  4. On my 48K speccy, I found that the component output was good just by hooking up to the 2 pins that go into the RF modulator. I then bought a +2 which had RGB output as standard (via a non standard socket). Later, I modified a monitor to use the Brightness line from the +2 via a potentiometer so I could adjust it and could therefore see all 16 colours (because the spectrum allowed you to have the brightness flag set on black). It was interesting to see that some games didn’t always set the brightness bit correctly because normally you couldn’t see it.

  5. Aaah, but does the FPGA implement DOT CRAWL ?!!? It’s not proper Speccy video without it. You can generate some fun animated effects by placing, say, red and yellow checkerboard patterns on the screen (or a moving water effect using Cyan and Blue checkered patterns).


  6. The spectrum had 16k to start with the zx81 or ts1000 if you prefer had 2k you could expand it with a 16k rampack when I was done with my ts1000 it had 64k but it wasn’t convenient

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