5

The original ZX Spectrum has a resolution of 256x192, for 32x24 text. Some later clones add a 512x192 mode. The most obvious thing to do with that is try to display 64x24 text. According to https://en.wikipedia.org/wiki/ZX_Spectrum_graphic_modes#512%C3%97192_monochrome exactly this was done:

"This mode was mainly used to display 64×24 or 80×24 columns text screen and only Timex Sinclair computers and some Russian clones can display it. There are two graphics editors for this mode and it is supported by BASIC64 and some CP/M implementations."

The Spectrum outputs video to a TV, or to a composite video monitor with the frequencies and resolution of a TV. Given that, the effective resolution of the display is limited no matter what the computer outputs. The consensus is that 80-column text is never going to be usable unless you have a monitor with frequencies and resolution designed for computer use.

How usable is the 64x24 text output in practice?

Would the answer change if you were adapting the machine to NTSC rather than PAL?

  • 1
    64 columns of 4×8 characters in 256 pixels are (under right conditions) perfectly readable, so as a starting point, if the (8×8) characters are designed in this way (i.e. if possible, fat, 2 pixel wide vertical lines), the end result would be the same. However, the quality of the TV makes a big difference - I never realized my small B&W portable TV had an excellent picture until I connected my Spectrum clone to a "proper" living room TV (original 32 column text was quite fuzzy on that one and 64 column was unreadable). – Radovan Garabík Dec 30 '17 at 14:43
  • 1
    The ZX Spectrum +3 displays 51 characters/line (5 pixels wide) in monochrome when using CP/M Plus and it is perfectly readable. – tofro Dec 30 '17 at 15:35
6

I had two micros when I was growing up: the SAM Coupé and the Acorn Electron.

The SAM Coupé has a 512x192 mode much like you discuss. It actually makes the display physically wider than a Spectrum though, pixels being approximately 1.25 times as wide as tall.

The Acorn Electron inherits full 80-column 640x256 video from its progenitor, the BBC Micro.

In both cases my recollection, across the TVs we had in the house during the early '90s, is that the high-resolution text was legible but uncomfortable. I spent a lot of time programming on the SAM in particular (because it's so much nicer when there's a floppy drive) but don't recall spending all that much time in its high-resolution mode.

To go beyond the qualitative, it just so happens that I'm an emulator author and that one of those includes an emulation of the Acorn Electron and is capable of displaying its output via a composite signal. This isn't one of those hacky "PAL filters" that tries to post-process a frame according to what I and an echo chamber feel like a TV did, it really generates a real composite signal, then really decodes it. So colour artefacts, frequency limits, occur naturally and accurately. Apologies for the sales pitch, but I hope it explains why I think the screenshot below is relevant to the conversation — that's what happens when I simulate the Electron's 640-pixel video mode via a composite signal.

enter image description here

Honestly, it looks worse than it is when pictured statically, as the PAL colour clock is out of phase with the pixel content, so it sort of swims frame by frame: the colour fringing moves around and your brain is able to get a little more of the form than it can from just a static shot.

NTSC is even more limiting than PAL: due to a different trade-off between available spectrum and quality of signal, the luminance part of the signal can contain only around 80% as much detail.

The Apple II, which acquired an 80-column mode not too long after launch, gained a quick fix for that: in text mode it declines to provide a colour burst. The colour burst is the part of a composite signal that tells a TV both the the current colour phase and indicates that this is a colour-encoded signal. High-end sets born into a world where they might receive a colour or black and white signal can use knowledge that what they're showing is black and white to apply less filtering to the signal. But all sets should decline to decode colour, so you should at least be spared the colour fringing.

Also, cheap black and white sets will usually give a much better picture: they make no effort whatsoever to remove the colour subcarrier. You get a full-frequency image. If the source image is black and white then that can be exactly as good an image as a real monitor. If it has colours then you'll see some chroma dots around, but both PAL and NTSC subcarriers were selected to reduce their visibility: that's why they're out of phase with the scanning frequency.

  • 2
    Got to agree. My old school had a bunch of BBC Bs, about half of which were hooked up to RGB monitors and half to cheap B&W TVs. While it wasn't as clear as it was on the monitors, the 80 column mode was usable on the TVs - but 40 column mode was much more comfortable. – Jules Dec 30 '17 at 23:56
  • 1
    The color burst for the Apple ][ was quite early (only the first 6000 rev 0 boards missed it) and would only fire for graphics modes (I'm pretty sure mixed modes turned back on the color killer for text lines), until the //e 80 column cards (which were 3rd party) would have been black and white. On the Apple //e board there is a turn off color burst switch which also turns it off for graphics modes (useful for those of use with green screen monitors) as the Woz pattern sending trick at least tends to produce useful edges. – PeterI Jan 2 '18 at 13:54
  • On the ts2068 (ntsc machine) the high resolution screen mode was very readable on b&w televisions. Colour tvs were another story where you started to see colour artifacts as Tommy mentions here. Timex provided a composite output in addition to rf output on the ts2068 specifically for usable display of the high resolution modes. – aralbrec Jun 2 at 4:28
2

I had a Timex TC 2048 (compatible with the ZX Spectrum 48k) with the text extended modes, never used them much.

Granted, they were not supported by the ZX Spectrum ROM in the first place and the software to support them was not that widespread; however I got a tasword copy that supported those modes, and in my cheap B/W (PAL) TV the result was difficult to read; in the bigger (PAL) TV at the living room it was better and clear too.

However you could not use it for larger periods of time due to eye tiredness.

[the analog modulator+(extended) ULA in the TC 2048 gave a better/sharper quality image than the original ZX Spectrum too, and the extended modes had better quality than it would be expected]

In practice, at the end of the day, those modes only created game incompatibilities for the mere common mortals.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.