Old CRT TVs - could it display 40 columns ?

[catmeows]

I read somewhere that CRT TVs sometimes could not display all 40 columns for Atari computers. I understand that it really depends on display clock and since video generation and memory read are synchronized, it seems to me that BBC micro could display 40 columns because of slightly higher clock but Atari with 1.77 (PAL) is (perhaps?) on edge. On other side, Spectrum showed rather big borders.

According of my calculations, pixel area for various computers are:
Spectrum 57% (of scanline)
BBC Micro 62%
C64 66%
Atari 70%

So i wonder, given that Spectrum retains same clocks (1.75 for 48K), what would be safe number of columns ? How do you remember your home TV, would be there place for 36 or 40 columns ? Or even more ?

[Joefish]

The border generally looked about 5 characters wide, so that would give you a total of 42 columns. But only at the widest point of a curved screen edge.

The ZX81 is actually very good at aligning its pixels with the phosphor dots of a good PAL TV, such that stipples show up pin-sharp. The Spectrum would be too if it didn't have the dot crawl caused by the colours being slightly out of synch. More importantly, the pixels do have a square aspect ratio.

I remember an Atari ST on the same screen would show vertical bands in any large areas of stippled graphics, as it squeezed its 320 pixels into an area slightly wider area than the Spectrum's screen, but still with substantial borders. As such, the pixels were squeezed to something narrower than the individual phosphor dots of the TV. I'd have much preferred it if it had gone right to the edges of the screen and maintained a square pixel aspect ratio.
I don't know what older Atari machines looked like on PAL.

[1024MAK]

I was going to write a reply saying that of course a CRT TV can display a 40 column text screen. Then I read the post again. And I thought a bit more about my own experiences…

I don’t have an 8 bit Atari. And apart from in the shops back in the 1980s and in computer museums, I’ve never played with an 8 bit Atari. And these places almost certainly used monitors rather than TVs…

I can however tell you that (a) using custom software, the humble ZX81 can display more than 32 text columns (although I can’t currently find any useful information on how many it could squeeze in).
And (b) I have used BBC Micros with CUB CRT monitors or other manufacturers CRT monitors and these have no problem displaying a 40 column text screen.

I had a Ferguson TX CRT TV back in the 1980s (and for many years after this).
Like this one:

But the only computer it was used with, was a 48K ZX Spectrum (via composite video).

So, I’m not sure I actually used a computer that had a 40 column text screen and which was using a CRT TV as a display…!

I do still have a portable colour CRT TV, so may have to test it with a computer at some point…

Oh, I have various Atari ST/STFM/STe computers, but I only ever use/used them with monitors or LCD TVs.

Mark

[Matt_B]

The Amiga can display 40 columns on a PAL TV with roughly square pixels, but the borders are very small. Even if you're not into the overscan area, you'd be getting towards the curved part of older sets.

I'd reckon that the Atari 8-bit would be about the same, although I don't have detailed figures for pixel clock speeds to hand; I'd imagine that they're both around 7MHz though.

Machines like the C64, Amstrad CPC and Atari ST had much faster pixel clocks in the 8MHz range, so you got a decent amount of border at the expense of slightly rectangular pixels.

[Seven.FFF]

The Timex Sinclair models like TS2608 could display 64 columns of 8x8 pixel text on a TV. They had built in RF modulator and built in composite video output. The manual recommended using the composite output in hi-res mode, but it also worked, less clearly readable, over RF.

It really depends what you mean by TV. My CRT TVs had composite input sockets, but I suspect the OP meant “TV using RF input” by TV.

It really depends what you mean by 40 columns too. That’s really old school text terminal terminology. Micro text display doesn’t have to be constrained to 8x8 raster pixels, we just think it is because we’re used to Spectrums. Other pixel configurations are possible, as are vector graphics. As are interpolated pixels made my a video chip. The BBC Micro Mode 7 used a Mullard SAA5050 “teletext” chip that output 40x25 interpolated characters at an effective pixel resolution of 480×500. Which is perfectly clear on a CRT TV over RF input.

It also depends what you mean by phosphor dots too. Most black and white TVs had continuous horizontal strips of phosphor coating, or a continuous sheet of it. Phosphor dots are a compromise introduced for colour TV. Vertical resolution still showed individually separated horizontal lines, but horizontal resolution was effectively continuous, only constrained by how fast the input signal and gun brightness transistor response times were able to change.

[catmeows]

It really depends what you mean by 40 columns too.

Yes, it is really about visible part of scanline.

[zup]

Microhobby (and some other magazines, I guess) published some routines for printing at 42 columns, using 6x8 fonts. It even published a routine to print at 36, 42, 51 and 64 characters per line. IMHO, 42 characters per line is easily readable on a ZX Spectrum with CRT (64 can be difficult at times, even on a TFT).

Also, PC CGA uses 40 character per line by default and most CGA cards have composite video output. Even you could buy a Sinclair PC 200 without monitor and use your on TV as display.

[Lethargeek]

i remember a real atari with crt, its visible area (with overscan) was quite wider than 40 columns, maybe even wider than 42

[catmeows]

To clarify the question: I really should ask what time of scanline could be expected visible on most (say 95%) of TVs.

So yes, some computers could display 40 columns but it really depends on pixel clock (usually 7 MHz or 8 MHz) and width of characters (6 px, 7px or 8px). What I'm curious about is if you could display safely 320 pixels with 7MHz pixel clock. Anecdotical evidence from Atari scene indicates that 320 px with 7 Mhz clock is little bit too much on some TVs.

This thesis (*) (very interesting reading about word processing on 8 bits) indicates that about one third of TVs had problem with 40 columns (on sample of 25 users). Also, scattered trough various forums, I found more people complaing about TVs inability to display 320 pixels without overscan.

So I wondered, if Spectrum could display 320 pixels across line without changing pixel clock. Or, how much area of side borders could be used for pixel graphics without introducing risk of overscan. Well, fact the the most (all ?) CRT TVs could be adjusted doesn't help either.

I found some pictures on the internet showing Spectrum attached to CRT TV. In most cases, it seems 320 pixels is on edge or even too much.


https://pbs.twimg.com/media/Ee0NFw4XoAA6lP-.png

https://images.app.goo.gl/bBSGRVpHrXcQoku6A

https://images.app.goo.gl/MyrFPCuoaaLCnd1v5

https://images.app.goo.gl/KCgWi2XjxqjexXpd6

https://images.app.goo.gl/dNBCMGATYZSjpwpZA

https://images.app.goo.gl/tWvNRxdxcq1dC9wo6

https://images.app.goo.gl/byGEG31maMST3e8WA

(*) - https://archive.org/details/REWRITE_wor ... r/mode/2up (page 22).

[1024MAK]

Yes, ALL CRT TVs (and CRT monitors) could be adjusted. However, not many had this as a ‘user’ control. On most, it was either a PCB mounted internal ‘preset’ variable resistor, or it’s in the engineers set-up menu.

The width of the image on screen on a particular CRT is actually determined by the line (or horizontal) timebase circuitry. This generates a sawtooth waveform signal which when amplified, drives the horizontal (line) scan coil, which in turn generates an increasing magnetic field to move the electron beam across the front of the screen.

Hence, by adjusting the line (or horizontal) timebase timing, the width of the displayed picture/image can be changed. There is also normally additional controls to adjust the horizontal and vertical position.

As for the ZX Spectrum, have a read of this page.

Mark

[1024MAK]

I can however tell you that (a) using custom software, the humble ZX81 can display more than 32 text columns (although I can’t currently find any useful information on how many it could squeeze in).

Found!

This demo "MaxDemo" by Bodo Wenzel (get it here) displays a 40 character (320 pixel) wide picture on a ZX81. The vertical size (if your TV can show it) is 30 lines / 240 pixels. Bodo Wenzel's web page is here.

It does need a 32Kb RamPack with the HiRes (WRX) mod. Recommend using a ZXpand.

As the ZX81 draws the video picture by using the Z80 and software, it's possible to ignore the BASIC ROM and use a machine code program to produce the video picture.

YouTube video from one user:


Another:


And one more:


Mark