It is currently Fri Oct 19, 2018 11:34 am

All times are UTC - 7 hours





Post new topic Reply to topic  [ 5 posts ] 
Author Message
PostPosted: Sun Aug 19, 2018 1:53 pm 
Online

Joined: Sun Apr 13, 2008 11:12 am
Posts: 7664
Location: Seattle
"System A" was a 405-line TV system used in the UK and Ireland up until just about 1985. Never had color, although there were experiments with all three established analog color encodings.

This puts its extinction just a little after the beginning of the personal computing and video gaming hardware.

So, has anyone heard of any late 1970s / early 1980s hardware that used a 405-line TV set as its monitor, instead of generating NTSC or PAL?


It occurs to me that the 2600 should be flexible enough to be adapted to generate 405-line video, since the CPU is responsible for generating the video timings. You could start with an NTSC stella IC, use a ≈2.3MHz clock instead of the normal ≈3.6MHz one, and then "just" generate 160 active scanlines (out of 203 total). The TVs are monochrome anyway, so the chrominance output wouldn't be very interesting anyway.


Top
 Profile  
 
PostPosted: Sun Aug 19, 2018 5:52 pm 
Offline

Joined: Fri Feb 24, 2012 12:09 pm
Posts: 654
Never heard of it (but I am not from uk/ireland). If somebody is interested, here's more info on wikipedia https://en.wikip8edia.org/wiki/405-line ... ion_system
As far as I can see, it's 50Hz, VHF. Interlaced 405 lines (including vblank), picture height is only 377 (excluding blank lines), and without interlace it would be only half that amount, ie. 188.5 lines.
That's giving a uncommon hsync rate of 50Hz*405 = 20250Hz, that won't match up with the Atari 2600's hsync generator (one of the few things that are generated by hardware). Don't know if the modulator outputs VHF (or UHF?) and if it's on a channel that could be received by those old TVs.
ZX81 also has hardware for hsync NMIs, and both ZX80 and ZX81 have all video output hardcoded in the BIOS rom anyways. Amstrad CPC464 (from 1984) was shipped with monitor, and didn't have any composite or RF outputs (I think the monitor was mostly because they wanted to make more money and/or to support 640x200 at good quality... though maybe they did also wanted to avoid troubles with people with old TV sets).
Anyways, wikipedia says that 625-line PAL launched in 1960's in uk/ireland, and that the 405-line broadcasts were only kept for backwards compatibility (or some remote regions), so I guess most people already switched to modern/color TVs at time when the first computers/consoles became available for home use.

But yeah, if you replace oscillators and create/patch software then you might get away with the atari 2600, and probably also with ZX80/ZX81, and CPC or MSX (both with CRTC 6845 for video timings/size). Do you have one of those old TV's around?


Top
 Profile  
 
PostPosted: Sun Aug 19, 2018 6:37 pm 
Online

Joined: Sun Apr 13, 2008 11:12 am
Posts: 7664
Location: Seattle
Hsync rate is 25×405 = 10125Hz. Wikipedia mentions that this lower pitched magnetostriction was conspicuously more audible than 16kHz sets.

System A was unique in being "positively" modulated, where a higher voltage on the baseband signal (closer to white, further from sync) produced more power being sent; all subsequent video standards flipped this around (so that bad reception conditions would lose sync last). So (unless I missed one), I doubt any existing RF modulator would be compatible.

I don't have one (being in the US). I admit I'm tempted to try to modify a 2600 to support it, though.

Given that Sir Sinclair invented the ZX80 in the UK, and he didn't seem to have deemed it relevant, you're assuredly right that it was too rarified by the time microcomputers started coming into the home.


Top
 Profile  
 
PostPosted: Sun Aug 19, 2018 9:10 pm 
Offline

Joined: Fri Feb 24, 2012 12:09 pm
Posts: 654
lidnariq wrote:
you're assuredly right that it was too rarified by the time microcomputers started coming into the home.

Yup looks so back then - but, yeah, increasing rarity is also making it even more attractive these days : )

Oops, you are right, it's 10125Hz.
Hmmm, that resulting noise must have contributed to some futuristic feeling when people got televized from their homes to distant places.


Top
 Profile  
 
PostPosted: Sun Aug 26, 2018 8:38 am 
Offline

Joined: Tue Nov 23, 2004 9:35 pm
Posts: 702
From what I can tell, by 1964 all new television sets, or "receivers" as they were known then, were dual-standard 405-line/625-line supporting sets and the capability to upgrade the receiver to support 625-line content was available in most sets made in 1962 and 1963. So by the time the Atari 2600 would have been available in the UK in 1978, playing the console would be problematic only if your "receiver" was over 17 years old at that point and still going strong. So I guess you probably weren't bring your shiny-switched console to "gran's" house.

See here for the best explanation of the transition from 405-line to 625-line TV :

https://www.transdiffusion.org/2017/04/ ... 25-line-tv

_________________
Nerdly Pleasures - My Vintage Video Game & Computing Blog


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 5 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group