Help Mark!!

Discussions about Sinclair ZX80 and ZX81 Hardware
Post Reply
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Help Mark!!

Post by Moggy »

Mark following your advice about 60 Hz conversion via a 10 ohm resistor for use with the USA Tree Forth ROM I did said conversion (posted elsewhere in hardware) and as it exhibited the same visual glitches as the UK ROM I ended the experiment content with having achieved at least that much.

Now here's the bit I cannot fathom.

Having removed the resistor to revert back to 50 Hz UK spec (I know you hate the use of PAL/NTSC for the zeddy!!)
I discovered that the US ROM will work with UK PAL TV (sorry) providing it's at 60 Hz display ,negating the need for the resistor.
I thought that once the resistor was added an NTSC standard of TV/monitor would be needed at this point whereas it seems that a standard UK zeddy connected to (in my case ) an old Aiwa CRT TV running at PAL 60Hz for the display will run the USA ROM just fine,only displaying visual corruption when switched back to PAL 50Hz display.

I was thinking that the ULA was taking some info from the ROM about frequency and switching its output frequency accordingly hence no need for resistor but that's about as far as I can go.

I think I read somewhere that there is only a fifty byte difference between the two ROMs and to my mind it seems the only difference is in the two copyright messages displayed at boot up which seem about right byte wise.

Sorry if this isn't coming across to well Mark but you are aware of my technical limitations, it's just that I would be very interested to add to my knowledge with this little conundrum.


I would add that I'm using Karls wonderful little composite board to connect to TV.

Many thanks in advance.



Moggy.
Last edited by Moggy on Mon Feb 06, 2017 3:57 pm, edited 1 time in total.
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Help Mark!!

Post by 1024MAK »

Okay, two things here.

First, the resistor to enable "USA" 525 line 60Hz "mode" is simply a setting that the software can read to see which TV system picture needs to be generated. As you may know, the TV picture is mostly generated by the Z80 CPU with some help from the ULA.

Now, if the Sinclair "BASIC" ROM chip code does not generate the picture, but instead, other software generates the picture (either a different custom ROM or code running from RAM), this machine code may, or may not take any notice of the "status bit" available from the ULA (that indicates if "USA" 525 line 60Hz "mode" is required). If the software code does not care about the "status bit" and therefore whether the resistor is fitted or not, then fitting / or removing said resistor will make no difference at all to the generated TV picture.

The machine code in the Sinclair "BASIC" ROM chip reads the "status bit" from the ULA as it generates the TV picture so it can produce the correct signal for the connected TV (it was assumed by Sinclair back then that European ZX81s would be connected to 625 line 60Hz TVs and in the USA, the connected TV would be 525 line 60Hz sets).

Second, some TVs (CRT and LCD) are multi-standard, and as such, will have no problem displaying 525 line 60Hz monochrome pictures. Because the ZX81 / TS1000 do not produce any colour signals at all, the TV does not have to use either the PAL colour decoder or the NTSC decoder circuitry. With no colour signal component to worry about, it's easy for the circuitry in (most) modern TVs to "decode" and display either 625 line 50Hz or 525 line 60Hz TV signals.

I hope this helps to explain things.

Mark

PS it's interesting to note that it is possible to use either colour encoding system on either TV standard (625 line 50Hz or 525 line 60Hz). NTSC encoded pictures have been broadcast on 625 line 50Hz transmissions. As have PAL colour encoded pictures on 525 line 60Hz transmissions :D
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Help Mark!!

Post by Moggy »

Excellently explained as always and I totally get the picture (groan) :lol: now.

Many thanks Mark. :D
Moggy.
Post Reply