PAL-NTSC compatibility

[GK]

As noted in this post (reply 36): https://www.eevblog.com/forum/reviews/any-commodore-64128-people-on-here/msg499545/#msg499545

I have an Acorn BBC model B microcomputer. It came with a 14.3 MHz chroma carrier crystal (the oscillator operates at 4x the carrier frequency). That gives a 3.58 MHz carrier. 3.58 MHz is the NTSC standard, so I initially just assumed that I had an NTSC computer. However the the sync timing conforms the the PAL standard, rather than the NTSC standard. The field frequency is 50.08 Hz and the line frequency is exactly 15.625 kHz (I measured it).

PAL timing with a 3.58 MHz chroma carrier was used for the PAL-N standard. So do I have here a PAL-N motherboard or an NTSC motherboard? I am reasonably, sort of sure that a 60 Hz NTSC television will still lock to the 50 Hz frame rate of a PAL video signal, so is it possible, so to simplify the electronics design, the PAL timing was retained for the NTSC market? According to Wikipedia the PAL-N standard was adopted only in Argentina, Paraguay and Uruguay, so I'm not sure how a PAL-N computer may have ended up down here, or if an export model for those countries would have even been contemplated???

I am just trying to figure out what I have here in order or convert this thing to display colour properly on my PAL TV. The PAL chroma carrier frequency is 4.43 MHz, so I will need to substitute the 14.3 MHz crystal for a 17.73 MHz one. However if this motherboard is NTSC rather than PAL-N, then I guess that alone won't work completely as the colour encoding itself is different for NTSC.

[GK]

I guess the other scenario is that I have a PAL motherboard that was incorrectly loaded with an NTSC colour sub-carrier crystal.

[Andy Watson]

As far as I can tell, the 17.73 MHz is asynchronously XORed into the video signal before it is sent to the modulator. The timing for the frame and line sync is derived from the processor clock at 16 MHz - and so would remain unaffected by a change of the colour carrier frequency. I half remember (? could have been a different machine) there being alternative ROMs available with presumably different  configuration values for the 6845 chip.

[GK]

I'm using the composite video output. I actually measured the chroma colour burst signal with the original 14.3 MHz crystal still in place. It was exactly 3.58 MHz.
I've downloaded a service manual for the Beeb, but although the circuit, upon initial, superficial examination, appears to be the same, the component designators and parts layout of my motherboard do not match (quite significantly) that in the manual, nor any other motherboard photo I've so far found on the net.
 
My board:

[GK]

As far as I can tell, the 17.73 MHz is asynchronously XORed into the video signal before it is sent to the modulator. The timing for the frame and line sync is derived from the processor clock at 16 MHz - and so would remain unaffected by a change of the colour carrier frequency. I half remember (? could have been a different machine) there being alternative ROMs available with presumably different  configuration values for the 6845 chip.

Perhaps I have an originally NTSC motherboard that has only been half converted to PAL - the socketed ROM changed over to give the PAL timing, but the NTSC chroma carrier crystal still left in place.

The computer as it was displays fine on a PAL TV or monitor, just in B&W only. This computers original owner was a paraplegic. It originally had a guide to aid the use of a mouth-held poker installed over the keyboard, consisting of a clear acrylic sheet with a circular hole above each individual key. The original owner probably wasn't concerned with colour.     

[Andy Watson]

I think you have a later version of the board - the early versions had the ROM chips under the keyboard. Also, I notice that you appear have all eproms - so somebody could potentially have reprogrammed them with a different OS. Appendix G of the Advanced User Guide has some notes on the "American MOS Differences".

http://stardot.org.uk/mirrors/www.bbcdocs.com/filebase/essentials/BBC%20Microcomputer%20Advanced%20User%20Guide.pdf

[GK]

OK, thanks, I'll download that and give it a read tomorrow. It's past 12am now and really need to go to bed. However my suspicion, given the half successful attempt at producing a colour display with the DG1022 substitution, is than once I get a 17.73 MHz crystal installed all will be fine and dandy.
 

[deephaven]

Look at the colour burst, triggering you scope off the hsyncs. If it's been done properly and it's NTSC, then you should get two distinct phases of the colour bust cycles due to the simple relationship between subcarrier frequency and h frequency. If you get a lot of different phases, chances are it's PAL.

[SeanB]

I have seen 2 variants of NTSC with 2 different subcarrier frequencies. Most likely you have either one of those or the SECAM version or MESECAM one. Most modern multistandard TV sets will happily detect and play them back with little issue, at least I know Panasonic sets are happy, as I used video tapes recorded with those standards and a multistandard pro player to play them back. Panasonic TV set wasa happy with anything I fed into it, and switched automatically between them.

[Alex Eisenhut]

Were there two distinct video chip part numbers for the two video standards?

For example, the Commodore 64 had

"VIC-II (Video Interface Chip II), specifically known as the MOS Technology 6567/8562/8564 (NTSC versions), 6569/8565/8566 (PAL)" (wiki)

For example, if you see a 6567 NTSC chip with some other crystal than 14.31818MHz, you've got a mutant.

(I've never seen that but with 17 million of the critters made, I'd bet someone somewhere maybe did)

[Andy Watson]

Were there two distinct video chip part numbers for the two video standards?

No (AFAIK). The user manual appears to confirm what GK has reported. The line scan and horizontal resolution were the same for both the American and UK markets. The frame rate and vertical resolution were changed to 60 Hz by using different configuration parameters for the 6845 CRT controller. The colour carrier signal was generated separately and mixed into the video signal as the last stage. The 6845 shares it clock with the processor and is intimately linked with memory such that it performs its video duty and dynamic RAM refresh.

[GK]

Yes, that would appear so:

Appendix G - The American MOS differences

This Appendix outlines the fundamental differences between
the English and American BBC Microcomputers. The hardware
is basically the same in both machines, except for minor
changes in the video circuitry. The software in the MOS is
however somewhat different to cope with the difference in
television display frequency.


I definately have the non-American MOS installed. It's a pity they don't mention what the actual miror differences to the video circuitry are, but I hope it is confined to just a different frequency chroma carrier crystal. Still trying to find a schematic of the American model to confirm.....

[GK]

Okay..... eventually found the answer in the text rather than the schematics of the English-model service manual I downloaded:


A 17,734,475Hz (+/-100Hz) oscillator is used to
generate the colour subcarrier master clock. VC1 allows adjustment of
the clock frequency. Two D type registers form a ring counter which
generates two phase shifted 4.4336MHz signals (1C79 pins 6 and 9). All
chroma signals are derived by mixing cambinations of the two master
chroma signals or their inverse with a bank of ex-OR gates. For PAL (
Phase Alternation by Line) the chroma signal reference phase is
shifted 90 degrees on alternate lines. An exclusive-OR gate (1083)
driven from half of IC69 modifies one of the ring counter outputs to
cause the required phase alternation.

For NTSC operation, link S28 can be changed to give a constant
reference phase (IC83 pin 13 to 0v) - R92 must be removed for NTSC,
and the crystal X2 must be 4 times the colour carrier frequency of the
NTSC broadcast standard (eg 14.318MHz for USA).


None of the component designators given actually match my computers later, revised PCB layout, but it should be easy enough the trace the circuit out and identify the correct components.

[GK]

For NTSC operation, link S28 can be changed to give a constant
reference phase (IC83 pin 13 to 0v) - R92 must be removed for NTSC,
and the crystal X2 must be 4 times the colour carrier frequency of the
NTSC broadcast standard (eg 14.318MHz for USA).[/i]

A few evening hours of probing and circuit tracing later and it turns out that the above is only partly true for converting a later NTSC board to PAL, rather than an earlier PAL board to NTSC.

It turns out that there are only 2 export models of the BBC model B - West Germany and the US. They both used the exact same PCB layout, which is what I have; namely board revision number 223.000. Mine is Issue 1, though I cannot find any information indicating that there may have been other issues of this board layout.

The PAL/NTSC colour burst reference on this board is jumper S38, rather than S28. There is no resistor to either remove or replace to convert between PAL and NTSC.

After changing the position of J38 from the NTSC position to the PAL position, I met with success, still injecting the correct chrominance sub-carrier frequency from my Rigol DG1022. I wrote a quick basic program as a colour test, which produced the screen below.

However, so far, the colour lock is only reliable and stable so long as I have the probe for my DS1202CA oscilloscope attached to a particular node of the chrominance modulation resistor matrix! I suspect that this is because the DG1022 signal generator output isn't clean enough for the job - a little probe capacitance in the right spot delivers just the right amount of HF filtering. When my 17.73 MHz crystal arrives from Mouser I report on the results.

I would still like to find a schematic for my actual PCB though, but so far the web has not turned anything up.

[deanclaxton]

Hi

I too have a BBC Model B 233,000 issue 1 that am trying to get colour output on the composite. After checking the diagrams of UK boards I realised that my board has the NTSC carrier crystal fitted. Then I started googling to see if that is all I need to change, and I found your thread

I've hunted around through my bits and pieces but cant find a PAL one so will have to order one, but did you get everything working ok?

On the later UK boards there was a jumper (S39) to add the PAL carrier to the composite output. Earlier ones had to be manually modded by adding a 470pF capacitor. Is this required for the 223,000 board?

Lastly, did you end up finding a circuit diagram? I've hunted high and low without success.

Thanks,
Dean

[GK]

Hi, Yes I did manage to this sorted exactly as described in the prior posts. It's been a while now since I looked at it but the requirement for an additional 470pF on this board doesn't ring a bell.

Haven't managed to find a schematic either.