Colour link on BBC Master

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
Post Reply
User avatar
geraldholdsworth
Posts: 482
Joined: Tue Nov 04, 2014 9:42 pm
Location: Inverness, Scotland
Contact:

Colour link on BBC Master

Post by geraldholdsworth » Wed Oct 17, 2018 12:34 pm

Ages ago I put in a cap across a couple of transistors on my BBC Master so I get colour out of the composite out. However, months later, I read that an Issue 2 board has a link ready made for this purpose.

So, as I've currently got the machine in bits to do some other long awaited upgrades, should I remove this cap (I'm actually using RGB to SCART these days, so it's not actually used)?

This isn't the machine that needs a 6522 replaced...so that is still to come (along with recapping the other PSU).

Cheers,

Gerald.
Gerald Holdsworth
Repton Resource Page
www.reptonresourcepage.co.uk

User avatar
Richard Russell
Posts: 804
Joined: Sun Feb 27, 2011 10:35 am
Location: Downham Market, Norfolk
Contact:

Re: Colour link on BBC Master

Post by Richard Russell » Wed Oct 17, 2018 2:41 pm

geraldholdsworth wrote:
Wed Oct 17, 2018 12:34 pm
I read that an Issue 2 board has a link ready made for this purpose.
The equivalent link on a BBC Micro is LK39, I assumed that all BBC Masters had one too (I thought it was a BBC requirement) so is that not the case?

User avatar
AndyF
Posts: 1241
Joined: Sat Feb 23, 2008 10:16 pm
Location: Derby
Contact:

Re: Colour link on BBC Master

Post by AndyF » Wed Oct 17, 2018 2:51 pm

The cap seems the standard way I'd always assumed, could be wrong! :mrgreen:

See: http://beebmaster.co.uk/CompColour.html and: http://www.sprow.co.uk/bbc/howto.htm#colourcvbs

And another one:
6.4. Master 128 issue 1 motherboards
A 470 picofarad capacitor should be connected between the east leg of R137 and the east leg of R153. These are located to the east of the modulator.
Source: http://mdfs.net/Info/Comp/BBC/Monitor/BBCtoTV.htm

I don't know if later (Issue Two) Masters have a link instead. I do remember the last M128 I had did have colour out on the composite but I can't recall if it simply had had said cap added, was ages ago now...

I do remember even though this is slightly off topic that adding this to the 'B (unsure on Master) does result in the quality of output being a bit less, I thought so anyway.
Last edited by AndyF on Wed Oct 17, 2018 2:52 pm, edited 1 time in total.
Andy

* NEW * The Jetset Willy and Manic Miner community :)

Adventure games ported across to the BBC (in progress) as soon as I can find some time!

User avatar
geraldholdsworth
Posts: 482
Joined: Tue Nov 04, 2014 9:42 pm
Location: Inverness, Scotland
Contact:

Re: Colour link on BBC Master

Post by geraldholdsworth » Wed Oct 17, 2018 4:07 pm

Richard Russell wrote:
Wed Oct 17, 2018 2:41 pm
The equivalent link on a BBC Micro is LK39, I assumed that all BBC Masters had one too (I thought it was a BBC requirement) so is that not the case?
Not Issue 1...apparently.
Gerald Holdsworth
Repton Resource Page
www.reptonresourcepage.co.uk

User avatar
geraldholdsworth
Posts: 482
Joined: Tue Nov 04, 2014 9:42 pm
Location: Inverness, Scotland
Contact:

Re: Colour link on BBC Master

Post by geraldholdsworth » Wed Oct 17, 2018 4:10 pm

AndyF wrote:
Wed Oct 17, 2018 2:51 pm
6.4. Master 128 issue 1 motherboards
A 470 picofarad capacitor should be connected between the east leg of R137 and the east leg of R153. These are located to the east of the modulator.
Source: http://mdfs.net/Info/Comp/BBC/Monitor/BBCtoTV.htm
Yep - it says issue 1, but nothing about issue 2. I can't see any obvious links...but could be hidden under the co-pro board. I might just remove the cap as I don't use composite anymore anyway.
I had thought about changing the BNC composite video out for an S-video socket - probably got the idea from mdfs.net.
Gerald Holdsworth
Repton Resource Page
www.reptonresourcepage.co.uk

Coeus
Posts: 1265
Joined: Mon Jul 25, 2016 11:05 am
Contact:

Re: Colour link on BBC Master

Post by Coeus » Wed Oct 17, 2018 4:13 pm

AndyF wrote:
Wed Oct 17, 2018 2:51 pm
I do remember even though this is slightly off topic that adding this to the 'B (unsure on Master) does result in the quality of output being a bit less, I thought so anyway.
I don't think this is specific to the BBC micro or any particular model of it, but just how composite video works. Colour was shoehorned into b/w TV by modulating the colour info onto a sub-carrier and inserting it into an area of the b/w luminance signal that wasn't used by "typical" TV pictures. That meant no more bandwidth was required but, as the space borrowed by the colour sub-carrier can be used for non-typical b/w pictures, the two can interact. The interaction can sometimes be seen if someone wears narrow striped clothing where the stripes can have completely weird colours.

In theory it could happen the other way too. A colour set would have circuitry to separate the colour sub-carrier and the luminance info that remains should then be zero in that region meaning the colour sub-carrier won't be misinterpreted as luminance. A b/w set, including, possibly, a "green screen" computer monitor may lack this filter and therefore the colour sub-carrier could cause visible effects.

So by default the BBC micro feeds the "composite" port with a luminance+sync signal suitable for monochrome display devices. Coupling in the colour sub-carrier (which does go to the UHF modulator) is optional.
Last edited by Coeus on Wed Oct 17, 2018 4:17 pm, edited 1 time in total.

User avatar
Richard Russell
Posts: 804
Joined: Sun Feb 27, 2011 10:35 am
Location: Downham Market, Norfolk
Contact:

Re: Colour link on BBC Master

Post by Richard Russell » Wed Oct 17, 2018 4:36 pm

Coeus wrote:
Wed Oct 17, 2018 4:13 pm
I don't think this is specific to the BBC micro or any particular model of it, but just how composite video works.
Not really. If you look at the circuit (all I have handy is the original BBC Micro's schematic) you'll see that the 'summing point' (where the red, green, blue, syncs and chrominance signals are combined, i.e. at the base of Q7) is not a low-impedance 'virtual earth' as would be desirable, so connecting the coupling capacitor C58 not only adds chrominance to the composite signal but also acts to attenuate high-frequency luminance. So it's entirely possible that there is a noticeable loss of quality as a result of joining LK39 which is specific to the BBC Micro rather than what is inevitable due to the characteristics of composite PAL.
The interaction can sometimes be seen if someone wears narrow striped clothing where the stripes can have completely weird colours.
Cross-colour.
In theory it could happen the other way too.
Not just in "theory" but very much in practice, this is Cross-luminance. But subjectively I would not expect cross-luminance to be described as a 'loss of quality' but rather as 'a pattern of stripes' (or dots). I well remember when colour TV started, but we still had a black-and-white set, when the chroma was very visible as cross-luminance. But I actually liked it! It resulted in a sort of textured effect on coloured areas, and the effective change of gamma was quite pleasing to my eye.
A b/w set, including, possibly, a "green screen" computer monitor may lack this filter and therefore the colour sub-carrier could cause visible effects.
A chroma notch filter is likely to make the subjective quality worse, not better! With the technology of the day any such filter would inevitably remove high-frequency luminance as well as chrominance, and could easily result in (so-called) ringing artefacts. This is precisely why such a chroma notch was generally not used (at least by the BBC) when B&W film recordings were made, for which we must be eternally grateful because it made my Colour Recovery process possible!

User avatar
1024MAK
Posts: 8862
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Colour link on BBC Master

Post by 1024MAK » Wed Oct 17, 2018 8:34 pm

In simple terms, a composite signal that does not include the colour carrier (be it PAL or NTSC) is equivalent to a single colour channel of an analogue RGB signal.

So for example, if you have a display device which accepts an analogue RGB input and there is an option for sync on green, you can feed a monochrome composite signal to it on the green channel only. You will normally be rewarded with a monochrome picture in green only :D

You can also do it the other way round, if you have a device which outputs an analogue RGB signal and there is an option for sync on green, you can feed it to a monochrome monitor via a composite signal input. It will then display the green video signal as a monochrome picture with the colour dependent on the phosphor of the screen :D

Mark

User avatar
geraldholdsworth
Posts: 482
Joined: Tue Nov 04, 2014 9:42 pm
Location: Inverness, Scotland
Contact:

Re: Colour link on BBC Master

Post by geraldholdsworth » Sun Nov 04, 2018 9:51 pm

What does LK16 do?
Gerald Holdsworth
Repton Resource Page
www.reptonresourcepage.co.uk

User avatar
1024MAK
Posts: 8862
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Colour link on BBC Master

Post by 1024MAK » Sun Nov 04, 2018 10:06 pm

geraldholdsworth wrote:
Sun Nov 04, 2018 9:51 pm
What does LK16 do?
Fitting a link in position LK16 would bypass the filter formed by L1 and C81. Not very useful.

Mark

User avatar
geraldholdsworth
Posts: 482
Joined: Tue Nov 04, 2014 9:42 pm
Location: Inverness, Scotland
Contact:

Re: Colour link on BBC Master

Post by geraldholdsworth » Mon Nov 05, 2018 1:15 pm

1024MAK wrote:
Sun Nov 04, 2018 10:06 pm
Fitting a link in position LK16 would bypass the filter formed by L1 and C81. Not very useful.
Cheers Mark - I was actually studying the Master circuit diagrams looking for the audio circuitry and found this link. For an Engineer, I'm not too good with most electrical circuits and what they do. I'm actually amazed at those people who can look at a circuit and say exactly what it does.
Gerald Holdsworth
Repton Resource Page
www.reptonresourcepage.co.uk

User avatar
Elminster
Posts: 3540
Joined: Wed Jun 20, 2012 8:09 am
Location: Essex, UK
Contact:

Re: Colour link on BBC Master

Post by Elminster » Mon Nov 05, 2018 3:01 pm

geraldholdsworth wrote:
Mon Nov 05, 2018 1:15 pm
I'm actually amazed at those people who can look at a circuit and say exactly what it does.
It is really easy. The pixies put the electrons in wheelbarrows, and push them around till they get bored, they then empty them into a big pit in an alternate dimension. Or that is how it seems to work.

User avatar
1024MAK
Posts: 8862
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Colour link on BBC Master

Post by 1024MAK » Mon Nov 05, 2018 4:11 pm

Elminster wrote:
Mon Nov 05, 2018 3:01 pm
geraldholdsworth wrote:
Mon Nov 05, 2018 1:15 pm
I'm actually amazed at those people who can look at a circuit and say exactly what it does.
It is really easy. The pixies put the electrons in wheelbarrows, and push them around till they get bored, they then empty them into a big pit in an alternate dimension. Or that is how it seems to work.
Actually the ‘pixies’ cart the electrons to the ‘load’, wait for all the energy to be used, then cart the ‘used’ electrons back to the supply...

They get confused by the various different components, each of which has a different effect! If they are running fast (high frequency), a coil or inductor, if in their way, will confuse them so much it stops them. Meanwhile if they are going too slow (low frequency) or very very slow (DC) a capacitor in their way will bring them to a complete stop.

Mark

Kazzie
Posts: 728
Joined: Sun Oct 15, 2017 7:10 pm
Location: North Wales
Contact:

Re: Colour link on BBC Master

Post by Kazzie » Mon Nov 05, 2018 4:16 pm

Resistors, on the other hand, are narrow corridors that the pixies have to go down two-by-two (or one at a time) instead of all at once. :D
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

User avatar
geraldholdsworth
Posts: 482
Joined: Tue Nov 04, 2014 9:42 pm
Location: Inverness, Scotland
Contact:

Re: Colour link on BBC Master

Post by geraldholdsworth » Mon Nov 05, 2018 5:02 pm

Elminster wrote:
Mon Nov 05, 2018 3:01 pm
geraldholdsworth wrote:
Mon Nov 05, 2018 1:15 pm
I'm actually amazed at those people who can look at a circuit and say exactly what it does.
It is really easy. The pixies put the electrons in wheelbarrows, and push them around till they get bored, they then empty them into a big pit in an alternate dimension. Or that is how it seems to work.
Ah...that explains it all. Thank you. :lol:

Is that why Windows 10 and Office 2016 refers to the computer as "We"? :D
Gerald Holdsworth
Repton Resource Page
www.reptonresourcepage.co.uk

Post Reply