Composite video from the BBC

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PitfallJones
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Composite video from the BBC

Post by PitfallJones » Fri May 31, 2019 11:23 am

Hi There,

Amazing I think I have never used the composite video output from the BBC until last week!
Once I got myself a BNC to RCA adapter (BNC - isn't that what 1970s networks cables used?) I was pleasantly surprised at the quality of the signal and the fact that RGB and composite output at the same time is surprising useful.

I have a few of questions:

1) Blue is coming through as black - is that normal?

2) This article details how to bridge a jumper on the BBC-B to get colour:
https://www.classic-computers.org.nz/bl ... or-bbc.htm
Can anyone post a picture of where that jumper is on a BBC Master?

3) I'm not very good at soldering and would be very sad if I screwed up my irreplaceable BBC with some ameutur attempt at soldering that jumper - can anyone recommend a simpler method? Would taping a bit of metal across the gap work?

Thanks for any help
PJ

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Re: Composite video from the BBC

Post by AndyF » Fri May 31, 2019 11:40 am

To clarify you have a Master not a Master Compact yes ? The Compact has a separate 'keyboard' (its actually most of the computer) and a large box with a drive or two in and a power supply, whereas the Master is a single box.

I think you might need to see what issue of mainboard it is, this is printed on the PCB near the bottom (as in near the edge connectors Tube/Disc etc) below the cart slots area about a couple of inches to the left of it.
On the Master 128, you can solder a 470pF ceramic capacitor between the
emitter (right leg) of transistor Q12 and the base (centre leg) of Q13. These
are located between the CV and RGB sockets, and although it is a slightly
delicate soldering job, it is nowhere near as tricky as the equivalent
modification on an early BBC B
https://www.beebmaster.co.uk/CompColour.html < That may help.

http://mdfs.net/Info/Comp/BBC/Monitor/BBCtoTV.htm < That too.

I'm not 100% sure on the issue concern, I do recall reading something about it not working or something different might be needed for Iss2 vs Iss1. Check to see what board you have. :)
Last edited by AndyF on Fri May 31, 2019 11:40 am, edited 1 time in total.
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Re: Composite video from the BBC

Post by AndyF » Fri May 31, 2019 11:43 am

Actually this is a bit better, covers most of them and Iss2 just needs a link apparently:
App Note Composite Video.pdf
(7.73 KiB) Downloaded 37 times
=D>
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Re: Composite video from the BBC

Post by 1024MAK » Fri May 31, 2019 11:57 am

I was going to say that in later issue boards, it’s a link... but Andy got there first... :lol:

If it helps, later I can post up a photo...

Mark

P.S. despite always seemingly always ordering more jumper shunts/links, do you think I can ever find any when I need them #-o

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Re: Composite video from the BBC

Post by 1024MAK » Fri May 31, 2019 1:49 pm

LK11 position on a BBC Master 128 to enable colour on the composite video output:

Image

:arrow: Note, this only applies to the issue 2 board.

Mark
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DBF5B885-B7A4-4381-81C7-309E4E011A71.jpeg
LK11 position on a BBC Master 128 to enable colour on the composite video output

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Re: Composite video from the BBC

Post by geraldholdsworth » Fri May 31, 2019 2:00 pm

I'd also like to chip in to say that there are, at least, two different variants of BNCs. There are 75ohm and 50ohm variants, which have different sized centre pins. The 75ohm ones are used for video, and the 50ohm are used for...well, actually, I can't remember - it's been 10 years since I worked on The Bill. I also can't remember which has the thicker pins.

EDIT: I seem to recall, but could be wrong, that the 10base2 BNC connections (to which you are referring) are 50ohm, which have the thicker pins. If you were to use these in a 75ohm socket, the thicker centre pin will splay out the centre connection in the socket so when you then use a 75ohm BNC, it won't make as good a contact.

As a point of interest, totally irrelevant here, but there is also a TNC connector (Twist, instead of Bayonet).

Cheers,

Gerald.
Last edited by geraldholdsworth on Fri May 31, 2019 2:14 pm, edited 1 time in total.
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Re: Composite video from the BBC

Post by AndyF » Fri May 31, 2019 2:14 pm

geraldholdsworth wrote:
Fri May 31, 2019 2:00 pm
I'd also like to chip in to say that there are, at least, two different variants of BNCs. There are 75ohm and 50ohm variants, which have different sized centre pins. The 75ohm ones are used for video, and the 50ohm are used for...well, actually, I can't remember - it's been 10 years since I worked on The Bill. I also can't remember which has the thicker pins.

As a point of interest, totally irrelevant here, but there is also a TNC connector (Twist, instead of Bayonet).

Cheers,

Gerald.
TNC I recall seeing on early video machines , can't recall if they used them for video or audio i/o though. But I'm aware of the type of connection. :) I did not actually know what they were called, tended to find an adaptor or two in one of those erm "80's video leads" sets you'd find floating about in electronic shops and the like. Those and some usually curious DIN to phono adaptors and things of that nature that never got used.

Was not aware of the different BNC's, I used to have a box of those BNC > Phono adaptors and each machine that appeared would have one attached regardless of if I wanted to use composite or not :lol: simply just as I found a phono cable easier! :oops:

EDIT... Now I do know a bit about the differing types of BNC's > https://en.wikipedia.org/wiki/BNC_connector
wikipedia wrote:Video (particularly HD video signals) and DS3 Telco central office applications primarily use 75 ohm BNC connectors, whereas 50 ohm connectors are used for data and RF. Many VHF receivers used 75 ohm antenna inputs, so they often used 75 ohm BNC connectors.
Last edited by AndyF on Fri May 31, 2019 2:18 pm, edited 3 times in total.
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Re: Composite video from the BBC

Post by geraldholdsworth » Fri May 31, 2019 2:17 pm

AndyF wrote:
Fri May 31, 2019 2:14 pm
Was not aware of the different BNC's, I used to have a box of those BNC > Phono adaptors and each machine that appeared would have one attached regardless of if I wanted to use composite or not :lol: simply just as I found a phono cable easier! :oops:
We were warned to look out for them, as both types were used in the production of The Bill. I think the 50ohms are also used for radio transmitters (e.g. radio mics, etc.) which is why we had both.

Cheers,

Gerald.
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Re: Composite video from the BBC

Post by geraldholdsworth » Fri May 31, 2019 2:19 pm

AndyF wrote:
Fri May 31, 2019 2:14 pm
EDIT... Now I do know a bit about the differing types of BNC's > https://en.wikipedia.org/wiki/BNC_connector
wikipedia wrote:Video (particularly HD video signals) and DS3 Telco central office applications primarily use 75 ohm BNC connectors, whereas 50 ohm connectors are used for data and RF. Many VHF receivers used 75 ohm antenna inputs, so they often used 75 ohm BNC connectors.
Ah yes...all comes flooding back now.

Gerald.
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Re: Composite video from the BBC

Post by PitfallJones » Fri May 31, 2019 2:48 pm

Thanks for the document and picture - very helpful - I actually have a Master 128 and a Model-B - I'll have to open them up to see what mother boards they have got.

Do you think my 'tape a bare wire' across the link is a sensible idea?
What I really need is some sort of conductive Plasticine...

Wow - I never knew that about BNCs - I was surprised I could still buy them in the shops actually!
Do I remember work having a PC computer network using those, with the t-bar connectors, back in the late 80s...

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Re: Composite video from the BBC

Post by AndyF » Fri May 31, 2019 4:46 pm

I think the 'best' way would be to fit a two pin header then a jumper, but myself I'd probably clear the holes out a bit with a desolderer/sucker tool and then simply use a wire link. Probably. :)

You might be able to actually put a erm blob across both those pads to just link them but I'd not really recommend this somehow.
Last edited by AndyF on Fri May 31, 2019 4:47 pm, edited 1 time in total.
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Re: Composite video from the BBC

Post by Richard Russell » Fri May 31, 2019 5:10 pm

PitfallJones wrote:
Fri May 31, 2019 2:48 pm
Wow - I never knew that about BNCs - I was surprised I could still buy them in the shops actually!
BNC is still one of the most common connectors in use in professional equipment today, and will probably remain so for many years. It's the standard connector for Serial Digital Interface (SDI or HD-SDI) video, when sent over copper rather than fibre, after all.

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Re: Composite video from the BBC

Post by Elminster » Fri May 31, 2019 5:20 pm

Richard Russell wrote:
Fri May 31, 2019 5:10 pm
PitfallJones wrote:
Fri May 31, 2019 2:48 pm
Wow - I never knew that about BNCs - I was surprised I could still buy them in the shops actually!
BNC is still one of the most common connectors in use in professional equipment today, and will probably remain so for many years. It's the standard connector for Serial Digital Interface (SDI or HD-SDI) video, when sent over copper rather than fibre, after all.
Plus BNCs are still pretty much standard on scopes, signal generators etc. Even if one buys a brand new £80,000 scope they often have BNCs, not until they get to bandwidth greater than anything you are ever going to need as a hobbyist do they start having other connectors

http://findoscilloscopes.com/s/oscillos ... lloscopes/

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Re: Composite video from the BBC

Post by 1024MAK » Fri May 31, 2019 5:29 pm

Forget any ideas about using conductive tape. It may work for a short period, if indeed it works at all. For a permanent solution, either a wire link needs fitting, or better still, a two pin header that a jumper shunt/link can be fitted to.

If you don’t feel confident, please find someone who is good at soldering.

75 ohm BNC have been in use for professional baseband video (component) connections for many years. 50 ohm BNC connectors (along with various other types, including TNC) have been used in radio and other RF applications, again for many years. This is the reason that they are used on oscilloscopes, frequency generators, and similar test gear.

50 ohm BNC is/was also used for the 10BASE5 Ethernet.

Edit: Ahh, Duncan got in just before me :mrgreen:

Mark
Last edited by 1024MAK on Fri May 31, 2019 5:30 pm, edited 1 time in total.

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Re: Composite video from the BBC

Post by Elminster » Fri May 31, 2019 6:01 pm

:D

Plenty of solders around Cambridge in a month. But you have to sneak in and pretend you are a normal person and you thought the classroom was the cafeteria. Oh and by the way you found this beeb with a note on saying make this link.

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Re: Composite video from the BBC

Post by 1024MAK » Fri May 31, 2019 6:21 pm

Elminster wrote:
Fri May 31, 2019 6:01 pm
:D

Plenty of solders around Cambridge in a month. But you have to sneak in and pretend you are a normal person and you thought the classroom was the cafeteria. Oh and by the way you found this beeb with a note on saying make this link.
But I think PJ is in America...

Mark

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Re: Composite video from the BBC

Post by Elminster » Fri May 31, 2019 6:33 pm

1024MAK wrote:
Fri May 31, 2019 6:21 pm
Elminster wrote:
Fri May 31, 2019 6:01 pm
:D

Plenty of solders around Cambridge in a month. But you have to sneak in and pretend you are a normal person and you thought the classroom was the cafeteria. Oh and by the way you found this beeb with a note on saying make this link.
But I think PJ is in America...

Mark
That could make it slightly more tricky.

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Re: Composite video from the BBC

Post by guesser » Fri May 31, 2019 7:41 pm

In which case you'll have to go to the one in Massachusetts and ask someone to sodder it instead :D
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Re: Composite video from the BBC

Post by PitfallJones » Sat Jun 01, 2019 9:35 pm

Soooo....
Turns out my Model-B has the link but a transistor is bent over it so there's no way I'm going to get to it...
modelb-link.JPG

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Re: Composite video from the BBC

Post by PitfallJones » Sat Jun 01, 2019 9:43 pm

The Master actually sort of worked!
There was no link present (I guess it's issue #1) so I tried the transistor to transistor bridge method
master.JPG
with a 50v 470pF capacitor...
50v.JPG
The result was a 'hint' of color - maybe the probe wires were too long I'm thinking - but everything is SO small it tooks me ages to attach the clips so I can't imagine poking a soldering iron down there...
snapper.JPG
PJ

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Re: Composite video from the BBC

Post by AndyF » Sat Jun 01, 2019 10:14 pm

Can you not see the issue number on the board itself then ? :)
AndyF wrote:
Fri May 31, 2019 11:40 am
I think you might need to see what issue of mainboard it is, this is printed on the PCB near the bottom (as in near the edge connectors Tube/Disc etc) below the cart slots area about a couple of inches to the left of it.
Pic may help ? :)
m128_issue_one.jpg
I've tried ( slightly badly it seems :D ) to highlight the 'Iss X' area for you.
Last edited by AndyF on Sat Jun 01, 2019 10:16 pm, edited 2 times in total.
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Re: Composite video from the BBC

Post by PitfallJones » Sat Jun 01, 2019 10:53 pm

Yes - looks like mine is an issue #1 - thanks for showing me exacly where that is.
20190601_152051.jpg

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Re: Composite video from the BBC

Post by AndyF » Sat Jun 01, 2019 11:29 pm

PitfallJones wrote:
Sat Jun 01, 2019 10:53 pm
Yes - looks like mine is an issue #1 - thanks for showing me exacly where that is.
Most welcome. :) Just pleased you could make sense of my quick pic. :oops:
Last edited by AndyF on Sat Jun 01, 2019 11:29 pm, edited 1 time in total.
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Re: Composite video from the BBC

Post by 1024MAK » Sun Jun 02, 2019 7:56 am

PitfallJones wrote:
Sat Jun 01, 2019 9:35 pm
Soooo....
Turns out my Model-B has the link but a transistor is bent over it so there's no way I'm going to get to it...
modelb-link.JPG
Gently bend the transistor away from the area of the link. The leads are made of copper, so they should bend without a problem.

Mark

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