Shimmering Teletext on BBC/Master

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maniacminer
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Shimmering Teletext on BBC/Master

Post by maniacminer » Thu Jan 09, 2020 5:44 pm

I had some free time over the holiday period to think some more about the problems I have been having with MODE7 (Teletext) and various add-ons (VideoNULA / BBC2DVI) I went through the entire clock circuit from top to bottom, replacing everything in the path and that made a huge improvement, but it still had that occasional shimmer and the TV, being far higher resolution than the BBC can display, assumes that shimmer is useful information and displays it next to where the "real" image should be.

So I decided to bite the bullet and replace the crappy 6MHz clock generation circuit with my own. I have a big box of 74F DIL logic from getting Transputer stuff working again, so I decided that would be the basis for the circuit. I then worked out from the SAA5050 datasheet that there couldn't be much error between the 1MHz register clock and the 6MHz pixel clock. Some simple maths and the only frequency that fits both 6 and 8 is 48.

What I did was to use a 48MHz clock generator module, use three flip-flops to divide the signal from 48MHz to 6MHz then use a divide by three circuit using another three flip-flops and some assorted logic (implemented with NANDs) to give me the 16MHz base clock. I then disconnected the on-board clock by bending up the output pin of the clock inverter (IC43 pin 2) and that stopped the signal going to the jumpers at LK60. I then connected the 16MHz from my board to LK60. I then lifted pin 19 of IC32 (SAA5050) and connected that instead to the 6MHz output from my board.

The result in a pin-sharp, totally steady MODE7 :D

The only issue was I named the pins in reverse from not noticing the pin numbers were backwards in the KiCad model I was using #-o
Master Teletext Clock Fix.jpg
Board in place
Shimmer Free Teletext!.png
No noise!

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SimonSideburns
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Re: Shimmering Teletext on BBC/Master

Post by SimonSideburns » Fri Jan 10, 2020 11:19 pm

I love that sharp image of Mode 7. I don't think I've ever seen one so clear and precise.
Just remember kids, Beeb spelled backwards is Beeb!

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maniacminer
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Re: Shimmering Teletext on BBC/Master

Post by maniacminer » Fri Jan 24, 2020 2:40 pm

SimonSideburns wrote:
Fri Jan 10, 2020 11:19 pm
I love that sharp image of Mode 7. I don't think I've ever seen one so clear and precise.
Yup :D it looks lovely and smooth, no distortion or wiggles.

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maniacminer
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Re: Shimmering Teletext on BBC/Master

Post by maniacminer » Fri Jan 24, 2020 3:22 pm

I have mounted another PCB into the Beeb, slightly more involved, but it works very nicely. It takes me about an hour to fit as a couple of ICs need removing and sockets installed, plus I use the power present on the serial port "overclock" jumper that is a hard-wired link (S28)

I took a before and after photo of asterisks, as they easily show up the malformation of characters on this screen (Sony KDL-46X4500) and shows my original problem with the VideoNuLA board in MODE7.
Board installed in BBC.jpg
Board mounted in the Beeb on top of the UHF modulator
Without clock board.jpg
Without the clock mod board fitted
With clock board.jpg
With the clock mod board fitted
VideoNuLA problem.mp4
The problem I was having with the VideoNuLA in MODE7
(4.04 MiB) Downloaded 25 times
BBC with NULA.jpg
A minor mechanical incompatibility with the IFEL RAM/ROM board :P

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Re: Shimmering Teletext on BBC/Master

Post by lurkio » Sun Jan 26, 2020 9:54 pm

Please forgive my utter ignorance of all things hardware, but is this a mod that can be done to a standard unexpanded Beeb (and, if so, would it help?), or is it only suitable for Beebs with the VideoNULA installed?

:?:

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Richard Russell
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Re: Shimmering Teletext on BBC/Master

Post by Richard Russell » Mon Jan 27, 2020 9:53 am

lurkio wrote:
Sun Jan 26, 2020 9:54 pm
is this a mod that can be done to a standard unexpanded Beeb (and, if so, would it help?)
The circuit that Acorn devised for deriving a 6 MHz clock from a 16 MHz clock (which initially I wasn't convinced was even possible without starting at a higher frequency!) inevitably gives rise to an output with lots of phase modulation, and that directly affects the quality of the MODE 7 characters. Whether that really matters depends on your sensitivity to the defects; I can't say I've ever found the legibility to be significantly impaired so personally I wouldn't feel the complexity of the mod is justified.

But if you're dissatisfied with the quality of MODE 7 I assume the mod is equally applicable to an unexpanded Beeb, although the OP will no doubt confirm.

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Re: Shimmering Teletext on BBC/Master

Post by maniacminer » Mon Jan 27, 2020 12:40 pm

lurkio wrote:
Sun Jan 26, 2020 9:54 pm
Please forgive my utter ignorance of all things hardware, but is this a mod that can be done to a standard unexpanded Beeb (and, if so, would it help?), or is it only suitable for Beebs with the VideoNULA installed?
:?:
I am using an un-expanded Master and Beeb to drive my TV. I couldn't use the VideoNuLA because of the problems I was having with MODE7. So I made the board and noticed the difference when driving the TV. I guess I didn't really notice until I could switch between the Acorn 6MHz-ish circuit and mine. It just gives a very smooth, as Mullard intended, Teletext display. My next steps are to get another VideoNuLA and also Hoglet's Pi RGB->HDMI and be able to use them without any of the graphical glitches. 8)

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Re: Shimmering Teletext on BBC/Master

Post by maniacminer » Mon Jan 27, 2020 1:00 pm

Richard Russell wrote:
Mon Jan 27, 2020 9:53 am
lurkio wrote:
Sun Jan 26, 2020 9:54 pm
is this a mod that can be done to a standard unexpanded Beeb (and, if so, would it help?)
The circuit that Acorn devised for deriving a 6 MHz clock from a 16 MHz clock (which initially I wasn't convinced was even possible without starting at a higher frequency!) inevitably gives rise to an output with lots of phase modulation, and that directly affects the quality of the MODE 7 characters. Whether that really matters depends on your sensitivity to the defects; I can't say I've ever found the legibility to be significantly impaired so personally I wouldn't feel the complexity of the mod is justified.

But if you're dissatisfied with the quality of MODE 7 I assume the mod is equally applicable to an unexpanded Beeb, although the OP will no doubt confirm.
Yes, I did look at the schematic many years ago (30+) to figure out why I couldn't genlock onto a Beeb MODE7 using the same SAA5050 that worked externally. The penny dropped rather quickly when looking for the source of "TR6" on IC5, traced it back and saw the flip-flop/capacitor/XOR glitch-switch and then realised it would be a lot more work. I do recall trying to make my own PLL to run off the 1MHz clock to try and keep the sync between the system clock and the 6MHz clock, they have to be very close or the chip goes wild, that didn't work all the time and always died when I wasn't looking at it #-o I ended up driving it from the 1MHz bus with 1kB of local SRAM and a sync separator, it was happy then :)

The circuit does work for an unexpanded BBC/Master. It is only once you've seen the difference that you'll notice it, as the old saying goes, if you've never had it, you'll never miss it :D To quote a mostly-reliable source Wikipedia - " This gave a particularly stable and flicker-free arrangement on interlaced displays." https://en.wikipedia.org/wiki/Mullard_SAA5050

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