Bad video output? But why?

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fharo
Posts: 4
Joined: Fri Jun 22, 2018 6:19 am

Bad video output? But why?

Post by fharo » Mon Feb 25, 2019 2:40 pm

Hi everybody from Seville, Spain.

I thnik I have an issue with my output video signal

I attach a photo I think you can see the problem.

Is it normal the gaps between pixels?


Thanks in advance!
Attachments
IMG-20190223-WA0045.jpg
video output
IMG-20190223-WA0045.jpg (113.54 KiB) Viewed 1507 times

Jeff
Posts: 331
Joined: Fri Mar 07, 2014 4:44 am
Location: British Columbia, Canada

Re: Bad video output? But why?

Post by Jeff » Mon Feb 25, 2019 11:43 pm

Yes.. the gaps are normal.
Screen Shot 2019-02-25 at 3.42.08 PM.png
Screen Shot 2019-02-25 at 3.42.08 PM.png (1.5 MiB) Viewed 1494 times
/Jeff
Image

dave
Site Admin
Posts: 561
Joined: Tue Sep 09, 2008 5:24 am

Re: Bad video output? But why?

Post by dave » Mon Feb 25, 2019 11:45 pm

Double checking the 600 Rev D schematics in the repository, it's supposed to be that way. The output of the video shift register is ANDed with a pulse derived from the system clock, called "Dot Shape" on the 540 boards, and "DB" (dot blank?) on the 600 board. It's derived from the crystal oscillator, so it's a fraction (1/4 or 1/2) the duration of the dot (depending on 32- or 64-char mode) and appears to be the same regardless of 32- or 64- column mode, so it might look more pronounced in 32-column mode.

I remember first seeing this when I converted a GoldStar 12-inch TV for a friend. I hadn't noticed it on my converted Boshei TV because the video amp of the Goldstart had a higher bandwidth. Modern LCD monitors have a much higher bandwidth, so you'll see the blanking very crisply. If you want to stretch the pulse, you could replace the U62 inverter with a 74ls123, or similar. Or, you could hang a small capacitor across the video out to smear the dots a bit. Or, switch to an old fashioned CRT monitor. . .

Cheers,

Dave

dave
Site Admin
Posts: 561
Joined: Tue Sep 09, 2008 5:24 am

Re: Bad video output? But why?

Post by dave » Tue Feb 26, 2019 12:00 am

dave wrote:
Mon Feb 25, 2019 11:45 pm
If you want to stretch the pulse, you could replace the U62 inverter with a 74ls123, or similar.
By that, I mean route the signal to a 74ls123 one shot, and use the /Q output for the DB signal, instead of routing the signal through the inverter gate in U62.

Cheers,

Dave

dave
Site Admin
Posts: 561
Joined: Tue Sep 09, 2008 5:24 am

Re: Bad video output? But why?

Post by dave » Tue Feb 26, 2019 12:04 am

Or, try cutting the trace between pin 8 of the U18 inverter and pin 18 input of the NAND at U56, and tie pin 18 of U56 high (with a 10k pullup). I'm not sure what artifacts the dot blanking signal is suppressing, but it may be worth a try.

fharo
Posts: 4
Joined: Fri Jun 22, 2018 6:19 am

Re: Bad video output? But why?

Post by fharo » Tue Feb 26, 2019 11:47 am

Thank you very much, Jeff and Dave!

I breathe calm now! My SBII is in perfect condition :-)

I will try to found an old monitor in the second hand market here in Spain!

Thanks again.

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