VOGONS


First post, by matze79

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Hi,

i trying to connect my CGA Card via Composite to JVC Video Monitor but the Picture is running, any ideas what i can do to get it fixed ?
It should output PAL.

i really would like to save some Space by using it, as its stackable.
I have similar issue with Raspberry Pi Zero regardless what setting i use (sdtv_mode=2... sdtv_mode=0..)

But C64 and KC85/2 Computers are working fine on it.

Thanks

Matthias

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Reply 1 of 13, by Jo22

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On my Commodore 1702 I used to use the V.Hold knob for this, I think.
It changed the internal Oscillator in a way that either a 50Hz or 60Hz signal could be displayed.
The monitor stayed PAL, of course, so an incomming NTSC signal looked monochrome.

Edit: Found a reference to the V.Hold thing here:
https://www.metv.com/lists/13-things-nobody-m … -about-1970s-tv
😀

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In what to one race is no time at all, another race can rise and fall..." - The Minstrel

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Reply 2 of 13, by matze79

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There 3 hidden Pots in the Front, with just 3 Holes but i have no Idea which one is Hold.

I try to find some Service Manual.

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Reply 4 of 13, by Benedikt

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The ATI Graphics Solution in your picture is definitely not going to output a PAL signal via its composite port.
It's either NTSC or black and white, both usually with 60Hz.
However, you could write a TSR driver that intercepts video BIOS calls and sets the CRTC registers to values that result in 50Hz.

Reply 5 of 13, by matze79

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Should not matter, the JVC Manual says it accepts PAL and NTSC.

As the same issue happens with Raspi Zero but not with any 8Bit Computer 😒

EDIT:
Seems the Handbook is incorrect.

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Reply 6 of 13, by matze79

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Raspi works now..

Cable was broken what a pitty
And Handbook is wrong, only PAL accepted.
Setting Raspi to NTSC yields same result as CGA Card.

Thanks for Help

now i would still need to search a Monitor with NTSC Support 😒

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Reply 7 of 13, by Jo22

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The Raspberry Pi's Composite output is tricky, though.
It is entirely software-based and uses PWM (pulse width modulation) method.

If memory serves, there are many tweaks for the config files.

Like overscan enable/disable etc.
Not sure if this is helpful, though.

Also, the CGA likely outputs progressive scan instead of interlacing.

This means, I assume, that the lines are not sent in an odd/even manner. Rather, only one of two fields is sent.

That's one of the reasons as to why old video games are limited to ~240/~288p, I assume.

https://en.wikipedia.org/wiki/Low-definition_television

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Reply 8 of 13, by mkarcher

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Jo22 wrote on 2021-01-17, 17:09:

Also, the CGA likely outputs progressive scan instead of interlacing.

This means, I assume, that the lines are not sent in an odd/even manner. Rather, only one of two fields is sent.

Yes, the CGA outputs progressive scan. A "real" interlaced video signal in system M (the official name for the 60Hz video standard used together with the NTSC color encoding) has 262.5 lines per frame. As one of the fields start with the beam doing the horizontal retrace, whereas the next field starts with the beam being just in center horizontally, in interlaced video, the scan lines of the two fields are shifted by half a line.

The CGA instead outputs 262 lines per frame, with the vertical sync starting at the same phase in the horizontal cycle, and at the same line number. This causes no shift between successive fields, creating 60frames per second progressive scan instead of 30 frames/60 fields per second interlaced.

Reply 9 of 13, by happycube

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I think you're confusing that with the analog audio on the combined port used by the Pi 1 B+ and later.

Jo22 wrote on 2021-01-17, 17:09:

The Raspberry Pi's Composite output is tricky, though.
It is entirely software-based and uses PWM (pulse width modulation) method.

Reply 10 of 13, by Jo22

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happycube wrote on 2021-01-17, 17:40:

I think you're confusing that with the analog audio on the combined port used by the Pi 1 B+ and later.

Jo22 wrote on 2021-01-17, 17:09:

The Raspberry Pi's Composite output is tricky, though.
It is entirely software-based and uses PWM (pulse width modulation) method.

Thanks for the hint!😅
Yes, I know that audio on the Raspberry Pi is PWM-generated up to this day (though it improved a bit). So I assumed it was similar on the CVBS output, as well.

I found out about Pi's audio when I considered starting my own high-altitude balloon project
using a Pi Zero for sending SSTV pictures..

It then became clear that these boards dropped the analogue audio and that the alternatives were to use an USB soundcard or the GPIO headers.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 11 of 13, by Benedikt

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Potential modifications:
If your JVC monitor uses a TDA4510 as PAL decoder, you could convert it to NTSC by replacing the TDA4510 and its support circuitry with a TDA4570 NTSC decoder and its support circuitry.
JVC is a Japanese brand (NTSC territory), so a sufficiently flexible PCB design would not surprise me. Caveat: You would loose PAL support.
Since the monitor converts the signal to analog RGB anyway, adding analog RGB inputs would be another option. Add a few logic ICs and you get proper digital RGBI inputs.
As far as V-hold is concerned, I would add a second trimmer pot in series with a little switch in parallel to the existing V-hold trimmer pot to simplify switching between 50Hz and 60Hz.

Reply 12 of 13, by matze79

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New Monitor New Fun.

Trying Funai Analog LCD TV now.

The Card seems to switch between Mono and Color all around.
If i push DIP Switch 3 up and down during running it sometimes gets stable in Mono.
But only for a short Period.

i cleaned switch with contact spray, checked Afterwards still no success.

i guess 74LS166 Shiftregister is used to generate Composite Signal on the ATI Card ?

Thanks

Matthias

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Reply 13 of 13, by matze79

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If you have Problems with Funai TV and CGA Output add 75 Ohm Resistor in your Video Cable and Composite work fine.
The Switch was bit molty and also causing Problems.

Got it running!

EDIT:
still not 100% stable, but much better.

https://dosreloaded.de - The German Retro DOS PC Community
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Co2 - for a endless Summer