| Posted By
Mince
on 2020-06-26
14:18:34
|  Quality of S-Video image from a Plus/4 - mine looks bad
Hello,
I have a Plus/4 with an S-Video cable to SCART and another to the mini-DIN. Connecting it via either cable, to a variety of displays, though a variety of analogue to digital converts, results in an image that isn't great: the S-Video looks much like composite, with ragged edges with high contrast and colours with patterns on them. The text on the power on screen also has purple stripes on one side and green on the other.
I've tried making a composite connection and it looks much the same. Connecting just the luminence part of the S-Video signal gives a good, monochrome image that looks clear, so I think the problem is with the chroma part.
I have heard mention that the voltage level on a Plus/4's chroma is sometimes high and you need to use a more powerful resistor on it for modern displays: my cable has a built-in 300 ohm resistor but I've heard it suggested you might need to go up to 20 kiloohms (from the C64 wiki site).
Before I start dismantling my cable and fiddling about, has anyone had this, or is it a known problem caused by someone else?
Thanks in advance.
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Posted By
siz
on 2020-06-27
02:13:17
| Re: Quality of S-Video image from a Plus/4 - mine looks bad
According to TLC:
There's also a small design flaw in all 264 series machines: there is a ferrite bead in series with the common ground point of the video out port, which (i.e. the common point that has non-negligible resistance above the 1-2MHz mark) causes crosstalk between the signals. (This is the main reason why the separate luma-chroma output image of these machines looks bad... there is so much crosstalk between luma and chroma that it introduces visible moire on the image). All in all... if you want to get rid of a disturbance factor, find and short the FB between the machine's and the socket's ground.
The fix:
Just short the FB with a piece of wire on the underside of the PCB.
Easily reversible that way.
On the C16 and C116 it's FB13 and on the Plus/4 it's FB7.
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Posted By
MMS
on 2020-06-27
03:51:32
 | Re: Quality of S-Video image from a Plus/4 - mine looks bad
Also worth to mention, that the Y/C output of the C64 and C+16/C+4 are not according to the standard.
It is evident because the S-video standard formulated in 1987, while the release of the C=machines were at 1982 and 1984.
From the "264 Hardware specification" book written by Dave Haynie and Bil Herd, the vieo output signals of the 264 series is defined as 1V P-P (pitch to pitch).
If you compare it to the S-Video standard (not that easy: most of the document refers to the IEC60933-5 standard, costs 25€ just to view it), it defined maximum 0.7V level for Luma, and ~0,85V maximum for Chroma.
Fortunately there are some other good document than IEC.
So, clearly the Commodore signal is too strong for the TVs, and it causes overshoot image and/or color burst (it depends on the TV's electrical circuits, how they manage is)
Probably that's why the picture is SO much better and crisper on a Commodore monitor, despite it uses the very same signals: because the Commodore monitor EXPECTS 1V level, and is shows the perfect picture as comes out, while the TVs ar just confused (my Dell Ultrasharp monitor with SVideo and Composite inputs just refuses to show anything from Commodore. for along time I thought it is the synch (it is told to be Dell is picky to that) but now for me it is evident it is just the too high signal level)
To become more compatible with the Svideo connector at side panel or with the Scart Y/C, you need to add a resistor too, to decrease the signal level from the 1.0V to max. 0.7V at Luma(Y) and max. 0.85V azt Chroma (C) lines.
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Posted By
siz
on 2020-06-27
03:51:20
| Re: Quality of S-Video image from a Plus/4 - mine looks bad
... and that's because Commodore Y/C output is NOT S-Video (the standard didn't exist back then when these computers were made)
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Posted By
Mince
on 2020-06-27
07:05:40
| Re: Quality of S-Video image from a Plus/4 - mine looks bad
Thanks everyone for your help...
Thanks @siz, I've tried shorting across the terminals on FB7 (just be holding some wire across the terminals, rather than soldering just yet: to see if it made a difference) and that makes the picture worse. What's interesting, though, is that it makes the picture worse in exactly the way that it's already bad, so there's even more colour bleed, so maybe I'm in the right area.
The voltage level problem that @MMS talks about, I think, is the problem described on the C64 Wiki page about the A/V jack and I assume is supposed to be solved by my A/V cable having a 300 Ohm resistor on the chroma line: I've checked that with a multimeter and it is there (and all the other lines have no resistor). Although that page also says it might need to be larger (at least on a C64, but I assume it applies to the Plus/4 as well):
https://www.c64-wiki.com/wiki/A/V_Jack#Pin_assignment_of_s-video_and_Scart
I think I might just have to live with it: I don't want get an old CRT monitor. Although any further suggestions welcome!
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Posted By
MMS
on 2020-06-28
03:56:56
| Re: Quality of S-Video image from a Plus/4 - mine looks bad
I just checked the C64 Wiki, thanks the info.
Actually the signal levels also not explained there, "just do it like this", but they do not explain, why.
The Composite could look better, as the standard (and the TV) also expect 1V peak signal, and this is what the Plus/4 provides too. On the other hand, the mixing of the synch, color and luminance could be problematic as the equipment and it' component ages (it is valid for the Commodore and the TV set too)
https://en.wikipedia.org/wiki/Composite_video#/media/File:Composite_Video.svg
I think this is the value what the project team (or Commodore) copied, due to some resons the SVHS ( S-Video) standard makers decided to use lower voltages for the signals.
Actually if you check the standard, the Luma line also needs the a resistor, even a higher valued one.
The Luminance (Y) signal peak is fixed 700mV at 100% white, while the chroma could increase to 850mV. (Well, actualy I am not 100% sure that 300/330 Ohm are the correct values, but some other ppl seems tried it out, so I believe they are correct, but the C64 Wiki also mentioned 2000 Ohm.
As I see it is more about the TV's circuit's tolerance (what does it do with above standard signals) than about the difference between the standards.
Maybe two independent fine potmeters with 300-3000 Ohm value could be the best solution on both Luminance and Chroma signals.
The cheapo solution is a normal liner potmeter, up to 1K or 2K or 2.5K
https://www.hestore.hu/prod_10028894.html
As I see, there are 3000 Ohm helipots, allow very fine adjustment from zero up to 3000 Ohm.
https://www.hestore.hu/prod_10036580.html
@siz may able to provide a proper value to close the gap between the 1V to 0.7V on the luminance, but it is beyond my humble skills, I did not count anything similar in the last 25 years. (75 Ohm should be the cable, but on DC it is zero, just on AC it is measurable (capacitance and impadance of cable together)
The quality of the cable (shielding) also plays a major part here. When I used cheap single, rigid AV cable juts for the video, I had much worse picture than with a properly shielded cable. But the picture with SVideo was always worse than the composite, but I've never applied any resistance to the Y/C signal (yet)
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Posted By
Chronos
on 2020-06-28
12:27:49
 | Re: Quality of S-Video image from a Plus/4 - mine looks bad
Apply TLC's hack, the improvement is dramatic!
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Posted By
Mince
on 2020-06-28
12:57:53
| Re: Quality of S-Video image from a Plus/4 - mine looks bad
I tried the TLC hack of shorting FB7 but it just made the picture worse on my Plus/4 (in the same way it was already bad).
I've ordered some 2kΩ potentiometers and I'll rig them up in line on the Luma and Chroma signals and then have a twiddle to see that does. If that improves things, I can make up a cable with some fixed resistors in it, when I measure the potentiometers.
If I can't get a good picture with that, I might make up a switch to turn off/on the Chroma signal then I can turn it off to get a monochrome image from the Luma only, which might be better for editing text, although that's not ideal!
I wonder if everyone's Plus/4 is different. Of course, there could be something gradually going wrong with mine.
As an aside, several people have said the S-Video output on the Plus/4 is not good. Do people mean that it's no better than composite? When I tried composite, I got pretty much the same quality of signal as I get from the S-Video. It's not unusable but just lots of red and green fringes to the characters and a checkboard pattern gives red and green colour bars through it.
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Posted By
Sigmoid
on 2020-08-11
04:31:00
| Re: Quality of S-Video image from a Plus/4 - mine looks bad
I'm wondering if anyone looked into removing the rf modulator from these machines. There are kits available for the c64 to replace the modulator with a good quality amplifier circuit for the luma and chroma lines, and | wonder if these might be compatible with the plus4 or c16 - I kinda doubt it though.
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Posted By
MMS
on 2020-08-13
16:50:22
| Re: Quality of S-Video image from a Plus/4 - mine looks bad
As I saw on the picture, they added one :-?
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