Almost the ultimate cheap RGB->HDMI adapter

[towmater]

I bought a Hyperkin Sega Genesis RGB-to-HDMI cable from Amazon for my 32X, and it came with a short adapter for use with a Model 1 Genesis, so I thought... this must be easier to use with an ST than dragging out my 15KHz monitor, or using an bulky wall-warted SCART-to-HDMI convertor.

 

In short, after soldering on an ST connector, it works on a 1040 STF, but I have an issue on which one of you may have some insight...

 

Wiring the comp sync line from the ST to the Csync line on the Genesis side results in the HDMI screen blanking out every few seconds. This is something I saw before with a Sophia on the 1200XL, and that was cleared-up when I used a DVI-to-HDMI dongle to insert the XL's audio.  In this case, however, the audio signal is available on the ST monitor port and is wired to the Hyperkin adapter, so it probably isn't a case of missing audio packets on the HDMI side.

 

I then pushed the ST Csync line into the Genesis CVBS line on the Hyperkin adapter, and the monitor image properly remains on, but jitters a bit too much, was in a half-inch from side-to-side constantly.

 

Hopefully our Wunderkind can suggest what might be off here? Some pinouts show the Csync line on the ST being labeled as "TTL" where the seperated H/V sync lines are not labeled as such, should I try combining those instead? 

 

Edited July 4, 2019 by towmater
clarification

[ParanoidLittleMan]

First question: from where it gets power ?  Surely not passive cable, so needs power - voltage ?  And I got answer thanks to allmighty search portal  - it is on HDMI connector, +5V from TV/monitor ?

 

Yes, it is worth to try with sync lines. However, that might be not simple because low levels - maybe with Schottky diodes - they have much smaller voltage drop. With 5V from monitor there are diverse ways how to solve it.

Or just to divide that C-sync from STf (without RF mod.) - some simple resistor based divider.

Here to add that Atari ST makes not interlaced video signal - so is not exactly by old TV standards. That caused problems even on some older TVs . Probably the reason for that jittering when connected to CVBS .

[Christos]

Your issue is that the ST's signal is incompatible with your TV. You need a TV that has more tolerances. I remember reading somewhere that there is a mod you can do on the ST, but don't quote me on that.

[ParanoidLittleMan]

It's not TV but monitor - and yes, it is incompatible with ST's signal, that's why it needs RGB-HDMI adapter ?  In other words, Christos is missing the point here.

 

I had same, periodical blanking with compatible TV, via Scart connector - even talked here about it. Problem was in driving switching line - pin 16 .

But in this case it is adapter designed for Sega, and that's most likely the problem. Still, idea about correcting C-sync may solve it.

 

[towmater]

2 minutes ago, Christos said:

 but don't quote me on that.

I tried 45 different TV's, I don't think I'll find one with more tolerances!

[towmater]

1 hour ago, ParanoidLittleMan said:

Still, idea about correcting C-sync may solve it.

 

I think it wants to see certain sync conditions. I'll have to look into some of the other Hyperkin HDMI convertors (I know there's one for N64 -which is not S-Video, and one for PS2, which definitely is RGB), so perhaps one of the other versions uses separated sync signals.  In any case, some success, I did just get the Genesis one to work by adding a 100Ohm resistor between the composite connector (on the Genesis end) and the comp sync pin on the ST side. Since the STF doesn't generate composite video, I'm certain what I'm viewing on my TV is an RGB image. I think the result can be cleaner, so I will examine some suggestions for combining H/V sync to see what happens, but for now, it is not a bad deal for US$29.

[towmater]

Interestingly, the "no signal" display from a $35 Amazon generic SCART-to-HDMI and the Hyperkin converter is the same, I gather some generic circuit is involved.

[towmater]

Since we are on the topic of Hyperkin HDMI convertor hacking... sort of, 

 

Looking for clues inside the Hyperkin cable, I spotted this unpopulated trace. According to the manual "if the cable doesn't self power, use the supplied USB cable."  

 

Without that diode, like Newman told Redford, "It ain't gonna happen."  The convenience of not needing a USB cable and wall-wart far outweighed the $29 risk, so I added the missing diode, and voilá, the thing is now self-powered. In order to use this with the ST, though, I'll need to drop the monitor port's available 12V to 5V using a regulator.

[Forrest]

I was able to connect a 1040ST to a Sony KV-20XBR by building a cable that connected HSync and VSync using 2 resistors and 2 diodes. This information may be helpful to you. I've attached my instructions from 1987

 

Atari_ST_to_Sony_Monitor_cable.pdf

 

[ParanoidLittleMan]

14 minutes ago, towmater said:

....

 

... I added the missing diode, and voilá, the thing is now self-powered. In order to use this with the ST, though, I'll need to drop the monitor port's available 12V to 5V using a regulator.

 

Did you check is there +5V under work really ?  I say this because that 12 V on STs monitor connector has very limited current - it goes via resistor of 1.2 K, so max current what it can supply is about 12-5=7 V/1.2 K = some 5 mA - very little . And that even could be what makes it working not good.

[towmater]

1 minute ago, ParanoidLittleMan said:

Did you check is there +5V under work really ?  

Sorry, I didn't state it very clearly. The 5V works on the original 32X for which it was made. I didn't plan tp power it via ST until I tried the sync-combo approach, so thank you for stopping me from making that mistake in using the 12V.

[towmater]

2 hours ago, Forrest said:

I was able to connect a 1040ST to a Sony KV-20XBR by building a cable that connected HSync and VSync using 2 resistors and 2 diodes. This information may be helpful to you. I've attached my instructions from 1987

 

Thank you Forrest, although that circuit still left me with sporadic blanking, when I tried combining your circuit with the original TTL composite sync signal, I have been left with a stable image, so for now this iteration is the best image quality, I'll see how it holds up when I run some games through it.

 

Not having to use a dedicated VGA monitor for the ST is a great advantage. Hopefully I'll be able to get this working with monochrome mode as well.

[ParanoidLittleMan]

When I talked about resistor based divider - meant that should try with dividing that C-sync signal - best and simplest is to use variable resistor of about 1 K Ohm, connect 1 end to GND, other end to Atari C-sync sugnal and middle (adjustable) pin to C-sync (composite in) of converter. Then may check by adjusting will it go stable in some position.