Now that many dbGrafx Boosters have found new homes with happy customers, I received several complaints that the dbGrafx Booster did not cooperate well with the XRGB-Mini Framemeister and several models of PVM. This was very surprising to me since I test and verify each dbGrafx Booster in my workshop with a Sony PVM and an HDMI upscaler before shipping – could the Boosters be deffective on arrival?
The main complaint was that the image was very dark on RGB outputs. Here’s an example to the right: screenshot of a dbGrafx Booster connected to a Framemeister exhibiting the dark image problem. This could maybe point to a deffective CXA1645 (video encoder on the booster) but all of these customers also verified that the S-Video output worked correctly which also comes out of the CXA1645. Dark images could also be caused by improper resistance on the RGB lines, but this is not the case since the Booster’s RGB outputs are directly DC-Coupled (just like the Genesis 2, hence use the Genesis 2 SCART cables) which essentially means the chips RGB pins are connected directly to the output connector. Maybe the problem lies in the SCART cable then.
SCART Cable Differences
I had run into an issue before where Genesis SCART cables I had purchased from several Chinese sellers on eBay did not include any 75ohm resistors on the RGB lines. This makes for a very bright image and is totally incompatible with the Genesis’s RGB outputs – stay away! After this experience I set out to find a reputable manufacturer of retro SCART cables, stumbling upon www.retrogamingcables.co.uk – looks good right! I bough a few cables, tested on my PVM, perfect!
The common thread emerging from everyone who was experiencing issues with their dbGrafx Booster was the Genesis 2 SCART cable from retrogamingcables.co.uk. – ok so what’s up with this cable?
To the right is a picture of the insides of a Genesis 2 SCART cable from retrogamingcables.co.uk. As you can see, it includes 75 ohm resistors on the RGB lines (hidden underneath heat-shrink). But wait! There are no AC-coupling capacitors on the RGB lines! This cable directly DC-couples the Genesis’s RGB outputs to whatever your connect it to (upscaler, PVM, etc…). Ok now, most equipment “should” be able to accept a DC-Coupled signal but this is not guaranteed nor is it particularly good design practice to rely on this fact. All in all, this cable should have 220uF capacitors on the RGB lines and without them does not respect the original specifications of the Sega Genesis 2 RGB output. It may “work” with most equipment on a Genesis 2, but evidently, on a new design with a different video encoder this cables shows its weakness.
Other Genesis 2 SCART Cables
I ordered a bunch of other Genesis 2 SCART cables for comparison. As I expected, all other cables (not from China) included both 75 ohm resistors and 220uF AC-coupling capacitors on the RGB lines. This particular cable below was purchased from the eBay store retro_console_accessories (which was highly recommended by reputable people). This particular cable meets the output specifications for Genesis 2.
AC-Coupling vs. DC-Coupling
What is this sorcery? Can’t we include both like the rock band and all be happy? Well no. I don’t really want to bore anyone with mountains of circuit theory (nor am I a video expert by any means). So I figured it’s probably best to link a professional article for this. All in all, the outputs on the CXA1645 (and CXA1145 for that matter) are not designed to drive DC bias current into a 150 ohm load; this happens when there is no capacitor in the cable to prevent this.
Using a Proper SCART Cable
With a proper SCART cable including AC-Coupling capacitors the dark image issue is no longer. The screenshot is a dbGrafx Booster through a Framemeister using a proper SCART cable. A big thank you to RetroRGB for testing with a Framemeister because I don’t own one. Many other users have also confirmed to me that switching away from retrogamingcables.co.uk cables has solved their problems with dark images output from the dbGrafx Booster. Awesome!
This problem has left several obvious questions:
Why Make a Cable Without Capacitors?
The most obvious reason is cost and labour. Even though 3 capacitors like the ones required are dirt cheap, they cost time in labour to mount and install inside the cable. I figure not installing the capacitors probably saves at least 2-3 minutes per cable assembly. Also, retrogamingcables.co.uk is only concerned with Genesis compatibility with his cables. I 100% disagree with “oh it works without capacitors therefore it’s ok to remove them” – this is not good design practice in my opinion.
Why Didn’t Sega Include the Capacitors in the Console?
Other consoles, like the Neo Geo for instance, include the capacitors and 75 resistors required on RGB lines directly in the console. So if they’re required, why not just put them there and thus only require a cheap SCART with no built-in components? After all, electrically speaking, it is best practice to place these components as close as possible to the video encoder – not 6 feet away in some cable. Again, the answer is probably cost. Let’s say there are 20 million Genesis consoles in the US – almost all of which never saw the use of their RGB outputs. By not including those capacitors and resistors Sega saved on purchasing and installing 60 million capacitors and 60 million resistors.