If retro gaming in RGB is a hobby you enjoy you have probably experienced the problem of having fewer inputs available on your monitor or upscaler than RGB enabled consoles you want to have hooked up. The natural solution is to find some sort of switch box and many of us have turned to using RGB matrix switchers made by professional AV equipment manufacturer Extron. The Crosspoint series of switchers by Extron have become incredibly popular among retro gamers due to their high number of inputs and outputs and their relatively low cost when compared to other RGB switching solutions.
Lately, there has been a lot of discussion in the retro gaming community about c-sync and its potential to do harm to video equipment if not properly implemented into a retro gaming set-up. This is because there are actually two standards for c-sync with different acceptable peak to peak voltage ranges. The standard we as gamers should all be using is called 75 Ω c-sync which has a voltage spec of 300 mVp-p to 1.0 Vp-p. The other standard is called TTL c-sync which has a much higher voltage range of about 4.0 to 5.0 Vp-p. Some consoles output c-sync at TTL levels, like the SNES and Sega Genesis for example, and this is why your SCART cables need to have resistors and sometimes other components as well, on the sync lines in order to drop the voltage down to 75 Ω c-sync levels. If this isn't done and the TTL sync is fed into a device that is expecting 75 Ω c-sync, as most consumer grade video equipment does, then over time this will wear out the device's protection circuits and damage the device. One such device is the XRGB-Mini Compact Upscaler more popularly known as the Framemeister.
The Framemeister is a very expensive piece of equipment, so when I learned of the potential damage I could be causing it I decided to make sure everything in my retro gaming setup adheres to the proper c-sync standard. This lead me to discover that one of my SCART cables was missing the proper resistor. It also lead made me do a little more research on my Extron Crosspoint and discover that it has a feature called ADSP that is actually processing the sync signal.
When trying to figure out if my Extron Crosspoint was damaging my Framemeister my research made it clear that a lot of the retro gaming community was just as confused as I was. I could not find any definitive answer anywhere stating whether or not an Extron Crosspoint, a professional grade piece of equipment, was safe to use with consumer grade devices. What I did find was a lot of people disagreeing on two major factors, what ADSP actually is/does and what the DIP switches on the back of the unit do. The only way to clear this up once and for all is to clearly explain both, and to test the output on an oscilloscope. I decided to tackle this question so I borrow an oscilloscope from a retired electrical engineer friend of mine. His oscilloscope is old but it will get the job done and before we get to the actual testing, let's clear up those two major points of contention.
WHAT IS ADSP?
When I bought my initial Extron Crosspoint I wasn't paying any attention to which model I was getting beyond the number of available inputs and outputs. It wasn't until I learned of the different types of c-sync that I noticed the text on the lower right corner of the front panel indicating that the unit includes a feature called ADSP. Not knowing what this was I looked it up and found it defined in the manual for the 300 series of Crosspoints;
ADSP - Advanced Digital Sync Processing – An exclusive, all-digital process that regenerates the sync signal waveform and restores incoming sync level to 5.0 V p-p specifications. This ensures a sharp, stable image for improved signal compatibility with any LCD, DLP, plasma, or other digital display device.
For some reason, the meaning of this statement has been debated on forums but it seems clear to me that Extron Crosspoints with ADSP take all non TTL sync inputs and boost them back to 5.0 Vp-p, unterminated. Whether or not this is true is what I will be testing. If true, supplying this high voltage sync signal may be fine for professional equipment such as PVMs but could wreak havoc on the Framemeister.
SYNC TERMINATION SWITCHES
The other source of confusion is the DIP switches found on the back of most Extron Crosspoints. Extron calls these sync termination switches. They have two positions labelled 510 Ω and 75 Ω and indicate that they are for the first four inputs of the unit. Many people, including myself initially, have confused these as switches for toggling 75 Ω c-sync output but again, the manual makes its true function clear;
Sync termination switches — The CrossPoint 300 matrix switchers have sync termination switches on the rear panel for inputs 1 through 4 (figure 2-7). The switches provide a way to condition non-TTL sync levels greater than 5 Vp-p, enabling the sync to be properly passed from input to selected output(s). The matrix switchers have two sets of sync termination switches; one for horizontal or combined sync and a second set for vertical sync.
Each switch provides the option of selecting either 510 ohms or 75 ohms. The 75 ohms position is required only for an input with non-TTL sync, greater than 5 Vp-p. The normal position is 510 ohms.
Both of my Crosspoints have these DIP switches and they only apply to the input side of the Crosspoint. They are actually to reduce non TTL sync signals with peak to peak voltages higher than 5.0 Vp-p back down to a level that the Crosspoint can through successfully.
PUTTING IT TO THE TEST
These are the two Extron Crosspoint models I will be testing. The one I got initially is a newer model with ADSP, and the second one I purchased is older and doesn't have ADSP.
Top - Extron Crosspoint xi Series 12 x 4 RGB matrix switcher. I could not find any literature online specifically about the xi series of Crosspoints but the model number found on the side indicates that it is likely part of the 300 series. This model has ADSP.
Bottom - Extron Crosspoint Series 12 x 8 RGB matrix switcher. This model is older and does not have ADSP.
Both of my Extron Crosspoint's were tested under the same conditions. The sync signal was generated by my SNES mini that I modded myself for RGB output, running Super Mario World. To generate a load to test under RGB output was sent to my Sony PVM-8044Q RGB monitor. The signal was always pulled from the SCART head of the SCART to BNC cable connected to the PVM. Here is a shot of the test setup and the RGB mod I installed in my SNES mini.
In order to establish a baseline I tested the c-sync voltage directly from my SNES under load. The SCART cable I used for all tests was built by Retro Console Accessories and has a 330 Ω resistor on the sync line.
C-sync reading is approximately 830 mVp-p and falls with in 75 Ω c-sync spec as expected.
As I expected, the DIP switches had no impact on the sync output levels at all. To my surprise however, both Crosspoints performed identically. The older model that does not have ADSP also boosted the sync signal up to just shy of 2.5 Vp-p when under load. I had gone into this testing expecting the older model to not affect the sync signal at all and to be the better choice for use with my Framemeister but the oscilloscope doesn't lie. After seeing this in action I checked both manuals again and under the specifications for sync both manuals state;
Input level............................................... 0.5 V to 5.0 V p-p, 4.0 V p-p normal
Output level............................................ AGC to TTL: 4.0 V to 5.0 V p-p, unterminated
Both models of the Crosspoint output 4.0 to 5.0 Vp-p unterminated regardless of whether or not it has the ADSP function and when a load is applied this voltage gets reduced to approximately 2.4 Vp-p. According to the manual this output boost is done through use of AGC which stands for Automatic Gain Control. ADSP must merely clean up the input sync signal and not affect the output at all.
This leads me to believe that Extron Crosspoints will output 2.4 Vp-p for any console outputting c-sync. To test this theory I tested my RGB modded NES under the same conditions. Without the Extron the NESRGB generated a c-sync wave at 400 mVp-p. When passed through the Extron the output was again bumped back to 2.4 Vp-p. I also tested my N64 and Sega Genesis which had baseline voltage levels of 400 mVp-p and 500 mVp-p respectively and again, the voltage was boosted to 2.4 Vp-p by the Extrons. I now feel confident in stating that Extron Crosspoints should not be used directly with equipment expecting 75 Ω c-sync. 2.4 Vp-p is no problem at all for professional grade CRTs but if one of your Extron's outputs goes to a Framemeister's RGB input or something similar, you better add some resistance to the sync line to drop it back down to specification.
UPDATE - AUG. 30th 2017
Shortly after publishing my original findings I decided to add a 470 Ω resistor to my BNC to SCART cable to reduce the c-sync voltage coming out of either of my Extron Crosspoints back to 75 Ω specification. I was told by reddit user u/Typicalnervecell and Bob from retrorgb.com that this should do the job and I had also read that YouTube user neozeed1964 has had success with this as well.
I ordered some resistors from Digikey and once I had the resistors in hand I got right to work adding a 470 Ω resistor to my BNC to SCART cable originally built by Wookiewin on ebay. Here is a photo of the installed resistor shortly before covering it with the heat shrink tubing.
I retested both Crosspoints with my RGB modded SNES mini under the same conditions as my initial testing. The only difference is the added 470 Ω resistor on the sync line of the BNC to SCART cable.
Extron Crosspoint with ADSP. BNC to SCART cable with 470 Ω resistor on the c-sync line.
C-sync reading is approximately 600 mVp-p.
The 470 Ω resistor did the trick. It reduced the sync voltage to 600 mVp-p which falls nicely into the 75 Ω c-sync specification range. Adding this to your own cable is very simple and anyone with a soldering iron can do it. If you are currently using an Extron Crosspoint with a Framemeister, OSSC or similar device this is a necessary bit of work that will only cost you a few bucks. This solution will allow you to continue to enjoy the convenience and functionality of an Extron Crosspoint while ensuring a long life for your more expensive gear. If you want to purchase a 470 Ω resistor or two for your own cables they can be found here. Thanks to retrorgb.com for the link.