GBS-82XX experiments part 2

GBS-8200 and GBS-8220 experiments part 2

Work has continued, experimenting with these low cost but potentially highly capable video adaptor boards.

Fixing the random speckles on the display

I believe I have found the cause of the occasional white speckles seen on the display. The default setting for the GBS-82XX board is to clock the 166 MHZ speed grade SDRAM at 162 MHz. It appears to be worse if the Hynix HY57V643220DT-6 is fitted compared to the EtronTech EM638325TS-6G device, which appears to be fine.

I measured the SDRAM clock at 162 MHz and recorded this:


Whilst the signal is a bit noisy, it does not violate the +/-2V overshoot/undershoot limits of the SDRAM devices used.

The simplest fix was to reduce the SDRAM clock speed to 129.6 MHz, with a single I2C write. This has fixed the issue. Halving the SDRAM speed to 81 MHz caused distortion on the video, the next speed increment of 108 MHz was sufficient for 1360×768 pixel output. A proper fix would be to adjust the timing of the DQM strobes with regard to the data bus as on SDRAM the DQM strobes clock the data out of the SDRAM. If this is adjusted, you also need to consider the timing of the SDRAM clock to the control bus (RAS, CAS, CKE, CS, BS0/1 and WE) and the Data bus. This is not too difficult if you have the PCB artworks as you can measure the PCB track lengths and adjust the timing by 7ps/mm. With the GBS-82XX board, it would be tricky and fraught with false starts. We could of course measure it and adjust accordingly, or reduce the clock speed by 20% and have more timing margin (an extra nanosecond) on the clock.

Now that the speckling problem appears to be fixed, I am looking at some general video quality issues. When using a 50 Hz PAL screen-mode, scan-converted to 60 Hz, there are some noise bands, particularly noticeable on a grey Workbench background. It is hoped that some digital filtering/sampling will help alleviate this.

Synchronising the GBS-82XX

I have experimented with different ways to synchronise the GBS-82XX device. As those of you that have experimented with the board now, sometimes it can be a bit ‘hit and miss’ with the video source (games console or computer).

When using the GBS-82XX with the Amiga, this is the cable I use:

The 680 ohm resistor is essential. It reduces the CMOS Composite sync signal from the Amiga to < 3.6V. The TVIA-5725, the device under the big heatsink, accepts a maximum voltage of +3.6V and a minimum voltage of -0.3V. The Amiga’s video sync, measured on the A1200, looks like this:Amiga_CSYNC_680R_GBS8220
The TVIA-5725 accepts a 3.3V TTL signal, via a Schmitt input, this changes logic levels slightly so a logic 1 is 2.4V to 3.3V,  and a logic 0 is 0 to 1.0V. The 680 Ohm resistor works as the GBS-82XX follows the Vesa VSIS specification, which requires synchronisation inputs to have a 2K impedance to ground. My 680 ohm resistor (+47 ohms in the Amiga), creates a potential divider, reducing the signal level to a safer level. This is cheap and easy to implement and does not degrade the video sync significantly.

A number of people use the venerable LM1881 video sync separator, any device or circuit that uses this, should have the 680 ohm resistor added as shown above. The LM1881 outputs 4-5V sync levels that are not compatible with the GBS-82XX/TVIA-5725. Alternative devices are the EL1883 or the LMH1980 but they are not available in hobbyist friendly DIP packaging.

I have tested the LMH1980 with the Amiga, I used the composite video output of the A1200 to create LVTTL (3.3V) HSYNC, VSYNC and CSYNC. The GBS-8200 I have synchronised with the CSYNC perfectly, the same as using a resistor. It did not work reliably when I supplied separate HSYNC and VSYNC, I had a very wavy display.

A final note, the GBS-8200 did synchronise to the Composite video output of the A1200 but it is not recommended. The 2V (approx) video contains colour information that is not filtered out and would in all eventualities, cause problems at a later date.

Cleaning up the power supplies

I have been reading numerous web-forums on GBS-8200/GBS-8220 problems to look for common trends and solutions. One topic that crops up a lot is related to the power supplies. I have already proven, in part one, that a 5V 2A supply is not required.

During my testing, I noticed an occasional, high frequency, burst of noise, on the +3.3V supply. I traced it back through the circuit to the power input. Changing the timebase of the oscilloscope, I spotted something important, it happened at 20ms/50Hz intervals. In the UK and Europe, the AC mains operates at 50Hz so when you see a 50Hz noise pulse, you know where it came from. Currently my GBS-8200 is powered from my bench power supply, built 20 years ago, with 3 linear regulators and still on the original electrolytic capacitors.

Also connected to the +5V output was my 5V to 3.3V TTL buffer board. Two of the eight inputs were in use, the other six were floating. I made a mistake here. You should never leave TTL inputs unconnected, they will pick up noise and oscillate, in this instance, they picked up 50Hz mains noise. Quickly dis-connecting the buffer board, removed the noise. The 3.3V (switchmode) and 1.8V (linear) regulator supplies now have about 20-30mV of noise, perfectly acceptable.

To ensure I do not have any further conducted noise issues I added a clip on ferrite bead:

You can see the buffer board in the top right of the photo. Whilst this ferrite was a little large, it did cut the noise out. If you are using the DC power jack (the black plug) either pick a PSU that has a ferrite fitted or measure the cable diameter and purchase a clip-on ferrite from a local supplier or ebay. I spent two hours trying to work out where the noise was coming from, reading datasheets and measuring the board.

To date I have changed a single capacitor on the board. I briefly touched on this in my first post but after additional testing, I am happy to confirm it needs changing.
Here it is:

The 1.8V regulator is used for the core supply of the TVIA-5725. with a ceramic capacitor, I was able to cause the power supply to glitch by switching my overhead inepection light on/off or my soldering iron transformer. Since changing it to a 16V, 22uF, tantalum bead capacitor, this has not happened. The original capacitor, shown under Kapton tape, had an ESR of 0.02 ohm, the recommended range for the LM1117 (similar to the AMS1117 used here) is 0.3 to 22 ohms! The part I fitted had an ESR of around 2 ohms.

The software settings solution?

Some people ask what the final software settings solution will be?

My preferred option is to use an Arduino Nano like this:


(Image from

This would be used to read and write the I2C commands to the TVIA-5725 device which provides the video scaling functions, among others. It is readily available and clones can be cheaply procured, finally it can be easily updated using the Arduino environment.

My aim is to make the design data readily available, for free. This will include the video settings. I’ve seen too many scammers on ebay selling ‘Amiga modified’ equipment for extortionate prices, I will not have this solution exploited.

Another option, still using the Arduino approach is a module that piggybacks on the 8051 microcontroller clone on the GBS board. This would allow access to the onboard switches and no wiring. The downside is it would be more expensive.

Until the final settings are known, the end solution is fluid.

Until the next update.


38 thoughts on “GBS-82XX experiments part 2

  1. Hi ! I have the GBS-8200. I use it wth my SNES(US) and it is working well but every 5secondes when the image is scrolling, there is always a glitch. Is this the SRAM speed problem ? My board got the Hynix chip !

  2. Hi,

    Are you using a NTSC RGB SCART cable or similar?
    Looking at some diagrams online, the only potential issue I can see is that you are feeding the Composite video into the Sync input of the GBS-82XX. This signal is borderline stable. You need a sync separator to create a Composite sync signal. Syncstrik or similar device, then apply the 680 ohm resistor as I detail on this blog.

  3. It’s a NTSC RGB cable. I am getting Sync from the AV connector pin 3 wich is CSYNC Composite Sync. Do that signal also need a syncstrike ? I was assuming that this was a clean sync signal.

  4. Well I just did some more testing and it only happen with vertival scrolling. And the other kind of glich I have is around moving sprite there is always some pixels that are not suppose to be there.

  5. Patrice,

    Looking at the SNES schematics, the CSYNC signal, on pin 3, is a 5V signal, too high for the GBS-82XX. Add a 680 ohm resistor in series.

    The glitch around scrolling sprites is related to the de-interlacing algorithm, even though you have a 240p video image.

  6. Tried the 680ohm resistor on Csync. It, didn’t stop my vertical scrolling issue but the image seem cleanner.

    Tonight I will test the Rapsberry Pi Interface to see if that fix my issues and will tell you how it went.

  7. I have a Sega Teradrive PC connected to a GBS-8200 V4.0 via the VGA input. The Sega MD side 15khz VGA output displays fine, but the PC side 31Khz VGA output refuses to display properly, all I get is a distorted frozen screen. Which setting will help me fix this issue?. I also have an Amstrad Mega PC which gives the same results.

  8. Hi, could I build a simple device that just disables the deinterlacer option?
    I’ve got an at90usb mini dev board, if it helps any?

    1. Hi,

      I’m developing some Arduino software that will detect the input video format and configure the board accordingly. It will detect interlaced/progressive video and enable/disalbe the de-interlacer and the motion compensation blocks/scaler as required.
      The software will be released for free, you just need an Arduino mini/nano.

      Currently it detects the input video types, just need to finish the code to setup the hardware for the best picture.


  9. Quick question: “able to cause the power supply to glitch by switching my overhead inepection light on/off ”

    What exactly was this power problem doing to the unit? Was the unit resetting? Was the unit losing sync / messing up the picture? Was it shutting down completely?

    Likewise you mention that you’re filtering the noise from the mains – what were the symptoms when you had the noise?

    1. Switching the light on/off caused the screen to flicker briefly. An annoyance and potential for problems with the card as it fails to respond to power transients.

      The noise filter was added to remove some random speckles seen on the display, traced back to my sync buffer board.

  10. Hi. The owner of this forum: (requires registration) came to a similar conclusion about the snow, and went with a hardware mod.

    “The SDRAM interface from the TV5725 scaler chip on the board has no damping resistors in the clock & control lines and fails to calibrate the timing of the interface properly as a result. It is marginal and causes bad screen refresh data. As such it took some time to track this down as the cause. I spent quite a bit of time talking to the chip manufacturer (TVia) and working through data, and also tried to contact the company that designed the GBS-8200 board a number of times but got no response (even when I had found the cause and suggested a fix).”

    Cheers Nick.

      1. Hi,

        Have you added a 680R resistor in series with the Composite Sync (CSYNC) if you use a sync stripper?

        I seem to remember that the PS2 outputs Sync On Green, this causes the amplitude of the Green video to be 1.0V peak to peak instead of the expected 0.7V. This can affect the Analogue input stage as the black level clamp and dynamic range can be affected. I plan to test my PS2 with the board, when I can find it 😦

  11. Hi ianstedman.

    I´m using sync stripper with 680R, i try both sync (CSync PIN6 and Y-Luma PIN5) from PS2 AV Port. The quality of both is the same, i don´t tried sync from green PIN12 😦
    No matter the type of connection, tried by video component the result is the same. I tried with a Wii connected by component and the result was the same, a lot of digitalization, noises and blocks, like on PS2. May i have a faulty GBS? Try using a GBS V3 and PS2 in System Configuration screen, the image is poor.

  12. Hi ianstedman. It´s me again 🙂

    “Have you added a 680R resistor in series with the Composite Sync (CSYNC) if you use a sync stripper?”. No i don´t use 680R in series with CSYNC, i use only the 0.1uf cap. I´m using 680k ohms in LM1881 pin6.

    I will try to use the 680R in csync and return with the tests.


  13. Hello,
    I got the device but it seems broken, beside the snow and motion adaptive deinterlacing artifacts I also get a bizarre wavy effect on the screen no matter what or even if something is connected. I cant capture it on photos or film since it looks like what happens when try to record a TV but I know it’s not suppose to happen. Also there is the problem that the image randomly jumps with some games mostly on FMV sequences on PS2 and PS1 (havent seen it happen with the saturn yet).
    What can I do, should I try get another one or can it be fixed?
    I’ll be changing the cap and adding that clip on noise reductor for the PSU (I’m sorry for my casualness) since allot of people say it helps. But I have no idea how I can change the processor frequency…

  14. I performed some lag testing of this device. By the third video, you can see that the scaler is not producing a noticeable amount of lag (my issue was due to the TV being used):

    As for image quality issues with the scaler, it’s vastly superior to even SVideo connections, but it does indeed have small issues with “deinterlacing” in that motion next to a static line will cause the line to move. It is an inexpensive scaler – and you do get what you pay for. For what it is, I think it does very well, especially with the 8 and 16bit systems.

  15. hello.

    My GBS-8220 V3 is on its way and Im lucky to have found this webblog of yours ianstedman !
    I hope you can test out the GBS-8220 V3 the same way as you did with the GBS-8200 V4 and give us advice of what to do, to get it running better. s you mentioned in part 1 of your experiment “highlighted areas show a crucial difference, the V4, GBS-8200 board, has a switchmode power converter, whereas the V3 board has a linear regulator” and I hope you will find a solution for the V3 as well as you did with the Tantal to the V4.


    Have you tried swapping the Hynix SDRAM with the Etron SDRAM and did it fix the problems mentioned with it?

    kind regards

    1. Hi,

      I’ve looked for a GBS-8220, they seem hard to come by in the UK. I can find GBS-8920, which look identical to the 8220 V3, in that they have linear regulators. I suspect adding a tantalum to a GBS-8220 V3 in the output will also help.

      I have not tried swapping RAM. All SDRAMs are JEDEC compliant, if the board is a bit flaky with one, another is not guaranteed to be better. The only possible difference is that the Etron may be more tolerant than the Hynix, when used out of specification.

      1. I just bought mine of amazon with morning express option from china, it will be here over the weekend.
        Could you point out which smd cap it is I will have to swap out on the linear regulator?

        I had problems with Hynix Rams on several devices, they are not very reliable.
        Samsung Rams where the best to work with.

        On the GBS-82xx there aren’t only Etron EM638325TS-6-6G and Hynix HY57V643220DT-6, there are Samsung-K4S643232H-UC60 on some revisions too !

        I maybe going to test several Rams with my board, if it has the common speckles problem.

      2. I’ve started to buy quite a bit of small electronics from, found a few of the boards you ask for, although I’m in Sweden so may be different ordering to the UK from that site?

  16. friday bought and monday here. amazon express, before 12pm, wow !
    bought GBS-8220 v3 and did get a HD9800 v5 -.- stupid chinese …
    i wanted GBS-8220 because of the two vga outputs, damn.
    the HD9800 has two vga outputs, but one is only via connector and they havent send one with the board.
    i think i might keep the board atm cause it has Samsung K4S643232H-TC60 SDRAM on it.
    i tested it only in the menu atm and i didnt get any speckles or other interference.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s