Blitterwolf: S-Video From A520

article from Dave's Amiga Hardware Page

The A520 video modulator was made by Commodore to allow TV sets to display Amiga graphics. It provided a standard UHF RF output as well as a standard composite video output. Unfortunately, with the exception of a few early versions, the A520 has atrocious video quality - being little better than VHS. Fortunately, by the deletion of a few components, and a little rearranging, it is possible to have it generate full bandwidth S-Video.

Why S-Video?

Below are some sample images showing a comparison between a standard A520 on the left and an A520 with the S-Video modification on the right. Images were taken from the A520, which was attached to an A1200, using an A4000 with a Vlab framegrabber. The unmodified images are darker than they should be because the ratio of the synch amplitude compared to the actual video signal was incorrect in the standard A520. The modification also corrects this. Click on the images to view the full sized versions.

PM5544 test pattern

The standard A520 is quite dark and very smeary. The S-Video version is much better, but shows slightly less colour saturation (see text)

Above is a multiburst pattern

The standard A520 is incapable of resolving anything above 2.4Mhz. The S-Video version is exhibiting severe moire in the 4.7MHz and 7MHz bursts. This is due to the different pixel sizes in the A1200 compared to the Vlab compared to your monitor. This moire is much less visible when viewing on an actual monitor, and does not occur at all on a CRT monitor. All images have been resized to the correct 4:3 aspect ratio.

Above is an Amiga Workbench screen. No need to comment here!

Another way to visualise the difference between the modified and unmodified A520 is to examine the output on an oscilloscope. Below you can see the two top images are from a standard A520, the two bottom ones are from an S-Video modified A520. In the top left is a colour bar signal. Note the rounding in the corners of the colour signals as well as the leading edges of the luminance portions. Also obvious is leakage of the colour oscillator signal used in the A520 as shown in the thickened traces in non-coloured areas, especially obvious in the sync tip and the back and front porches. In the top right is a multiburst signal. As you can see the quality is terrible. The colour leakage is especially noted here, as I have enlarged the trace for easy viewing. Also easily noted is jitter in the colour burst.

The bottom left image is the same colour bar signal, but now through the modified A520. Note the composite signal is now two separate signals, the luminance (B&W) signal on top, and the chrominance (colour) signal on the bottom. Note the absence of rounding in both signals as well as the absence of colour leakage in the luminance portion. Jitter is much reduced, being barely visible in the colour burst and in the colour bar signal. On the bottom right, the multiburst signal now looks the way it should....almost flat to 14MHz on the right. This means that you have potentially the same luminance resolution as if you had a RGB monitor. The standard A520 was lucky to get to 3MHz. Incidentally, another feature not readily noted from the traces above is that the sync signal is now at the correct 0.3V level. The standard A520 had it at about 3.5V.

One problem that existed with the earlier version of the S-video modification was weak colour saturation. Investigation revealed a design fault by Commodore - the MC1377 colour encoder chip used in the A520 requires 1 volt RGB signals for full saturation. The Amiga provides 0.7 volt RGB signals, which strictly speaking is correct as the addition of the 0.3 volt synch pulses gives the correct 1 volt video. To cure this I originally intended to modify the Amiga to generate 1 Volt RGB signals, however, as this requires a different mod for each Amiga model, and also results in non standard RGB signals, I chose not to proceed with this. While not a 100% solution, I was able to get a worthwhile improvement in colour saturation by a simple alteration of the existing modification.

How to do it - step by step

Firstly you need a standard PAL A520. You will also need an S-video compatible monitor, otherwise there is no point to doing this. Apologies for those in NTSC countries - I live in Australia, and I have never seen an NTSC A520. These modifications may not work on NTSC units. I do know however, that pin 20 on the MC1377 IC is grounded for NTSC, there is a solder pad for this on the PCB. On the other hand, from what I understand, the way the NTSC A520 handles the synchronisation of the colour subcarrier to the Amiga is completely different. In any case I tried changing the jumper and replacing the 4.43 MHz crystal with a 3.5MHz crystal with no luck. All I got was B&W. The oscillator circuit was not working.... Be aware that conversion to S-video will render the RF modulator inoperative, as of course, there is no way to broadcast S-Video on a standard RF carrier. The existing composite output connector on the side will now become the Y or luminance output. The existing audio input socket will become the C or chroma output. If you have an Amiga with a monochrome video output, such as the A500 or A2000, you can get a further slight improvement in quality by connecting the Y lead to this instead of the Y output on the A520.

Items to be changed are highlighted in coloured boxes. Red boxes denote components no longer required, and they are simply removed. Yellow boxes denote items that are either changed or are added. Refer to the designations on the silkscreening on the PCB.

Remove C1 and L1 (may already not be present)
Remove K2 (the coil), C8, C20, C21, C24, R1, R2, R3, R6, R8, R14, R16, R19
Remove K1 (the delay line) and place a 270 ohm resistor between pins 2 & 3
Add 0.1 uF capacitor between pin 10 of the IC and ground (underneath the PCB)
Remove C18 and replace it with a 330 ohm resistor
Cut PCB track to base (B) of transistor Q1 (marked with a cross on the picture of the PCB underside)
Connect with wire the C20 end of R15 to the now isolated base (B) of transistor Q1 (underneath PCB)
Cut the wire leading to the audio input socket (inside the modulator shield - as denoted by red square in picture)
Connect the now isolated audio socket to the leftmost pin on the white header (with letter "V" marking - as denoted with yellow square)
Cut track to pin 8 of the IC (marked with cross on the picture of the PCB underside)
Connect with wire pin 8 of the IC to pin 3 of the former K1 (now occupied with 470 ohm resistor - refer step 3)
Change R7 to 180 ohms
Remove R20 and R13 and replace each with a wire link
Change R11 to 1K
Add a 220 ohm resistor to the formerly vacant R18 position (there are no holes for this, you may need to drill them or place the resistor under the PCB)
Add a 1M resistor from pin 17 of the MC1377 to +12 volts (underneath PCB)
Remove R5 (if present) and replace with a 2.2M resistor
Add a 470K resistor from pin 19 of the MC1377 to +12 volts. (underneath PCB)
Add a 220uF capacitor to pins 3 & 4 of the former K2. Positive of capacitor to pin 4
Add a 56pF capacitor from the base of Q1 to GND.(underneath PCB)
You will need to make an appropriate S-video lead with 2 RCA plugs to suit the new A520