TTL signal level for sync signals

[necroware]

On VGA the color signals are analog values between 0V and 0.7V (actually all monitors are at least 1V tolerant) but the HV-Sync values should be at TTL levels between 0V and 5V. Unfortunately some devices are not 5V tolerant and could be damaged over time when using normal TTL levels. One of such devices is GBS-8200, which is only 3.3V tolerant.

In the past I experimented with different levels and it tuned out that all of the monitors and converters which I tested did actually work with the same levels for colors and sync signals with up to 1V, so I've chosen the resistors accordingly to stay under 1V for all signals.

Recently I saw slight instability on the H-Sync on some monitors when the computer attached was under load. A slight blurry shift at particular frequency. This is most probably because the H-Sync gets disturbed/unstable. Probably this can be mitigated by raising voltage levels of the sync signals.

Going up to 5V would be VGA specification conform, but as I wrote, some devices don't like it. The best would be to try to stay below 3.3V. This can be achieved by replacing 470 Ohm resistors R1 and R2 with 33 Ohm values.