How are the hsync and vsync signals encoded in the sync signal in a 15 KHz RGBS? I'm trying to view an old 8-bit computer with RGBS on an VGA monitor through an RGB to VGA converter. As it displayed a lot of noise on the margins, and also some blurred lines, I extracted the following waveform of the sync signal.

enter image description here)

I'd say that the positive polarity pulses are the hsync and the negative ones, the vsync, as there are much more positive ones, but I'm not certain if this waveform is correct because I can't find how both signals should be encoded.

PS. Notice also the small pulse before transitioning to vsync (an indication of vsync?), and the 4 pulses of vsync (why 4? shouldn't be just one?)


2 Answers 2


Using your own image (annotated):

Annotated sync diagram

The negative pulses are 64usec apart (15.625kHz) which is a standard line period, so those are line sync.

The longer period where the line goes low (and has positive edges) is the vertical sync. The vertical sync itself is just the low period; here you also have the (usual) inverted line sync signals which helps the system maintain the horizontal sync lock.

The vertical sync signal is not always aligned to the horizontal sync which is why you see the small pulses at the start and end of the vertical sync pulse.

The composite sync signal you have here is described by HSYNC XNOR VSYNC.


Looks like a bit non-standard sync format compared to TV transmission (not surprising for a home computer), as the double-rate serration pulses are missing near VSYNC lines. Which home computer this is by the way?

Anyway, the falling edge is always the start of HSYNC pulse, there will always be a constant time between two falling edges of HSYNC pulses. So normally during the picture, the CSYNC is high for the most time and there are short low pulses for indicating HSYNC. When it is time for VSYNC, the CSYNC signal is mostly low during the period between HSYNC pulses. But it needs to go high some time before HSYNC, as the falling edge signals HSYNC. So it's not just HSYNC XNOR VSYNC as it would invert the HSYNC edge polarity.

Basically HSYNC can be separated with a highpass filter that is sensitive to falling edges, and VSYNC can be separated with a lowpass filter that ignores the short spikes but responds to the slower average of VSYNC low time.

The VSYNC is active for more than 1 line period because it is easier to detect longer pulse with the lowpass sync separator.

  • \$\begingroup\$ It's an Amstrad CPC 464, from the 80's. \$\endgroup\$
    – perencia
    Apr 13, 2019 at 17:35

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