How to convert analog Composite Sync to TTL Composite Sync for RGB signal?

Hi,

I have an old computer (ZX Spectrum +2A) that outputs RGB signal (0.7Vpp 75 ohm) and composite sync signal at 0.48V (not composite video, only combined vertical + horizontal sync). I have a display device that can accept the RGB signal as is, but requires the sync signal to be TTL sync signal and not analog sync signal. Does anyone have any ideas how to take the analog sync signal and convert it to TTL sync.

Thank you.

You should be able to put the analog sync through a comparator, such as an LM339, to convert it to TTL levels. You connect the signal to one input to the comparator, and the other comparator input is connected to an adjustable voltage to set the trigger level of probably about 0.24V or so (can be generated by a 1kΩ pot connected through a 10kΩ resistor to the plus supply). The output is open-collector so it should be connected by a 1k ohm resistor to +5V, which will generate the TTL signal.

Which comparator input you use for the signal depends upon the polarity desired for the output pulse. If the signal goes to the plus input, the output will have the same polarity as the input.

Thank you Crutschow. I will attempt to put that together.