Unfortunately our favourite input signal conditioner IC, the MAX7450 is end-of-life’d and gets hard to find. For the Synkie to work properly, all video input signals must have their black level DC clamped to ground. the MAX did this very nicely, but for future revisions we must find an alternative. So I tried once again to build a decent input clamp circuit on my own. This led to a complete redesign of the Sync-Splitter and Resyncer modules, and finally to a new attempt for the Microsynkie which incorporates both modules on a small PCB.
Analog video input signal is limited for protection between the power rails before it is buffered and AC coupled. A LM1881 extracts all interesting timing signals which are fed to an analog switch. During H-Sync and Backporch we pull C15 to ground – effectively removing the negative going sync tips and clamping the backporch to ground. The result is buffered again and ready to be messed around with (Siff signal in Synkie-speak) The remaining switch lets us extract the colorburst neatly centered around ground as well. V-Sync can be used as a simple means for showing that there is a video signal present on the input.
First the potentially very weird signal that is coming back has to be clipped to meaningful video levels, that is 0V-0.7V. The following circuit seems to do the trick, this time without any potentiometers to set the clipping level. Thanks Max!
Another MAX4053 triple analog switch is used to fake a normal video signal. A simple resistor divider creates about -0.3Volts that can be inserted as sync tips during the H-Sync Phase. Colorburst is cut back in during the backporch and the clipped SIFF-Input signal during the active video period.
We tried to filter out some of the resulting transients with a low-pass filter before the output buffer, some nasty spikes can still be seen on the oscilloscope but even our very picky Analog Video To SD Recorder doesn’t seem to bother.
Color Phase Corrector
Before feeding the colorburst back to the signal, an all pass filter lets us delay the burst phase to correct for the time delay of the messed-with SIFF signal in order to get correct colors again (or any color at all)
|Power Draw on +5V:||N/A|
|Power Draw on -5V:||N/A|
Schematic & PCBView on Github
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