Some months ago we decided that internally, synkie’s signals should reference 0V for black (white is still not really defined, is it? 0.7V? 1V?) – while a standard PAL video signal has 0V for the sync pulses and black is somewhere around 300mV.
Having complete black at GND simplifies a lot of further signal processing. And the Maxim MAX7450 signal conditioner we used for the SK01-sync-splitter module does exactly that: it lowers the whole signal as such as black level is at 0V. Nice.
Now we wanted to add a lot of auxiliary inputs to the synkie to build a multichannel mixer. All these inputs should have black level at 0V. MAX7450 to the rescue. But of the 5 samples we’ve ordered, two went up in flames*, and the others are already in use.
But we cannot find these chips anywhere else than directly at Maxim, and they told us lead times of about 14 weeks…
* why did they burn? Maybe because I completely overlooked that they’ve got an exposed pad on their belly that wants to be soldered to VSS for cooling!!
Don’t build the sk01 like it is now!!!!
Looking for a simpler and quicker solution I’ve come up with this:
The differential amplifier IC1A shifts the signal to a level set by TRIM1 (not sure that we need voltage follower IC1B, but is was left over anyway).
Max’s clipper (around IC3A) cuts off anything below 0V (everything but the high frequency color information that can easily go blow 0V) so the sync pulses from our aux inputs get out of way, and the output s buffered by IC3B…
The result looks quite ok, except that the color is completely out of phase. But as we’ve seen before, the LM833D seems a bit too slow to handle color.
I’ quite confident that the Microchip MCP662‘s I’ve ordered yesterday will handle this.