This is a spin-off of the "Hellcat MkI" 625-32 converter. The thread is called "Video to NBTV converter"
In the past it has been advocated that 'porches' be included in the NBTV video signal to eliminate modulation of the sync pulse width and position. It's quite a subtle thing, but it's there all the same. Graham Lewis has often introduced at least a front porch into some of his designs saying that it does help with reliable sync extraction.
In developing the "Hellcat" the timing is such I will end up with with some spare time during the line so I decided to add porches, both front and back.
In the traces below the cyan trace is the TTL output composite syncs and the yellow trace is the filtered NBTV PWM output. The left trace is the reference for the other two, a totally black frame, it's the same with or without porches - as you'd expect. The sync width is 129us.
The centre frame is an all-white frame, note there is no change in width or position of the sync pulse when porches are present.
The right trace is with no porches as is common. Note the delay in position relative to the TTL syncs, which by inference also means a shift relative to the video. The pulse is also narrower at 116us at the same sync-slicing point.
As I said, it is subtle, and most of the time will not be of any great concern. But it will happen wherever there are filters, in and out of PC sound cards for example.
I'm NOT suggesting that this be a standard, but just raised the point which may be of interest to some.
Steve A.
Sorry if you have to download the image, it's wider than my screen allows here...