This one keeps coming around again and again and it’s not well understood by many.

When the standards for HDSDI and connecting HD devices were originally set down, almost everyone was using interlace. The only real exception was people producing movies and films in 24p. As a result the early standards for HDSDI did not include a specification for 25 or 30 frame per second progressive video. However over time 25p and 30p became popular shooting formats, so a way was needed to send these progressive signals over HDSDI.

The solution was really rather simple, split the progressive frames into odd and even lines and send the odd numbered lines in what would be the upper field of an interlace stream and then send the even numbered lines in what would be the lower field. So in effect the progressive frame gets split into two fields, a bit like an interlaced video stream, but there is no time difference (temporal difference) between when the odd and even are were captured.

This system has the added benefit that even if the monitor at the end of the HDSDI chain is interlace only, it will still display the progressive material more or less correctly.

But here’s the catch. Because the progressive frame split into odd and even lines and stuffed into an interlace signal looks so much like an interlace signal, many devices attached to the PsF source cannot distinguish PsF for real interlace. So more often than not the recorder/monitor/edit system will report that what it is receiving is interlace, even if it is progressive PsF. In most cases this doesn’t cause any problems as what’s contained within the stream does not have any temporal difference between the odd and even lines. The only time it can cause problems is when you apply slow motion effects, scaling effects or standards conversion processes to the footage as fields/lines from adjacent frames may get interleaved in the wrong order. Cases of this kind of thing are however quite rare and unusual.

Some external recorders offer you the option to force them to mark any files recorded as PsF instead of interlace. If you are sure what you are sending to the recorder is progressive, then this is a good idea. However you do need to be careful because what will screw you up is marking real interlace footage as PsF by mistake. If you do this the interlaced frames will be treated as progressive.  If there is any motion in the frame then the two true interlace fields will contain objects in different positions, they will have temporal differences. Combine those two temporally different fields together into a progressive frame and you will see an artifact that looks like a comb has been run through the frame horizontally, it’s not pretty and it can be hard to fix.

So, if you are shooting progressive and yet your external recorder say’s it’s seeing interlace from your HDSDI, don’t panic. This is quite normal.

If you are importing footage that is indicated as being interlace, but you know it’s progressive PsF into most edit packages you can normally select the clips and “interpret footage” or similar to change the clip header files to progressive instead of interlace.