What is PsF, or why does my camera output interlace in progressive?

XDCAM-sliding-system-v2 What is PsF, or why does my camera output interlace in progressive?

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 from 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.

24 thoughts on “What is PsF, or why does my camera output interlace in progressive?”

  1. Well done Alister – that has to be the clearest explanation of PsF that I’ve read and should clarify the issue for many – including me. Keep up the good work!

  2. I’d echo the above comment – that was superbly written, and makes the whole PSF thing easy to understand. Such an easy to understand and clear explanation.

    – I sort of understood before… now i properly understand.

    thanks Alister!

  3. Thanks for the information, but i have two questions
    1. Many of the cameras from Sony say 25p, 50i (25i should i say) but does the ‘p’ mean ‘true progressive’ or ‘PSF’. You find on some Panasonic cameras they write ‘True P’ for true progressive recording.
    2.If the camera is set to 1920×1080 50i and you plug up an external recorder and have that set to 1920×1080 25p, and then record it as progressive will the recorded feed be ok?

    1. From a camera prospective 25p would mean the camera is 25p and the internal recordings would be 25p. However there is no such format as 25p or 30p so far as the HDSDI output standards are concerned. So there is no way to send 25p/30p over HDSDI. So a 25p camera will use PsF to output 25p over HDSDI.

      If you have a camera set to 50i and you record that as 25p then you will have interlace recorded inside progressive frames, so if there is any motion in the shot you will have unpleasant combing artefacts and it won’t look good.

  4. I am using a Panasonic_AVHS400 vision mixer (which only takes interlace feeds) but if the camera is set to 25p then the output will be 25psf and the reason that the mixer still sees the video feed is because like you said’ system will report that what it is receiving is interlace, even if it is progressive PsF’. So does this mean vision mixing the feeds will still be ok in ‘p’ format for a finial output than using interlaced?

  5. Thank-you for this, it’s the best description of PSF I can find on the internet. One query:

    When shooting on the Sony PMW F3 into the AJA KiPro Mini, the AJA displays an interlaced signal is being received as expected. Do I need to change the settings on the AJA to PSF or is the camera doing this automatically? I ask this because when viewed afterwards frame by frame, the video appears to be progressive in appearance.

    I hope this makes sense… thanks.

    1. If you don’t tell the KiPro that the footage is PsF the the files will be marked as interlace (even though they will contain progressive). In most cases this makes very little difference but to be technically correct you should set the KiPro to PsF. Just remember to set it back to interlace when shooting I as otherwaise your interlace material will be treated as progressive and this really screws things up.

  6. Hi Alister,

    I am using the new FS7 and my marshall monitor is saying 1080i 50hz. I am using the HDSDI connection. I’m guessing that rule applies to this circumstance,and it is Progressive footage, but the monitor is not reading it like that??!?!
    As far as I can see, I have everything set to P to i.

    Although when I wave my hand or any harsh movement in camera, the object becomes made up of lines?!?!
    So is it Interlaced??!?!

    Thanks!

    1. That’s normal. You cannot output true progressive over HDSDI, the standards do not allow for it, so you use PsF which is progressive inside an interlace stream instead.

  7. Hi Alister,

    I hope my question does fit this article. In a few days I’ll be shooting a comedy with two FS7 for a local TV. I primarily asked to have the XDCA-FS7 on both cameras in order to record in prores and to have better balanced cameras for shoulder shouting. It happens that the renting place just called me today to say that they don’t have the XDCA-FS7 for both cameras, only one is available, but they said to not worry, they will give me an Atomos recorder or a AJA mini kipro instead… reading you I was wondering if I have the chance to have the same quality picture in both cameras, one with XDCA-FS7 and the other with external recorder, with the PsF story… A recomposed P signal (P to I back to P) will look the same to a P recorded as is, especially in movement ?
    I’m about to choose to shoot in XAVC-I and not get troubled with “different but similar” codecs.
    Your response will be much appreciated, thanks a lot for all your work.

    Aïdan

    1. Provided the external recorder is set to accept the PsF correctly the ProRos external recordings should be more or less the same quality as the ProRes internal recordings, certainly nothing to worry about. It’s just the issues of cables, extra batteries and no record confirmation in the camera I would be more worried about.

  8. Hi Alister,

    I have a BBC job coming up in a couple of weeks. I normally film on a PMW500 set at 50i.
    My question is, can I use an F5 and produce technically acceptable 50i rushes?

  9. Thanks so much for this Alistair, I’ve been searching for this answer for a while now and you’ve explained it perfectly! The comments above with explanations also are of great benefit!

  10. Hi Alister
    On this subject you hopefully can clear something up for me.
    I have both an f5 & fs7 (in 25p) and when connecting to a BM video assist I am able to output psf on the fs7 (and thus get 25p on the BM) but can’t on the f5 (the BM shows interlaced – as it has no 2:2 pull down). The output menu on the f5 says 1080p & the fs7 offers psf.
    Am I missing something with output settings on the f5?

    Cheers

    1. PsF is Progressive, even if the monitor says it is interlaced, the signal is progressive, so it really doesn’t matter

      1. Hi Alister

        Thanks for getting back to me.
        Is there a reason that the BM would see the signals from the f5 & fs7 differently – seeing as they are both apparently the same?

  11. Hi Alister,

    Not sure if you have used a Sony FS5 to record externally to a Video Devices PIX-E5.

    The FS5 user manual states that HDMI output is sent as “50p (Pull Down)”. When I output via HDMI to a Video Devices PIX-E5, the input is flagged as 1080p50, when what I want is 1080 at 25p.

    Using SDI, the PIX-E5 has no problem seeing the input signal as 1080p25.

    I figured out I need to choose the correct setting in the FS5’s Video Out > SDI/HDMI setting.

    Choosing “2160p/1080p/576i” results in the PIX-E5 seeing the input as 1080p05. Only when I choose “2160p/1080i/576i”, and then setting the PIX-E5’s “Input PsF Detect” to “interpret 1080i as P”, along with “Input to File Conversion” to “PsF to P”, can the PIX-E5 record a 25p file.

    Is this the correct way to do it?

    Thanks in advance!

    1. I think you are doing it correctly. If the camera is set to 50p then HDMI will be 50p. But 1.5G SDI cannot do 50p so it will output 50i containing 25psf.

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