A37 - 60i is actually broken 30p

[MovingSun]

Summary: I have a Japanese A37, up to date firmware. I can select movie recording at 24p or 60i. Movies recorded in 60i appear interlaced, but they are actually more like 30p with out-of-step fields. The specifications are incorrect since it appears this camera does not shoot 60-anything per second.

Has anyone else noticed this issue? Does it affect cameras from other teritories?

To reproduce:

Film anything moving horizontally, use a double-rate deinterlacer to give 60p and there will be near duplicate frames, this wouldn't happen if the output was really 60i.

Detail (a bit hard to explain, if you're familiar with interlacing you should understand and be able to test):

60i should record one half-height image every 1/60 second and interlace these into full-height images at 30fps. But the A37 records one full-height image every 1/30 second, splits it, then re-interlaces it out-of-step (top-field of frame 2 matches bottom-field of frame 1). This gives an interlaced "look" but it's actually just a kind of "broken" 30p with out-of-sync fields. With true 60i you can deinterlace to get 60p because there are 60 different snapshots per second to start from (use a good motion compensated deinterlacer). With this broken 30p you cannot do that, the best you can do is recover correct 30p.

It's disappointing that the camera doesn't appear to meet it's published specs. Although I don't shoot movies with the camera often, I was hoping to use a high quality deinterlacer to give 60p output (I personally prefer higher frame rates). I am hoping it's a firmware mistake and not a cynical attempt to make fake 60i just to bolster the spec sheet.

[Mick2011]

Hi, welcome to the Sony Forums

I'm not entirely sure there's a firmware problem there, as that sounds to me to be normal for Sony's implementation of interlaced capture. I'll pass the question on to the tech support guys and see if they can explain the preferred method of deinterlacing for the alphas.

If I hear anything back I'll post it here.

Cheers

Mick

[cmosse · Accepted solution]

The SLT-A37 (USA/Japan models) sensor output is 30p, so it actually captures 30 progressive frames/s. To adapt to the common 1080i standard the 30p frames are divided into 60 fields, a technique known as progressive segmented frame (PsF). A progressive frame is divided into two segments, with the odd lines in one segment and the even lines in the other segment. Technically, the segments are equivalent to interlaced fields, but unlike native interlaced video, there is no motion between the two fields that make up the video frame: both fields represent the same instant in time. This technique allows for a progressive picture to be processed through the same electronic circuitry that is used to store, process and route interlaced video and is commonly uses even by top level cameras.

[MovingSun]

Thanks for the answer, which clearly explains the issue.

It is disappointing, regardless of whether other cameras do it. It might be for compatibility but it's quite misleading to call this 60i given the camera actually captures 30p. It would be better called "60i compatible", although that compatibility comes at a cost, see below.

Does this affect 50i models? Are they really outputing a 25p variant in a similar way?

A couple of observations:

Given that this 30p is masquerading as 60i, then most user's workflow will be to run it through a deinterlacer. That might be as part of their editing, or playing back though a TV or other device that deinterlaces on the fly. However, deinterlacing this kind of output will reduce the video quality, potentially losing up to 50% of the captured data depending on the method. The deinterlacer will attempt to intepolate/blend/compensate fields that it expected to be at different moments in time - the result will be worse than the captured 30p. And the deinterlacing is entirely unnecessary since the output is already progressive. Ironically, the most advanced deinterlacers (motion compensated) will be particularly affected given that they rely on motion (that isn't there in this case).

The PsF 30p frames that are captured on the A37 don't match up very well. When the fields are matched back up to give the original 30p, there are discrepancies between the fields, short horizontal lines of mismatch.

The best quality approach to work with this output is to undo that strange PsF mangling, then clean up the field mismatch to give a pure 30p stream. Not many would know to do that. Of course I could just shoot at 24p...

Thanks anyway!

Message was edited by: MovingSun

[cmosse]

it is the same for 50i models.

If you need a higher frame rate you have to use a 50p model (e.g. SLT-A57)

[zmp2000]

That´s a Shame, I bought an A37 just because in the specifications it was explicit that the camera could shoot movies in 60 FPS, I just feel mistaken!