Frame Rate Puzzle

[Graham Jones]

I am using the JVC GR-PD1 to shoot a low budget feature which will finish on 35mm.

I selected the PD1 over the HD1/HD10 because of frame rate. I explored frame rate conversion programmes to convert the HD cams 30p to 24p but didn't feel confident from the tests I saw that I would be protected.

The PD1 shoots a nice image in true 16:9 at 25p.

I just have one query.

The cam outputs at 50p (or 50i if you change a setting). I haven't made a mistake and shot at 50p, which is indeed possible, I am definitely shooting at 25p - but it outputs at 50p.

This is explained by the manual - recording is at 25p but playback 50p.

I have experienced MPEG2 NLEs getting confused by this and playing footage too fast and saying it's 50p... though not the bundled NLE MPES PRO - it knows that it's really 25p.

I assume that 50p playback doesn't truly affect frame rate - that the frame rate is still 25p, as there are only 25 differences a second.

However, I am transferring to 35mm, where 25p will be vital, and I just wondered if anyone had any idea whether the fact that technically it's coming in at 50p will matter.

I'm doing a test in less than a month, so will find out... but I just wondered whether anybody had anything to say in advance.

It seems to me it won't matter?

G

[Rob Lohman]

I don't know this particular camera, but it does sound weird
indeed. On what media is this being recorded? It sounds like it
is just 25p. Not sure where this whole 50p thing is coming from.
Sounds like that is a printing error and it should read 50i. Yes,
interlaced.

For example. If we take the plain DV format it only understands
interlaced. But you can still store 25p in it for example. It makes
no difference in how it is stored. You can still see it as two fields,
only without the time difference between them (that's what
make's it progressive).

If your edit package handles it good you should be okay. To what
format will you be exporting for the film blow up?

[Graham Jones]

There are 3 modes I can record in

DV
PS50
HI-RES

'DV' is old DV - no explanation needed
'PS50' is 50 Progressive Frames a second in either 16:9 or 4:3 depending on how you set it
'HI-RES' is 25 Progressive frames a second in 16:9 only

On the subject of this latter 'HI-RES' setting the manual says:

'Allows you to record on a tape in MPEG2 format (576p). The camcorder records signals in the progressive scan mode (625 scanning lines at 1/25th of a second for recording and 1/50th of a second for playback at one time).'

Rob, not sure of the precise format/system we will be exporting to for the film blow up. However the lab requested 'separate reels on DigiBeta' and were then in agreement when I suggested avoiding that DigiBeta stage and transfering separate, locked reels directly from respective Mini DV tapes from the Cam via it's component Y/Pb/Pr output to their system... i.e. component is an acceptable input to their system. This is the test we will be doing in less than a month...

The component Y/Pb/Pr output has two settings:

'576/50P: signals are output in progressive scan mode (FACTORY PRESET)'

576/50i: signals are output in interlace mode.'

This is when playing out HI-RES 25p footage!

[Rob Lohman]

After reading your post and doing a bit of research it looks like
there are 3 modes indeed. DV SD 25, HD 25 en SD 50. I'm
wondering why when you want to go out have you shot in an
SD like resolution with 50p instead of HD at 25p?

[Graham Jones]

no, I've shot in HI-RES 25p.

Output is only 50p/50i.

[Rob Lohman]

Output from to where? I'm assuming if you shot it in HD 25p
the output over firewire to your computer will be the same.

I'm assuming the output THROUGH YOUR ANALOG connection
will be either 50i or p. That makes sense.

Then again, I know nothing about this camera. So I'm only
able to apply what I hope is some common sense. Sorry.

[Graham Jones]

'Output from to where? I'm assuming if you shot it in HD 25p
the output over firewire to your computer will be the same.'

As I said above:

'transfering separate, locked reels directly from respective Mini DV tapes from the Cam via it's component Y/Pb/Pr output to their system... i.e. component is an acceptable input to their system. This is the test we will be doing in less than a month...

The component Y/Pb/Pr output has two settings:

'576/50P: signals are output in progressive scan mode (FACTORY PRESET)'

576/50i: signals are output in interlace mode.'

This is when playing out HI-RES 25p footage!'

'I'm assuming the output THROUGH YOUR ANALOG connection
will be either 50i or p. That makes sense.'

This is interesting. Why definitely 50p/i?

[Rob Lohman]

Ah... now I understand. Sorry for not reading more closely.

Well with analog they will need to conform to specs and basically
that means 50i. Which if you have recorded to just plain DV 25p
is no problem since 25p will look exactly the same in 50i.

My remark wasn't entirely correct since 50p is a bit weird indeed.
In theory analog should not have the bandwidth to do 50 full
frames a second. But then again, this perhaps only works with
compliant equipment and all others will just see 50i?

What I'm really wondering is whether you are getting HD out of
it or not. If you do, then it is probably following some other
standard which can also do 50p. Just look at 50i as 25p and that
should answer your 25p question.

" 25p is exactly the same as 50i only without the time difference between the fields "

[Graham Jones]

'In theory analog should not have the bandwidth to do 50 full
frames a second.'

yeah, the manual says that often it will be downsized but that the 'latest display and recording equipment' can handle 50p.

I've been thinking about this and I think I should be okay.

I mean, I'm shooting at 25p.

Outputting via ananlog at 50p.

Receiving 50p with up to date equipment I think.

Surely every frame will be repeated once in the 50p output and it won't be noticable?

Thanks - this is really helpful.

No, it ain't HD. But it's nice I think.

[David Newman]

It is not an issue. 50p analog output is simply so that the frame doesn't flicker like crazy. Even 24p film is displayed at 48Hz, with each frame being flashed on the screen twice. You are shooting 25p, if you are editing from the IEEE1394 signal you are editing 25p.

However, the PD1's MPEG transport stream does indicate 50 discrete frames -- this is where some software is getting confused. The MPEG stream marks repeat frames, just the same way 24P film is encoded onto 30fps only NTSC DVDs. The repeat frames do not take a significant slice of the bit-stream, but editing software does need to be aware of it.

[Graham Jones]

That's illuminating David, thanks!