720p24 Mode Theories

[Nate Weaver]

Here's my theory about what's going on in 720p24 mode on the HD100:

1-The file is really 720p60, with only 24 active frames. The rest are null

2-VLC plays it fine at the correct frame rate.

3-Quicktime attempts to play it at 60fps, but it can't keep up (on my machine) so it settles for about 40fps

4-There are no "pulldown" frames to remove like the DVX in 24PA. Only "null" frames to ignore in the 60fps stream

If all this is truly true, it looks like Apple will only have to update how Quicktime/FCP looks at the MPEG stream from the HD100. It also looks like this approach is pretty clever on JVC's part...before observing all this I was trying to figure out how one would remove pulldown from the 720p24 (actually 720p60) stream without a re-compress.

I suspect Apple will have this wrapped up shortly in an update.

Does anybody have any evidence to the contrary? Thoughts?

[Stephen van Vuuren]

Originally Posted by Nate Weaver

1-The file is really 720p60, with only 24 active frames. The rest are null
Thoughts?

How would this work with shutter speed and motion blur? Do you think the image is being exposed at 60 times a second only giving you a shutter speed of 1/60th as the lowest possible? No 1/48th shutter?

Or is it just being written to tape at 60p?

[Nate Weaver]

Originally Posted by Stephen van Vuuren

How would this work with shutter speed and motion blur? Do you think the image is being exposed at 60 times a second only giving you a shutter speed of 1/60th as the lowest possible? No 1/48th shutter?

Or is it just being written to tape at 60p?

I'm talking ONLY about the MPEG stream format, as it's imported into a computer.

All evidence so far is that the CCD/camera section of the camcorder captures and processes images just like a DVX, with the exception of having the option of Motion Smoothing. Default shutter speed is 1/50th, and you can dial in 1/48th no problem.

[David Newman]

Nate,

You are totally correct. The MPEG transport stream is a 60p wrapper with 24 active frames. Repeat flags indicate to player and capture devices what to do.

[Nate Weaver]

Originally Posted by David Newman

Nate,

You are totally correct. The MPEG transport stream is a 60p wrapper with 24 active frames. Repeat flags indicate to player and capture devices what to do.

Wow. I sounded smart then. I honestly was kinda winging it.

I know you probably wouldn't presume to speak about an Apple product, but it sounds like getting real 24p support is not a huge underlying issue?

[David Newman]

I can try a feel superior that we do support 24p and 25p from the new JVC camera, but it wasn't that hard. I'm sure Apple is just around the corner with their update. Large companies natural take longer with their relaeses.

[Nate Weaver]

Naw, I'd feel superior. That's a very real advantage of staying small!

[Stephen van Vuuren]

Originally Posted by Nate Weaver

I'm talking ONLY about the MPEG stream format, as it's imported into a computer.

All evidence so far is that the CCD/camera section of the camcorder captures and processes images just like a DVX, with the exception of having the option of Motion Smoothing. Default shutter speed is 1/50th, and you can dial in 1/48th no problem.

Okay, I assume then it's just a matter of figuring where the valid 24p frames are and decoding them correctly. But might not GOP issues cause problems in extracting the frames from MPEG stream?

[Nate Weaver]

Okay, I assume then it's just a matter of figuring where the valid 24p frames are and decoding them correctly. But might not GOP issues cause problems in extracting the frames from MPEG stream?

Kinda. All frames are valid in the stream because they're the only ones there. JVC distributed the 19mbs across 24 and ONLY 24 frames per second. The "filler" frames, to bring it up to 60fps, are empty virtual frames. They're not even repeat picture data...they're just placemarkers instructing the decoder to repeat the last frame for either 2 or 3 60fps frames.

As far as GOP issues go, it sounds like the way an MPEG stream works, it's a non-issue. I don't know that though, I'm just guessing based on David's response above.

My question for you David is if you think the JVC method is a clever, legal use of the MPEG spec, or a hack. It seems not all software reads this correctly, so at the very least it seems like new use of an obscure part of the spec. Barry Green notes that the MPEG decoder in his Sony XBR HD set correctly decodes it though! (via firewire/TS streaming)

[Stephen van Vuuren]

Interesting - thanks for the info.

[David Newman]

Originally Posted by Nate Weaver

My question for you David is if you think the JVC method is a clever, legal use of the MPEG spec, or a hack.

It is all standard MPEG. The these new ProHD streams can be captured and played on old DVHS decks, nothing but the way MPEG works. However NLEs have never been good at MPEG, so there is some work to do.

[Steve Mullen]

Originally Posted by David Newman

It is all standard MPEG. The these new ProHD streams can be captured and played on old D-VHS decks, nothing but the way MPEG works.

I read several versions, in this thread, of how ProHD works. As far as I know:

CAMERA
Always running at 48fps, 50fps, or 60fps with "correct" shutter-speeds of 1/48th, 1/50th, and 1/60th.


CAMERA ANALOG OUTPUT
1) 720p50 or 720p60 direct from the CCDs.

2) I haven't read what what happens in "24fps" mode. Perhaps it is not output via analog. Perhaps every other frame is dropped and 2:3 pulldown is added to yield 720p60. (I've got no idea what happens in Region 50?)


MOTION FILTER OFF
Every other frame is dropped, yielding 24p, 25p, or 30p.

1) If the HD1/HD10 MPEG-2 encoder is employed, it still may not be able to encode 24p. (The original could NOT.) So some scheme may be used to create 30fps from 24fps. This may be where the idea of REPEAT frames comes from. In fact, the NTT encoder couldn't encode 25p either. So maybe 25p is converted to 30fps by using REPEAT frames. All recording is 720p30. As far as I know there is nothing recorded that has anything to do with 720p50 or 720p60

2) 24p, 25p, and 30p are MPEG-2 encoded and recorded. As far as I know there is nothing encoded or recorded that has anything to do with 720p50 or 720p60.


MOTION FILTER ON
Pairs of frames are "conbined" into one -- so there is 24p, 25p, and 30p output from the filter. (MF possibly IN the encoder.)


PLAYBACK ANALOG OUTPUT
1) 720p25 and 720p30 are frame-doubled to yield 720p50/1080i50 or 720p60/1080i60. This is just like the HD1/HD10. 2:3 pulldown is used to yield 720p60 from 720p24. (I've got no idea what happens in Region 50?)

2) If only 720p30 is recorded, the REPEAT FLAGS are used to obtain 24 and 25fps. p25 and p30 are frame-doubled to yield 720p50/1080i50 or 720p60/1080i60. This is just like the HD1/HD10. 2:3 pulldown is used to yield 720p60 from 720p24. (I've got no idea what happens in Region 50?)


i.INK OUTPUT
There are many possibilities:

1) Do the obvious: frame-double 25p and 30p to 720p50 and 720p60. Add 2:3 pulldown to 24p to get 720p60. NLEs understand 720p50, 720p60, and 720p60 carrying 24fps. 720p60 (w/wo 24fps) is ATSC legal. I assume this is what is meant by a 720p60 "wrapper."

2) Output 24p 25p, and 30p. Modify NLE's to capture 720p24 -- ready for Timeline. HDV NLE's already accept 25p and 30p. Both 720p24 and 720p30 are ASTC legal.

3) Frame-double 25p and 30p to 720p50 and 720p60. Output 24p as 720p24. NLEs can accept 720p50 and 720p60. Modify to capture 720p24 -- ready for Timeline. Both 720p24 and 720p60 are ASTC legal.

4) 24p, 25p, and 30p are frame-doubled to yield 720p48, 720p50, or 720p60. NLE's are ready for 72050 and 720p60 -- but not 720p48 so they must be modified to capture and then drop every other frame. Only 720p60 is ASTC legal.

5) Output only 30p. Use some kind of scheme to embed 24fps and 25fps in 30fps. NLE's could drop the REPEAT frames to get back 24p and 25p. Certainly not ATSC legal. (This matches the 30fps only encoder.)

6) Output only 25p and 30p. Use some kind of scheme to embed 24fps in 30fps. NLE's could drop the REPEAT frames. Certainly not ATSC legal.


I would sure love to know what is really recorded and what come down the i.LINK!!!

[Nate Weaver]

Wow. That's a lot of questions and suppositions for one post. I'll add info where I can from my experience with the camera last week.

Originally Posted by Steve Mullen

I read several versions, in this thread, of how ProHD works.

Really? I was only talking about 24p mode the whole time, and David confirmed my theory.

Originally Posted by Steve Mullen

CAMERA
Always running at 48fps, 50fps, or 60fps with "correct" shutter-speeds of 1/48th, 1/50th, and 1/60th.

I'm almost positive that the camera section runs at the FPS going to tape in all modes. The available selection of shutter speeds in any given mode supports this theory

Originally Posted by Steve Mullen

2) I haven't read what what happens in "24fps" mode. Perhaps it is not output via analog. Perhaps every other frame is dropped and 2:3 pulldown is added to yield 720p60. (I've got no idea what happens in Region 50?)

Pulldown is definitely added via the component outputs, and AFAIK, the composite output also. During our quick motion tests at Clairmont, it looked to me however that this might be a "quick/easy" method of pulldown added, and frame based, not field mixed. I saw a little stutter live, but not from the files pulled off that tape.

Originally Posted by Steve Mullen

2) 24p, 25p, and 30p are MPEG-2 encoded and recorded. As far as I know there is nothing encoded or recorded that has anything to do with 720p50 or 720p60.

There is though. The m2t and m2v files pulled off tape shot in 24p mode are definitely 59.94fps. That's what David just confirmed for me.

Originally Posted by Steve Mullen

MOTION FILTER ON
Pairs of frames are "conbined" into one -- so there is 24p, 25p, and 30p output from the filter. (MF possibly IN the encoder.)

I agree. It states in the manual that one cannot see the effect of Motion Smoothing live through the LCD, finder, or any analog output. This supports the theory that it relies on extra DSP built into the MPEG encoder.

Originally Posted by Steve Mullen

1) 720p25 and 720p30 are frame-doubled to yield 720p50/1080i50 or 720p60/1080i60. This is just like the HD1/HD10. 2:3 pulldown is used to yield 720p60 from 720p24.
2) If only 720p30 is recorded, the REPEAT FLAGS are used to obtain 24 and 25fps. p25 and p30 are frame-doubled to yield 720p50/1080i50 or 720p60/1080i60. This is just like the HD1/HD10. 2:3 pulldown is used to yield 720p60 from 720p24.

I'm not sure why they'd go with frame doubling for some formats, and MPEG repeat flags on others, but who knows? David only confirmed how it works in one mode.

Originally Posted by Steve Mullen

i.INK OUTPUT
There are many possibilities:

1) Do the obvious: frame-double 25p and 30p to 720p50 and 720p60. Add 2:3 pulldown to 24p to get 720p60. NLEs understand 720p50, 720p60, and 720p60 carrying 24fps. 720p60 (w/wo 24fps) is ATSC legal. I assume this is what is meant by a 720p60 "wrapper."

Again, I have no idea about frame doubling in the 25 and 30 modes. 24fps inside p60 is the repeat flags vs. pulldown as we know it from the DVX. I know this because the files Barry pulled live off the camera via 1394 while shooting match the files we later pulled off tape.

[Steve Mullen]

Originally Posted by Nate Weaver

I'm almost positive that the camera section runs at the FPS going to tape in all modes. The available selection of shutter speeds in any given mode supports this theory.

That's the one thing I'm sure of because JVC always claims you get true 720p50/720p60 via the analog output. In fact, they are making an HD-SDI option so you can use the camera in a studio. Also, double frame-rate is the key to the Motion Filter.


"The m2t and m2v files pulled off tape shot in 24p mode are definitely 59.94fps. That's what David just confirmed for me."

I agree -- but David hasn't said if the 24fps has 2:3 pull-down. The NLE world understands 2:3 pulldown so I assume that's what is used even with i.LINK. What's interesting is that i.LINK is the one connection where pure 24fps could be used right into a disk file -- just like film.


"I agree. It states in the manual that one cannot see the effect of Motion Smoothing live through the LCD, finder, or any analog output. This supports the theory that it relies on extra DSP built into the MPEG encoder."

If you go to : http://www.gyhduser.com/

you'll ind my short version of how I think an MPEG-2 encoder can do the MF. Like you, if it was done in DSP I can't see why we can't see it. (Wow, that's a sentence!)

"24fps inside p60 is the repeat flags vs. pulldown as we know it from the DVX. I know this because the files Barry pulled live off the camera via 1394 while shooting match the files we later pulled off tape."

Can you explain more. DVCPRO HD uses Repeat Flags but the DVX simply uses 2:3:3:2 pull-down. Which did you find?

[Nate Weaver]

Originally Posted by Steve Mullen

That's the one thing I'm sure of because JVC always claims you get true 720p50/720p60 via the analog output. In fact, they are making an HD-SDI option so you can use the camera in a studio. Also, double frame-rate is the key to the Motion Filter.

There's some Motion Smoothing filter clips on the web. The one I examined looked like it just overlayed the previous captured 1/24th frame, not one captured "in-between". I absolutely could be wrong on that though.

Originally Posted by Steve Mullen

"The m2t and m2v files pulled off tape shot in 24p mode are definitely 59.94fps. That's what David just confirmed for me."

I agree -- but David hasn't said if the 24fps has 2:3 pull-down. The NLE world understands 2:3 pulldown so I assume that's what is used even with i.LINK. What's interesting is that i.LINK is the one connection where pure 24fps could be used right into a disk file -- just like film.

and the following are related:

Originally Posted by Steve Mullen

Can you explain more. DVCPRO HD uses Repeat Flags but the DVX simply uses 2:3:3:2 pull-down. Which did you find?

The statement "but David hasn't said if the 24fps has 2:3 pull-down" is not really true because the method of using repeat flags would BE the "pulldown". I've found that JVC inserts their 24 frames per second into a 59.94 stream with repeat flags. That way they get the entire 19mbs the stream allows. Why they didn't go with a 24fps stream to begin with I'll never know...existing compatibility, perhaps? It seems a good portion of MPEG decoders already decode it properly, and without worrying about pulldown methods, which is a technical coup I'd think.