Why is over/undercranking in SD technically impossible?

S

simismo

Member
Hello everybody,

I've recently acquired this great camera after many months of research (I come from the film market, used to work with 16mm mainly) and I need to state above all things I'm fascinated. Everything is as great as I had foreseen.

That being said, there's something I could not figure out yet... that is, why is it not possible to get a variable framerate stream in SD? I mean, we are actually dealing with a higher bandwidth signal in 720p and pN. So, considering that the SD mode can also achieve progressive frames -then why cannot we over/undercrank footage when recording SD to a P2 Card (I can sort of figure out the issues of printing to tape)?

Using common sense, one would say we should even be able to DOUBLE the framerate speed, as it occurs with the file sizes. So, I conclude there must be some issue I'm unaware of.

And it's a shame because when we are facing SD projects that for any reason need the slow motion effect our gear is capable of, either dubbing in-camera the cranked footage or importing the HD files into the NLE SD timeline, are both time consuming procedures that compromise our workflow.

I'll appreciate any light over this interesting topic. I vainly searched for a thread talking about it, and I must say I find it quite strange nobody ever mention this.

(I'd love to hear the camera is perfectly capable of delivering cranked footage in SD and I'm the only idiot that got confused... so if that's the case then it's good news!)
 
The tape in the camera always rolls at a constant speed, so it can't be over or under-cranked. The camera can record at different speeds to solid state media because it doesn't need to move at a constant speed.
 
I suppose it could be posible. I'd suspect with the idea of HDTV surpassing SDTV in the near future, the big companies would think of it as a step back. They all want to sell there HDTV's. They already think of SD as on it's way out.
 
the HVX can only record under over cranking because of the PN mode. there is no SD pn mode.... only DV or DVCpro or DVCpro50
 
I understand it might be really difficult with tape, but the HVX records standard def to P2 as well. I'm guessing Brian's right. Panny just didn't see the value in spending the $$ on making it possible in SD. If their decision made it easier for me to afford the camera, or meant I could get it sooner, I guess I'm okay with that.
 
mrWr0ng said:
The tape in the camera always rolls at a constant speed, so it can't be over or under-cranked. The camera can record at different speeds to solid state media because it doesn't need to move at a constant speed.
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You wouldn't alter tape speed to over or under-crank. You would always record 60fields/sec, but at lower frame rates you repeat frames to fill in fields until the next 'real' frame. This is how the Varicam does undercranking, and why there are only certain speeds available.

For overcranking, you can store frames in a buffer and then lay them to the tape at normal speed. Using a buffer, you could also undercrank at any frame rate.

This can be done with any format, including SD, but I suspect there was never enough demand to warrant the complexity and cost for SD cameras.
 
well no because you can do it with the 720p and down res in camera, or in post... its not like its impossible...
 
Oh well, I said I understand why it was not possible to do on tape. But being able to record SD in the cards... well, then, why is it that we are unable to shoot variable framerates standard def to the P2?

And I don't think it's entirely related to the pN mode, since we can crank our footage in mere 720p.
 
The reason it's possible on the HVX is because DVCProHD in 720 mode is a 60 frame per second codec. This makes it possible to shoot 60 progressive frames a second and lay them down to tape or P2 or Harddrive, and play them back at 24fps, or 30fps, or 60fps.

You can do the same with SD if you like... shoot 29.97 frames per second and slow it down to 24 frames per second, but the difference obviously will be minimal.

This is also why we don't see slomo in 1080 mode on the HVX. DVCProHD in 1080 mode is made up of 60 interlaced fields (or 30 frames) per second. You just don't have the 'room' in the codec to shoot higher framerates.

EDITED TO ADD:
To add another note about tape speed - the Varicam has been doing variable frame rates for years...on tape.
So clearly tape is not an outright obstacle to variable frame shooting.
 
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I know that, but not MiniDv tape. Of course the 'tape' mechanism isn't an obstacle in itself.

However, I guess it is definitely related with the codec nature you wrote about. The chance to get variable framerates must be a direct consequence of the dvcpro-hd architecture and its 100mbps.

Although, having been able to downsample the original 1080 signal to 720 and doubling the fps (or else reduce the file size by half), I still can't get why weren't they able to downconvert again to 480p and get space for 4 times the fps of the 1080 mode.

Maybe the question is getting too technical. But hey, would have been nice to get 60fps in SD without the need of converting/dubbing (no wonder 120fps!!).
 
The sensor runs at 60fps, so 60fps will always be the max for the HVX.

The data rate is the other bottleneck. The cards are rated for 640Mbps so you can only put so much data through them at once.

The third issue is that the codec used on MiniDV is a standard that made no allowances for alternate frame rates. DVCPro25 is the same codec. They could have added overcranking ability for the P2 card but they'd have to hire a programmer to write the code around a 10-year-old standard.

Fourth is that your 720 footage should match perfectly with any of your MiniDV footage when you convert it, so there's really no need, just a matter of convienience.
 
ProfessorU said:
Fourth is that your 720 footage should match perfectly with any of your MiniDV footage when you convert it, so there's really no need, just a matter of convienience.
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I agree. Just a bit of extra-time.

You made your points clear. I guess it will not be that simple after all. It'll be cool though.
 
simismo said:
The chance to get variable framerates must be a direct consequence of the dvcpro-hd architecture and its 100mbps.
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It's a direct consequence of the architecture in so far as DVCProHD 720P is a 60fps codec.
The data rate is inconsequential in relation to the ability to do variable framerates.

Although, having been able to downsample the original 1080 signal to 720 and doubling the fps (or else reduce the file size by half), I still can't get why weren't they able to downconvert again to 480p and get space for 4 times the fps of the 1080 mode.
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Because the DV codec is the bottleneck - it won't allow for more than 29.97 frames per second. To be able to do what you're talking about would involve the creation of a new SD codec....which means worrying about third party capture and post compatability and a whole other slew of headaches. Why go through all that if you can simply record slomo in 720 and downconvert after the fact to DV in camera if need be?

At that point its possible because the slomo clips are being played back at 24 (or 30) frames per second - which is within the confines of the DV codec and can easily be dubbed down to tape in real time.

I suppose there may have been some sort of way to rig the camera to record directly slomo in SD to P2 cards (forgetting tape)...but its just not worth Panasonic's time to work something like that out. The camera can already do 720 slo mo and realtime downconverts to DV in camera. Adding the functionality to record slomo in SD to P2 media would have just made the camera more expensive and added a feature that very few people would probably use (just a guess).
 
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Yes, but the bigger problem is: the DV codec doesn't allow for "frames" at all. DV (and all standard-def) is defined as interlaced, inherently. And the 1080 codec is interlaced. Interlaced doesn't play well with variable frame rates.

Variable frame rates happen in 720p because it is not interlaced and it's not based on fields. It's individual discrete frames. Those are easy to record at variable speeds, because they're individual discrete frames.

Of course it's possible to embed progressive footage within an interlaced stream (that's how we get 24p and 30p on the DVX and HVX 480/1080 modes). And it's possible to transcode slow-mo footage into the field-based nature of the DV codec (which is how you can dub slow-mo footage from 720p onto DV tape). But the idea of directly recording variable frame rates into an existing field-based codec is not necessarily that a trivial task.

Sony tried to do this on their HDV camcorders (HDV 1080 = field-based) and ended up with a goofy hack system where they take 1/2 or 1/4-res footage, buffer it up in an internal six-second buffer, and then transcode it out to the tape. So you could only shoot in six-second bursts (or however-many seconds it is), then you had to stop while the camcorder fired up the tape drive and dubbed the frame-based footage out of its buffer onto the field-based codec.

If, instead of DV, Panasonic had made their own new frame-based standard-def codec, then yes they could have put VFR in standard-def. But then it would have been incompatible, and I'm guessing they didn't think that tradeoff was worth it.
 
can't you overcrank at 720pn to p2 then transfer to dv on tape in cam? i know it's an extra step but it gets the desired result.
 
TimurCivan said:
the HVX can only record under over cranking because of the PN mode. there is no SD pn mode.... only DV or DVCpro or DVCpro50
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According to the manual (PDF pg. 35, you can also record VFR in 24P or 30P at 720. But on the previous page, the manual seems to contradict itself, and says something about "without the use of a frame rate converter", it must be PN mode.

Can somebody clarify this?
 
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