Which frame rate is better on GH1?

rundavids

rundavids

Well-known member
Hi, I'm wondering what is the image quality recording in 1080p vs. the 720p? I hear a lot of people are using the 720p and reducing the frame rates to 24fps. Is it because of the "jello" issue?
 
720 is more stable in general depending on what your shooting. Jello and mud is reduced when shooting 720 but it all comes down to how and what you shoot. theres not much difference to my eye other than one being bigger than the other in resolution. the look is still the same if that makes sense.
 
If you capture at 60p, each frame is going to have less skew. The people you're reffering to drop frames from the 60 frames/second down to 24 frames/second. This does decrease the skew, but its not going to allow as much detail per frame.

The camera is allotted (lets just pick a number for example) let's say 120 mb/sec. I know its wrong but for my example its gonna make sense. If that's the case, then each frame can use 2mb for all the data in the frame. Lets say that frame picks up 70% of the actual resolution. If it was native 24p, then each frame would be able to use 5 mb per frame. Get the picture? More data equals more detail resolved in the compressed images.
 
Joe Calabrese said:
The camera is allotted (lets just pick a number for example) let's say 120 mb/sec. I know its wrong but for my example its gonna make sense. If that's the case, then each frame can use 2mb for all the data in the frame. Lets say that frame picks up 70% of the actual resolution. If it was native 24p, then each frame would be able to use 5 mb per frame. Get the picture? More data equals more detail resolved in the compressed images.
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Logical, yes. But that's not really how it works.

Because it's a long-GoP codec, the codec does not, in fact, allocate a certain amount of megabits per frame, at all.

The system encodes the first frame in the group, as a standalone frame (the Intra frame, or I-frame). That particular frame will get a huge chunk of the available bandwidth; I forget the average number but it's probably on the order of about 40% all goes to that one frame.

Then, every frame that comes after, is only encoding the changes between frames. Since most frames are pretty much the same, very little changes from frame to frame, so very little bandwidth is necessary per frame.

Now, here's where it gets tricky: you might think that 24 fps means fewer frames, and therefore more megabits per frame, meaning higher quality per frame, but that's not necessarily true! Because when you're dealing with 24fps, there's 1/24th of a second between frames, which is plenty of time for something to change between frames. Much more time, for example, than there is between each frame in 720/60p. In 60p, only 1/60th of a second elapses between each frame, so actually the compression may be MORE EFFICIENT even though it's handling more frames! In 1080/24p, there might be rather significant changes between frames (on a pan, for example) but in 720/60p, there might only be 1/3 as much change -- and therefore the frames will encode more efficiently.

Add onto that, the notion that a 720p frame is less than half as many pixels to keep track of than a 1080 frame, and you can start to see why in theory you might think that 1080/24p would have less compression, but in reality, the 720/60p mode has less compression!
 
Joe Calabrese said:
Didn't know that the GH1 used a Long GoP codec...
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It actually has both. It has an intraframe codec (the MJPG mode) and a long-GoP codec (AVCHD).

All AVCHD, pretty much all MPEG-2 (HDV/XDCAM-HD/XDCAM-EX) and pretty much all h.264 (Canon h.264 mov files) are all long-GoP. It's kind of hard to find an intraframe codec out there in the low-cost equipment. The D90's JPG codec is intraframe (but at way too low of a bitrate for my taste) and the GH1's MJPG is intraframe (but unfortunately only at 720/30p, and still too low a bitrate).
 
On the note of H.264, in Apple's Compressor, can't you "create" an intraframe h.254 compression codec by setting the bit rate to "Automatic" and the key frames to "All"?
 
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