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GH2 AVCHD Encoder Motion Testing
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  • we could mount a cardboard with chart on it to a dc-motor, set with an adjustable power supply slower or faster
  • Vitaliy,

    Go ahead and remove the old chart file if you want.
  • DRDave,

    I think the variable rate swing (oscillation) is actually a good thing as it will allow for a spectrum of change rates. One thing comes to mind, though. Instead of swinging it to outside the frame, maybe it would be better to swing it from edge to edge inside of the frame. Experimentation will tell...

  • Here's an improved res chart. It includes a star target so we can see symmetry.

    288 x 360 - 32K
  • Chris--a standard LP has two grooves, one on the back and one on the front. Any object placed on the outer part will move faster, proportionally, than one on the inner part. The groove itself can be said to move at one speed if considered to be one single groove, or at different speeds if it is imagined that it is many grooves. A cylinder is of course a superior design, as the groove moves uniformly, however this design was more expensive to mass produce and has variance across a frame for video purposes. A metronome of course has the same issue. A large hedge trimmer would have uniform motion, as opposed to a drum which would be no problem at a single point, as in an Edison device, but which would present similar issue in a cross-section when videoed. A hedge trimmer is better for uniform video than a weed whacker, basically, although both are interesting. Perhaps for this purpose it does not matter, or perhaps the velocity differential, which can be easily calculated, is good for finding a specific speed at which the codec breaks down.
  • What I mean by "from frame edge to frame edge" is to swing it so the target image just swings out of view.

  • Here's a simple res chart I built. Align the arrows to match up with the vertical frame size. I added diagonal items because I've noticed that strange things happen with diagonal lines sometimes. Print it, glue it to a card, and hang the card with two strings from the ceiling. Then swing it so it goes from frame edge to frame edge. With strings of 1m the swing period should be about 2 seconds. The image is 8"x10" at 720dpi.

    [EDIT] Use the newer file below...
  • I just bought a metronome. I want to see how it affects codec break-up with high shutter speed. I have a powerful video light to use with it as well.
  • The convenient thing about a metronome is its repetition rate is calibrated and easily adjusted. For me, the most challenging aspect of fine-tuning patch settings on the GH1 has been the problem of producing consistently repeatable test shots. Thanks for starting this topic, it's prompted me to consider setting up a more systematic test rig for the GH2.
  • DrDave,

    Are you suggesting that other people have record players where all grooves move at the same speed? Unless, of course, you're alluding to those of us who own Edison drum record players.
  • Come to think of it you could just hang a res chart from the ceiling on a string and swing it - cheaper than buying a metronome. If it was swung from the same distance at the start of each test the results would be pretty consistent. Actually, two strings would be better; that way you wouldn't have to worry about twisting, etc...
  • On my record player the grooves all move at different speeds. Well, technically, it is one groove, of course.
  • The metronome sounds good. I assume you would tape a small res chart or some other pattern picture to it. I think we want there to be a lot changing in the image field.

    I'm using incandescent lighting.

    I suspect using several shutter speeds would be good - all the way up to 1/500. The faster the shutter, the more extreme the fine detail changes.
  • How about standardizing on a classic analog metronome:

    For high-bitrate motion testing, I typically use a 1/125-second shutter speed in sunlight. However, it's difficult to maintain consistent lighting between takes. With indoor fluorescents, consistency is not a problem, but flicker-free shutter speed is restricted to 1/60-second. Since PAL would require a different shutter speed, I'm not sure what the best lighting solution would be?
  • I've tried the screen thing - it doesn't really work. First, resolution is low, and there are moire problems. Actually, I'm not sure pan speed is all that important, as long as it is slow. I think what we're looking for is I frame quality vs. P and B frame quality in motion. Results should be pretty visible no matter what the pan speed is - so long as it's reasonable.
  • Hi Chris,

    I was being slightly facetious about the record player, but actually a spiral printed on a circular bit of paper on the turntable would produce a quite slow movement (depending on the pitch of the spiral). But I absolutely agree the importance of having a moving target of some sort. I wonder how many of us have record players anyway...

    ...or some sort of rotating pattern on screen? Either way they would be pretty consistent speed-wise.
  • I have a motorized pan-tilt head, so I can do my own testing with it. The record player is a good idea, except I think the rotation will be too fast to produce usable results. The absolute pan speed is less important than doing it approximately the same speed for each test. We may find, for example, that resolution drops considerably between I frames with low bitrates, but doesn't with higher bitrates. It's the resolution in I frames vs. B and P frames that seems important to me - especially with high shutter speeds.

  • Shame no-one still has record players. You could make a nice round 12" test chart and rotate it at a known speed (33-1/3) and there you are. Difficult to otherwise think about how you could do a standardized speed.