Converting for Speed Adjustment Degrades Video Quality

Allowing iMovie to make this conversion visibly (and permanently!) degrades the quality of the video for that clip.
Files not having a frame rate the same as the standard in which you are editing must be converted (i.e., re-compressed) to change the frame rate to that standard before a "speed" effect can be applied.

Does anyone have a work-around to allow the speed adjustment and keep the original video quality?
The only work-around is to shoot your footage at the frame rate for the standard in which you edit.

I have Visual Hub and MPEG Streamclip - can I use one of these to convert my original .mov files to whatever iMovie is looking for in order to do the speed adjustments without having the video degraded?
Yes but you want to use the application providing the best frame rate conversion strategy. Any re-compression has the potential to degrade your video content. As to the "main" thing iMovie is looking for -- that would be the frame rate. Before iMovie can apply a speed change effect -- the frame rate must be standardized in terms of the reference clock used by QT. All other settings are arbitrary as long as they are compatible with iMovie '09.

What "recipe" should I use when converting the video?
That would depend on the target compression format you plan to use since some setting are "fixed" for some formats while others are user adjustable. Recommend you compare the newly converted clip with the original to see what changes were actually made so you can better decide which settings you may want to change and which you want to keep the same as the original if possible. As I said before, if you shoot the footage at the editing standard frame rate, no conversion would be required. Since changes in frame rate of cause "skips" or "halts" to smooth playback, I normally don't recommend changing the frame rate if it can be avoided.

Of course to change the frame-rate compressed video must be uncompressed and re-compressed.

And, of course this causes a slight degredation in quality.

To change speed also may use frame blending which alters the clarity of fames.

I'm sure changing the speed of interlaced video will produce even uglier results.

You keep mentioning the "editing standard frame rate". What is that frame rate for iMovie 09?
Good morning, Dagda. Based on what you posted last night, I think you've already worked it out for yourself. The "standard" I was referring to was either NTSC (29.97 fps) or PAL (25 fps). Sorry if this was confusing you. Would normally assume you were using NTSC based on your logged location but have been wrong in the past and so try to provide a more generalized answer that applies to users "on both sides of the big pond."

As for What Steve said, he was trying to add some detail to what is actually taking place no matter which strategy is being used while I was focusing more on the possible strategies which may be used. Remember, some applications change the frame rate by simply adding duplicate frames or dropping selected frames while others may use "blending" or "interpolation" techniques. Not really my area and one of these days I need to sit down and examine Compressor more closely which is supposed to employ a very slow but very good "optical" strategy about which I know absolutely nothing.

Dagda wrote:
In order to adjust the speed of a clip, iMovie must first convert the clip. Allowing iMovie to make this conversion visibly (and permanently!) degrades the quality of the video for that clip.

Are you sure about this or assuming so? iMovie's speed change is not a rendered effec and it simply doubles frames for slowdown and skips frames for speedup. It is not the best way to get fast/slow motion but it's cheap and doable in realtime. The only time when your source movie is converted is when it's exported to a movie file.

I don't think it's the case. I just applied speed change to a 24p clip (true 24p, not in 60i so it's not NTSC standard). iMovie 09 changes speed on the fly and it doesn't generate any intermediate file in NTSC format.

Can you explain a little more what happens when you do it? Why do you think it requires conversion?

iMovie does not convert your 24p movie. The only conversion iMovie performs is for conventional HD camcorders such as HDV and AVCHD. Your LX3 movies are 720 24p MJPEG movies and iMovie imports them natively.

If you use 24p or other frame rate movies in iMovie, iMovie will playback with it's default timeline fps. 24p movie will be read as 30 by duplicating every 5th frame, but it's done on the fly and not by recompressing it to a new movie.