OT: EX1 format vs file size

Ros wrote on 3/22/2008, 8:16 AM

I have been shooting some 15 second test clips with my EX1 at 720-24p, 720-30p, 720-60p, 1080-24p, 1080-30p and 1080i, all at HQ-35mbps. I was testing various things, but I also wanted to check the file size versus the format used.

To my surprise, all the .mxf files (ex1 .mp4 file also) have all the same file size, about 70mb, less than a mb difference between each shots (1920x1080 or 1280x720).

I haven't done further testing on this, basically running out of time, but does someone know a little more about it?

Thanks,

Rob

Comments

Chienworks wrote on 3/22/2008, 8:35 AM

"15 second test clips"
"all at HQ-35mbps"

Yep. The file size is determined by the duration multiplied by the bitrate. No other factors come into play. Use the same duration and bitrate for each file, and they'll all be the same size. Why would you expect anything different?

Ros wrote on 3/22/2008, 8:44 AM

Shooting at 720 shouldn't make a smaller file size than shooting at 1080?

megabit wrote on 3/22/2008, 9:47 AM

No. But from some basic maths it can be deducted it will be less compressed.

Chienworks wrote on 3/22/2008, 2:23 PM

720 lines isn't part of either the bitrate or the duration, so no, this will have no effect on the file size.

However, 1024x720 is only 0.356 as many pixels as 1920x1080, so you could presumably lower the bitrate to 12.44Mbps to get the same quality with 720 as you would with 1080 at 35Mbps. Then the file size would be 25MB instead of 70MB. Note however that the change of frame size doesn't alter the output size; it's only the change in bitrate that does this.

Ros wrote on 3/22/2008, 5:25 PM

So why would I want to shoot 24p-720 as opposed to 24p-1080 on the EX1 at 35mbps?

Chienworks wrote on 3/22/2008, 5:36 PM

Good question. Maybe because you wanted to. *shrug* I suppose if you knew that the final destination was 720 then you would get a much higher quality recording with 720 at 35Mbps than 1080 at 35Mbps.

Ros wrote on 3/22/2008, 7:37 PM

So shooting with my EX1 at 720-24p gives the least compression, best quality picture? That could also be the best option for chromakey?
Shooting at 720-60p gets more compression?

farss wrote on 3/22/2008, 7:52 PM

The 24p Vs 60p question is a really tricky one. Because mpeg-2 is using temporal compression it's pretty hard to say for certain which would be better. Yes at 24p there's less frames to compress however there'll likely be greater differences between the frames than at 60p.
One reason for shooting 60p is to get good slomo when you play the frames back at 24fps. As far as I know ther's no delivery system for 60p, pity really.

Bob.

Ros wrote on 3/22/2008, 8:00 PM

As for slow motion on the EX1, is 720-60p better or the EX1 slow motion mode or who knows, both equal?

deusx wrote on 3/22/2008, 9:41 PM

Keying? It will depend on what's being shot.

I'm pretty happy with the results I got with HV20 shooting at regular 60i. Decided to use that instead of 24p because it was a basketball shooting motion, so 24p would be blurrier.

I got better results with this, and using only sunlight ( no lights whatsoever ) than when we shot at ESPN studios with perfect studio lighting with their blue screen ( they shot it on DV though ).

farss wrote on 3/22/2008, 9:55 PM

I haven't dug too deep into that part of the EX1 as yet however as I understand it they might be the same. The only difference is the essence that says what the playback rate should be.
.