Ok, don't understand compression...

F

FatBird19

Well-known member
DVCPRO25 is 5:1 right?
and DVCPRO50 is 3.3:1 right?

I heard that DVCPRO HD is 6.7:1. why would it be so high? shouldn't it be the same as dp50? After all, dp50 is 720x480 and 720p is twice that(720x960) and twice the bandwidth. So shouldn't DVCPROHD have the same amount of compression as DVCPRO50?

eh?
 
Emanuel said:
With more resolution?! (= more needs of data, so...)
Click to expand...

Yes, DVCPROHD 720p has double the image resolution of DVCPRO50, but it also has double the bandwidth. So why is there a compression difference?
 
oh, I should have clarified.
the compression differences between DVCPRO25 and DVCPRO50 don't confuse me(by asking as a question I was looking for confirmation of whether I got the right compression amounts), it's when we get to DVCPROHD that I get confused. At least for 720p.
(I know that 1080i uses a greater amount of pixels per image, so the compression increase is obvious)
 
Maybe the equation is not so proportional...or it works like that. But I'm not the right person to enlight that, maybe a techie member may do it. Is there anybody there or just waiting for HVX 3CCD specs? :cheesy:
 
Emanuel said:
Maybe the equation is not so proportional...or it works like that. But I'm not the right person to enlight that, maybe a techie member may do it. Is there anybody there or just waiting for HVX 3CCD specs? :cheesy:
Click to expand...

are those going to be released soon or something?
 
I'm just joking! :) Maybe yes, my friend, but there's a private joke (similar jetpack mode) as you can follow since here (1st case) and there (2nd one: Copyright ©2005, brianluce - one of ours jokers...).
 
FatBird19 said:
I know that 1080i uses a greater amount of pixels per image, so the compression increase is obvious
Click to expand...
OK! Regarding 1080i, you already have the answer...
 
Concerning HVX 3CCD specs release, it's a long, long talk, even more seriously, that you can preview @that link above-mentioned.
 
Emanuel said:
OK! Regarding 1080i, you already have the answer...
Click to expand...

yes, but the 720p compression is still an enigma.

My only thought is that it's because it encodes 60 full frames per second instead of 60 interlaced half frames per second. That could account for the increase in compression, and it just about works perfectly(3.3 to 6.7).

I don't know if that's how interlaced video is encoded though.
 
And as I could and can see, you are a dvxuser.com member that don't like needless enigmas. OK! Welcome to the club and join to!
 
FatBird19 said:
My only thought is that it's because it encodes 60 full frames per second instead of 60 interlaced half frames per second. That could account for the increase in compression, and it just about works perfectly(3.3 to 6.7).
Click to expand...

You're right...

1080i is 100Mb/s (actually 100.61Mb/s for video plus 6.15Mb/s for audio)
720p60 is 100Mb/s
720p30 is 50Mb/s
DVCPro50 is 50Mb/s
 
In order to understand the compression ratios, you have to take into account the colour sampling. While DVCPRO-HD appears to have "2x the resolution" you have to remember the 4:2:2 business. A colour image isn't a 2D matrix, it's 3D.

Observe:
DVCPRO-50 720x480 4:2:2 @ 29.97 fps and 50 Mbps
Y: 720x480
Cr: 360x480
Cb: 360x480
Total pixel count: 691200 (pixels/frame)
Total bits per second = 691200 (pixels/frame)*29.97 (frames/s)*8 (bits/pixel): 165 Mbps.
Compression = 165 Mbps/ 50 Mbps = 3.3:1

DVCPROHD-720P 4:2:2 @59.95 fps and 100 Mbps
Y: 960x720
Cr: 480x720
Cb: 480x720
Total pixel count = 1382400 (pixels/frame)
Total bits per second = 1382400 (pixels/frame)*60 (frames/s)*8 (bits/pixel): 663 Mbps.
Compression = 663 Mbps/ 100 Mbps = 6.63:1

Hence DVCPRO-HD is much more compressed than DVCPRO-50. Now, you can do the calculations for any other colour sampling scheme and compression algorithm... If you do DVCPRO-HD at 1080(i/p), you'll find the compression ratio is exactly the same as DVCRPO-HD at 720p.

Note that this math does not indicate the effect of the pixel aspect ratios, and compression is based upon the resolution of the format. For example, this tells you that a 4:2:2 pixel is compressed 6.63:1. A 4:4:4 pixel has 2x the information, and is therefore being compressed 13.3:1. Furthermore, taking into account a square-pixel final image, you can multiply by the pixel aspect ratio... Which gives:
DVCPRO-HD @ 720P is 17.65:1
while
DVCPRO-HD @ 1080P is 19.9:1
when up-sampled to the final display resolutions of 4:4:4 1280x720 and 1920x1080 respectively.

-Spiff
 
Exactly. DVCPRO-HD in 720p has twice the pixels per frame as DV50, and twice as many frames per second as DV50, which means four times as much data as DV50. But only twice as much bandwidth. So it needs to be compressed twice as hard, hence 6.7:1 vs. 3.3:1.

For 1080, there's four times as many pixels per frame as DV50, at the same frame rate as DV50, but only twice the bandwidth. Again, it is necessary to compress it twice as much to make 4x as many pixels fit in 2x as much bandwidth.
 
Barry_Green said:
Exactly. DVCPRO-HD in 720p has twice the pixels per frame as DV50, and twice as many frames per second as DV50, which means four times as much data as DV50. But only twice as much bandwidth. So it needs to be compressed twice as hard, hence 6.7:1 vs. 3.3:1.

For 1080, there's four times as many pixels per frame as DV50, at the same frame rate as DV50, but only twice the bandwidth. Again, it is necessary to compress it twice as much to make 4x as many pixels fit in 2x as much bandwidth.
Click to expand...

it still looks pretty good though, right? I hope that doesn't have as big of an affect.
 
Spiff said:
In order to understand the compression ratios, you have to take into account the colour sampling. While DVCPRO-HD appears to have "2x the resolution" you have to remember the 4:2:2 business. A colour image isn't a 2D matrix, it's 3D.

Observe:
DVCPRO-50 720x480 4:2:2 @ 29.97 fps and 50 Mbps
Y: 720x480
Cr: 360x480
Cb: 360x480
Total pixel count: 691200 (pixels/frame)
Total bits per second = 691200 (pixels/frame)*29.97 (frames/s)*8 (bits/pixel): 165 Mbps.
Compression = 165 Mbps/ 50 Mbps = 3.3:1

DVCPROHD-720P 4:2:2 @59.95 fps and 100 Mbps
Y: 960x720
Cr: 480x720
Cb: 480x720
Total pixel count = 1382400 (pixels/frame)
Total bits per second = 1382400 (pixels/frame)*60 (frames/s)*8 (bits/pixel): 663 Mbps.
Compression = 663 Mbps/ 100 Mbps = 6.63:1

Hence DVCPRO-HD is much more compressed than DVCPRO-50. Now, you can do the calculations for any other colour sampling scheme and compression algorithm... If you do DVCPRO-HD at 1080(i/p), you'll find the compression ratio is exactly the same as DVCRPO-HD at 720p.

Note that this math does not indicate the effect of the pixel aspect ratios, and compression is based upon the resolution of the format. For example, this tells you that a 4:2:2 pixel is compressed 6.63:1. A 4:4:4 pixel has 2x the information, and is therefore being compressed 13.3:1. Furthermore, taking into account a square-pixel final image, you can multiply by the pixel aspect ratio... Which gives:
DVCPRO-HD @ 720P is 17.65:1
while
DVCPRO-HD @ 1080P is 19.9:1
when up-sampled to the final display resolutions of 4:4:4 1280x720 and 1920x1080 respectively.

-Spiff
Click to expand...

Are these figures valid for the PAL version as well?
 
For PAL you have to change the framerate to 25 fps, and the resolution to 720x576. Likewise, for HD you'd have to change the resolution and framerate to 1440x1080 and 25 fps I think? I'm not entirely sure if there is a PAL version of the 720p codec.

At any rate:
DVCPRO-50 PAL: 720x576 4:2:2 @ 25 fps and 50 Mbps gives
829440 (pixels/frame)*25(frames/second)*8(bits/pixel) = 165.9 Mbps
(carrying the fourth figure for NTSC gives 165.7, so I had a rounding error)
The compression is identical to NTSC here.

DVCPRO-HD PAL: 1440x1080 4:2:2 @ 25 fps and 100 Mbps gives:
3110400 (pixels/frame)*25(frames/second)*8(bits/pixel) = 622 Mbps
Making the final image compression per pixel 6.22:1.
Comparing this to the upsampled 4:4:4 1920x1080 output, you get a final ratio of 16.6:1.
Marginally better than the NTSC version.

-Spiff
 
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