EX1 Data Rate - Shoot 720 or 1080?

HowdyDoo

HowdyDoo

Well-known member
Unlike P2/DVCProHD where data rate is tied to the frame, XDCAM data rate is based on time. 1 second of video, regardless of frame size or rate is 35mbits VBR.

So, when you shoot 1080 you're spreading that data over more pixels on a frame compared to 720.

If I decide to finish my final project out to 720, would it better to shoot 1080 and then resize the image to 720 in Final Cut or would it be better to shoot straight to 720? I suppose if I shot 1080 and resized it, that would give me the option to crop the frame if I needed to fix the composition of a shot.

Which would yield better quality footage from the EX1, a 1080 resized to 720, or a native 720?
 
Some maths..


1080.00​
​
​

35.00​
​
​
MBS​
​
​
25.00​
​
​
FPS​
​
​
1.40​
​
​
MB per frame​
​
​
1080.00​
​
​
Vertical Pixels​
​
​
1920.00​
​
​
Horizontal Pixels​
​
​
2073600.00​
​
​
Total Pixels​
​
​
6220800.00​
​
​
three colours​
​
​
0.23​
​
​
Compression Ratio​
​
​

------------------
720.00​
​
​

35.00​
​
​
MBS​
​
​
25.00​
​
​
FPS​
​
​
1.40​
​
​
MB per frame​
​
​
720.00​
​
​
Vertical Pixels​
​
​
1920.00​
​
​
Horizontal Pixels​
​
​
1382400.00​
​
​
Total Pixels​
​
​
4147200.00​
​
​
three colours​
​
​
0.34​
​
​
Compression Ratio​
​
​

-------------

The different compression ratios is the key to quality surely - by compression ratio I mean shortfall from actually capturing the required DATA for an uncompressed image

at 1080 nearly 80% of the pixels are 'invented' !

whereas at 720 less than 70% of the pixels are 'invented'

There is a pretty big shortfall between the MBPS captured and the MBPS actually required to 'properly capture' the data - ie the camera is doing a lot of interpolations as far as I can work out

I would guess that either - there is not much difference of maybe 720 is better being 'purer'

This is pure speculation howver -

I think a lot more testing is required (or more links to sources of info please ) and most likely it is to do with subject matter that compresses well (or not)

In stills high compression can cause things like banding in the sky (smooth transitions turn to steps) whereas some images - blocky complex ones with simple edges (rather than gradied changes) compress well

Of course when watchng video one has less time to analise each image so some compression will go untoticed

My personal preference is fast moving towards 720 - 25p where one can switch to overcranking in a coulple of clicks and sustain a renderless timeline in FCP between the regular footage and overcranked footage

So shooting style comes into it too

But what do I know about the Ex1 - very little right now

SMM
 
I not sure what school you went to, but your math needs some work. And we need to talk about the formatting too... Dang that was hard to read.
 
yeah done in Exel - didnt copy right into the post - sorry

how many mbs per second do you think it requires to produce ?

25 FPS * 1080PX * 1920PX* 3 colours appreears to be 155million bits of data

meaning 35million is approx 20% of that

meaning the compression is heavy - that does not mean it is visually heavy

S
 
Your math would be correct *IF* it were an intraframe codec. But because it's not trying to capture 1920x1080 pixels with each frame, the math falls down. Also, it doesn't have to try to capture 3 separate colors per pixel, so that falls down too.


morgan_moore said:
yeah done in Exel - didnt copy right into the post - sorry

how many mbs per second do you think it requires to produce ?

25 FPS * 1080PX * 1920PX* 3 colours appreears to be 155million bits of data

meaning 35million is approx 20% of that

meaning the compression is heavy - that does not mean it is visually heavy

S
Click to expand...
 
Interesting - doubtless you are correct and I am wrong - compression algorithms are beyond me and most non PHD maths types

Anyway I dont genuinely dont want to ruin the thread

- Im interested in extracting the optimum performance from the EX1

- both myself and the OP seem to beleive that there may be a trade off in that 1080 may be more commpressed and therefore 'artifacty' while obvioulsy having higher resolution than 720 which one would assume is less compressed - and less 'artifacty' both being 'HQ' settings on the camera alledgedly using the full data rate

so which looks better and in what conditions.. especially conditions that dont actually display at the full 1080 size ?

you may ask, legitimately, why I dont just test my self - of course I am - but Im not convinced my MAC can actually play 1080 properly

S
 
Last edited:
morgan_moore said:
25 FPS * 1080PX * 1920PX* 3 colours appreears to be 155million bits of data
Click to expand...
You're calculating with 3 bits per pixel. But you should do it with at least 24 bit, if we assume 8 bit per color-channel.
So we have 1920*1080*25fps*24bit = 1244160000 bit/s = 1.24 giga bit/s
morgan_moore said:
- Im interested in extracting the optimum performance from the EX1
Click to expand...
You'll get the optimal performance with 1080p. Theoretically there should be more artifacts, but practically there aren't any visible even with 1080p. Also with 720p you can't use the whole latitude of the sensors, because some gain (about a half stop) will be added implicitly by the downconversion-process.
 
Last edited:
Dont worry about my dumbo maths and please note the phrase 'pure speculation' in my first post

Lets get back to how to extract the most visually pleasing performance from the camera in situations where..

- display size is 1080

- display size is 720 or smaller

S
 
Last edited:
Ok Morgan, Let's examine the fact that the camera actually has 1920x1080 photo sites. So in order to get to 720, the camera must do a down-conversion. So the question becomes would you rather do that inside the camera, or outside the camera?
 
mrjazz said:
You'll get the optimal performance with 1080p.

Theoretically there should be more artifacts, but practically there aren't any visible even with 1080p. Also with 720p you can't use the whole latitude of the sensors, because some gain (about a half stop) will be added implicitly by the downconversion-process.
Click to expand...

Nice info thanks

S
 
PerroneFord said:
Ok Morgan, Let's examine the fact that the camera actually has 1920x1080 photo sites. So in order to get to 720, the camera must do a down-conversion. So the question becomes would you rather do that inside the camera, or outside the camera?
Click to expand...

Err - outside - 1080 it will be from now on

I think we are in agreement

S
 
I don't think anyone addressed your original post properly. Because you never told us why exactly are you rendering out from Final Cut Pro at 720p?

After experimenting with the camera enough I only use 720 now if I need true slow motion or if I want the look of 60p. For anything else it's 1080.

720 was my choice before due to increased light sensitivity but now that I've seen enough footage from both I've realized that 720 is actually noisier and I can push +18dB out of 1080p and still have nice looking footage (for run and gun type situations). If I need more light I just drop the shutter speed to 1/24 or 1/32.

So...there's your answer from me. Shoot 1080p...downrez to 720p if that's your output format. The XDCAM EX Codec is really amazing and even in what some would call its worst mode (1080/60i) I haven't had any issues with it.
 
I didnt ask the original question..

My reasons for using 720 are (where)

-Ease of switching to S+Q overcranked mode while shooting

-Ease of FCP workflow for footage at both frame rates

-General theory about high compression on the 'higher res' (debunked by this thread)

-realisation that my work is unlikely to be seen anywhere but on the web for some time

-apparent choppyness of 1080 25p in intial testing that I now think may be computer playback issues

Indeed I drop those shutter speeds in low light
 
Morgan,

I totally see what you're saying in regards to 720p if you're distributing on the web only. 1080-sized video could appear choppier if you don't have a relatively new computer. 720p always plays back much smoother/without dropping frames. That's why a lot of sites are using 720 instead of 1080 for streaming (besides the obvious data compression benefit/trade-off.

1080 is just sluggish on the vast majority of systems. Within Quicktime/Windows Media Player specifically, not within an editing program that is dedicated for that type of encoding/decoding of HD material. And even then most are displaying HD at a 1/4 of the full size that 1080 can get you...unless you're outputting to an external HD monitor.
 
What I was saying is that I thought 1080 looked choppy but now understand that my computer cant play it properly - I dont know what it actually looks like !

I have a mac that is about three years old - it was top spec at the time

Before I would output to web it would be recomressed and downsized anyway most likely

S
 
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