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Recording in a less compressed/uncompressed format.


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#1 Daniel Smith

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Posted 22 September 2008 - 04:09 PM

Hi.

I've been pretty keen on the idea of recording material uncompressed or onto higher bandwidth tapes, but how big an advantage to this is there?

How much more flexibility in grading will I get? Will I see any difference before grading?

Possibly the most important aspect to me is latitude, will there be any real advantages when adjusting highlights/shadows with raw footage?

I haven't got any examples nor the computer to run the examples unfortunatelly, but if anyones got input on this I'd like to hear it.

tnx.
Dan.
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#2 Phil Rhodes

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Posted 22 September 2008 - 05:42 PM

The difference between really, really good compressed formats and uncompressed ones, such as a really careful 2:1 h.264 against uncompressed, can be pretty subtle in a lot of circumstances, which is why HDCAM-SR is workable.

However, in comparison with practical compressed formats you're actually likely to encounter in the wild, such as HDCAM and DVCPRO-HD, the differences are rather clear, and you'd generally only have to crank up the brightness of a monitor to see mosquito noise in the already noisy shadows.

It helps latitude not so much by giving you more latitude per se; the limit on video camera latitude is actually more noise in the blacks than it is highlight range, being as highlight range is determined only by where you choose to shoot mid-grey. It helps latitude because you can then use very aggressive gamma ramps - depending on the camera - which allow you to expose for highlights a lot more. You can do that on a compressed format, but a lot of your highlight range will be in such a small brightness range and end up so degraded by the compression that by the time you drag it all back down to a sane grade, it'll look like junk.

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#3 Daniel Smith

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Posted 23 September 2008 - 05:28 AM

cheers Phil.

when it comes to bluescreen work do you think I'm better off shooting 1080i or 720p, in terms of keying? down rezzing to standard def after.

dan

Edited by Daniel Ashley-Smith, 23 September 2008 - 05:29 AM.

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#4 Phil Rhodes

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Posted 23 September 2008 - 08:39 AM

Test. I suspect you'd get better results, though.

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#5 Scott Fritzshall

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Posted 23 September 2008 - 09:51 AM

You frequently won't see the difference unless you really go hunting for it- the thing about most compression is that it's designed to be visually lossless so that you can look at even a highly compressed image next to an uncompressed one and say "wow, that's great," but when you start pushing the colors around or doing keying, you realize that the compressed image falls apart very quickly. There's an old episode of fxphd (which is a great resource if you're interested in learning about effects, even as a DP) in which Mike Seymour demonstrates keying fine hair detail off of an HDCam source. He points out that it looks fine to the naked eye initially, but as soon as he starts changing the values, these huge ugly compression blocks show up which he ends up calling "horrendous."

Latitude is probably affected more by the camera than by the recording format generally. Most cameras are designed to output to a format that makes pretty good use of what they're getting off of the sensor. For instance, you can plug an SR deck (10bit) into an F900(HDCam, 8bit), but you're not actually capturing a greater bit depth; you're just mapping 8bit into a 10bit color space as far as I know. Compression is generally spatial; it looks to not record pixel data based on the surrounding pixels, but it's not going to go around changing brightness values.

What camera(s) are you thinking about using? A friend of mine recently shot a short with a Varicam and recorded it with a disc array- not totally uncompressed but I think ProRes, which is still a lot better than DVCProHD. He had a pretty good experience with it from what I heard.
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#6 Daniel Smith

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Posted 23 September 2008 - 05:19 PM

ok tnx.

This is more a computer question than anything, but as for building a server that can process hd-sdi, I've worked out the data rate to be about 1.054 gbits per second including 20% room. What's really confusing me is western digital hard drives, it says on their web site that even their 7,200rpm hard drives have a transfer rate of 3gbits per second.

http://www.wdc.com/e...veid=552#jump11


But in every server I've seen built to take high quality video there has been a raid 0 array, but these hard drives appear to have a transfer rate which can handle hd-sdi.

Either I'm looking at the wrong thing here or new sata II hard drives can handle hd-sdi viewing in realtime.

what am I missing here?
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#7 Scott Fritzshall

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Posted 23 September 2008 - 06:35 PM

I believe that uncompressed 4:4:4 HD is closer to 1.5Gbps. The confusion you're having with the drives is that their interface is SATA II, which can handle 3Gbps. The drives themselves, at least individually, will neither read nor write anywhere close to that speed. If you load them up in a RAID configuration, then you'll start to saturate the channel and get the speeds you need. I don't know how many 7200RPM drives you need at once to get realtime uncompressed HD read speeds. I've got a little RAID 0 on my computer with two 320GB drives, and it seems fast enough to read uncompressed SD in realtime, but not even close for HD. I tested them once and I think they had a read speed of like 100MBps, so I think I'd need to at least double it before being able to handle uncompressed HD.

Edited by Scott Fritzshall, 23 September 2008 - 06:36 PM.

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#8 Phil Rhodes

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Posted 23 September 2008 - 07:50 PM

I don't know how many 7200RPM drives you need at once to get realtime uncompressed HD read speeds


About six, but you don't want RAID-0.

I wrote an article on it.

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#9 Daniel Smith

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Posted 24 September 2008 - 08:52 AM

great article.

Does anyone know of any performance data giving actual read/write speeds of hard drives? I've had a search but nothings come up.

It would be nice to not use 6 hard drives and get some 10,000 rpm drives instead but as they give no clue of the speed I don't know how many would be fast enough.
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#10 Phil Rhodes

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Posted 24 September 2008 - 12:04 PM

Practical speeds are affected by filsystem, controller, degree of fill and fragmentation; it's pretty much impossible to try and state what you'll really get. Buy one and find out. I regularly get 70MB/sec out of decent modern SATA drives but they won't do that all the way in to the middle.

RAID-0 does tend to increase speed linearly, that is, two disks are twice as fast as one, but other RAID arrangements can have wildly varying speed between controllers.

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#11 Daniel Smith

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Posted 24 September 2008 - 03:34 PM

I regularly get 70MB/sec out of decent modern SATA drives


is this from a 7200?
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#12 Phil Rhodes

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Posted 24 September 2008 - 05:17 PM

Yes, but that makes less difference than you'd think.

What makes the biggest difference in my experience is perpendicular recording, which is designed to increase density but as a side effect increases speed simply by causing the same speed of rotation to move more data beneath the head per unit time.

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#13 Daniel Smith

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Posted 24 September 2008 - 05:36 PM

ok thanks guys.

I'm looking forward to those solid state drives coming down in price, writing at 85meg per second and reading at about 130, SATA II with RAID support, four of them should go in a fairly cheap raid 5 motherboard and cover hd-sdi.

But really I'm only looking into being able to process hd-sdi, I'm not splashing out on a blackmagic card when I know it's just not worth it at the moment.

I just wish my college would realise that they're only getting about 10% of the three HDX900's potential by recording onto tape right now.

Edited by Daniel Ashley-Smith, 24 September 2008 - 05:41 PM.

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#14 Daniel Smith

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Posted 24 September 2008 - 05:44 PM

writing at 85meg per second and reading at about 130


- not to mention the stupidly quick seek times, less than 0.35ms.
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