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Old May 28th, 2007, 08:24 AM   #1
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One drive short?

I'm getting dropped frames when I playback uncompressed 10-bit 1080/50i. Usually plays for about 10 or 15 seconds before it hits the snag. Now the first thought is that my disk array is too slow, and this may well be the problem. But my Blackmagic speed test shows a disk write speed of 219 MB/s and read speed of 214 MB/s. Shouldn't that be high enough? And if it's not, why was I able to capture uncompressed in the first place?

OK, the details... I'm using a 2.0T three-disk internal RAID on a Mac Pro (quad 2.66GHz, 5 GB RAM, ATI X1900, 3X750GB array), with a Blackmagic Multibridge Pro. After reading through this forum and elsewhere, I realize three internal disks is not the optimal configuration for uncompressed HD. But first let me just be sure that it's the RAID and not something else that's causing this problem. I was hoping to be able to take care of a couple of projects and get some revenue before I plopped down another $10K on a fiber channel RAID. Is my BM giving me faulty figures, or what is causing the slowdown? Thanks.
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Old May 28th, 2007, 08:50 AM   #2
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Assuming your raid has an interface that can handle that speed, it sounds about right for three drives. A typical read speed on modern drives is in the neighborhood of 75MB/s and triple that is about what you are getting. What is the interface of your RAID? I assume it is PCI or PCIX card. Is there any sort of buffering involved with the card?
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Old May 28th, 2007, 09:37 AM   #3
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Thanks Marcus,

I'm using a RAID of three internal SATA drives - 7200 rpm. I set a RAID block stripe of 256 KB. I don't know how much buffering goes on there.
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Old May 31st, 2007, 02:17 AM   #4
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OK. Let's assume that despite my Blackmagic speed test, the disks are too slow. Would I have an easier time capturing at 8 bit uncompressed? What will losing the 2 bits do to the quality of the video?
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Old May 31st, 2007, 03:44 AM   #5
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Quote:
Originally Posted by Steve Nettleton View Post
OK. Let's assume that despite my Blackmagic speed test, the disks are too slow. Would I have an easier time capturing at 8 bit uncompressed? What will losing the 2 bits do to the quality of the video?
If you are capturing from tape, nothing. Material stored on tape is 8 bits, so adding 2 bits will not improve quality, since those bits were never there in the first place. However, with color correction and keying the (fake) 10 bit material will hold up better over time.
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Old May 31st, 2007, 09:08 AM   #6
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"Let's assume that despite my Blackmagic speed test, the disks are too slow."

I wouldn't say the disks are too slow, but if your RAID controller is built into the motherboard it may use too much CPU.

I'm not exactly sure what you are trying to accomplish but you might want to look into the Cineform codec for your work. It is a very low loss codec and goes up in price with resolution and bit depth. I think the product you might want to investigate would be their Neo HD which goes up to 1920x1080 with 10-bit precision. It retails for $599. There is a cheaper model with HDV-type specs for resolution and bit depth, but it is still less lossy and costs only $249. I haven't used these products, but it seems like they may be along the lines of what you want to use.
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Old May 31st, 2007, 09:33 AM   #7
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Shouldn't be a processor problem. I've seen quad 2.66 GHz computers handling uncompresssed 4:4:4. And Activity Monitor shows only about 20 to 25 percent CPU load during playback.

I think I may have the solution. I downsampled a segment of my existing footage to 8 bit 4:2:2 and it played fine. So perhaps I should just capture it at that rate and work in 8 bit throughout. Besides, as I just learned, it was only recorded in 8 bit anyway (Sony HDCAM 750). I'm doing HD news production, so I don't need chroma keying or heavy compositing work. Maybe just some minor color correction and simple effects and subtitles. I get the feeling 8 bit will suffice.

I'll give Cineform a look. Don't think I want to chance it with a Mac Beta version just now but when it's gold master I'll definitely check it out.
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