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Canon XL and GL Series DV Camcorders
Canon XL2 / XL1S / XL1 and GL2 / XM2 / GL1 / XM1.


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Old July 13th, 2004, 04:31 PM   #1
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Anyone with some 8bit DSP insights?

I noticed this on one of the Skinny pages that the DSP is 8bit (Like all the Canons from the looks of it) as opposed to the DVX's 12 bit. Anyone know if this is going to have any real world effect on the quality level of the post processing that the camera will do? I'd imagine it would enhance the chance of banding.


Aaron

Moderator note: a while after this thread started Canon
came forth with the information that the XL2 actually has a 12 bit
DSP, not 8 bit! More information can be found in this thread
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Old July 13th, 2004, 06:06 PM   #2
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"I'd imagine it would enhance the chance of banding."

I dont' know about that, but I would like to hear from anyone else with expertise on this too.

DV is an 8 bit format, so in the end even Panasonics extra 4 bits of information is getting discarded at some point in the DV compression.

But, I can't help but think that there is something to the idea of having a higher bit DSP. Sony uses higher bit DSPs in some of their cameras. And, as you pointed out, Panasonic uses a 12 bit DSP in the DVX100a. Even more interesting to me is that they had a 10 bit DSP in the DVX100.

If Panasonic is upping the DSP that was already more bits than DV footage can handle, it seems like there is a reason for it. I would have a hard time believing a company like that would throw money away for no reason (I'm assuming here that a 12 DSP would cost more than a 10 bit).


Can anyone here explaing the advantages and disadvantages of the 8bit vs. 12bit DSP?


Thanks,

-luis
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Old July 13th, 2004, 06:19 PM   #3
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Fewer rounding errors take place if adjustments are done in the higher bit space. If the cameras internal adjustments (black level, cine gamma etc.) are done in a higher bit space, higher quality images may be obtained. It's too early to tell if the 8 bit/12 bit differences will be visually apparent.
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Old July 13th, 2004, 06:20 PM   #4
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Well it would seem that you get more dynamic range. So if you're applying colour multiplication, addition etc, then you've got a higher resolution without rounding off the values. Then once all the effects are applied you can scale it down. If you start with 8 bits, do 8 bit multiplication, addition etc you'll see a loss, clamped values etc. This would be compounded by multiple passes.

It's the same principal why software programs will work in higher resolution when graphics and sound. You lose less as you work.

If you imagine you're tweaking an 8bit picture, you can still only have 256 possible values, regardless of the processing, so a value that should be 47.5 is made 47. If you were using a higher bit range (16) it might be 12079. Do this multiple times and you're losing less resolution in the 16bit than the 8 bit. Then you scale it down to 8 bit and you've got a closer representation of what it should look like than if you had done all the processing in 16bit.

This is how it seems to me. Anyone really know?

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Old July 13th, 2004, 06:29 PM   #5
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In reality I think it is more marketing hype (smoke and mirrors) than a make it or break it mandatory feature. I look at 16 bit RAW and 8 bit JPEG files from 11 plus megapixel cameras. It is very difficult to ever see much of a difference. The processing power inside these video cameras is very limited and probably negates any advantage that the larger bit files have over the smaller files.
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Old July 13th, 2004, 08:58 PM   #6
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RAW image files and moving video is 2 completely different ballgames, although You would think video was just moving stills :)

Anyways, there by no stretch of the imagination is a big difference between 12 and 8bit sampling, The best place to ask about this is a transfer house, because its what they see (and Hate) when they even do Downsampling from 10 bits to 8.
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Old July 13th, 2004, 09:03 PM   #7
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There are many reason to use high bit depth files. But the conversion from 12 bit to 8 bit takes place in camera and the tape records an 8 bit file. The camera performs a relatively crude down-sampled file because of the limited processing capabilities in camera.
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Old July 13th, 2004, 09:32 PM   #8
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Jarred, why do they hate downsampling?


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Old July 14th, 2004, 04:20 AM   #9
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Just to note there are some mixups happening here. An 10 or
12 bit DSP will not give you more "dynamic range" on its own.
You need a 10 or 12 bit CCD for this first. If I'm not mistaken the
DVX has a 12 bit CCD chip. The XL2 has 8 bit CCD's. So it is not
of much use to have a 10 or 12 bit DSP with this.

It could yield *some* improvements, but I think those would be
marginal. Now if you have a 10 or 12 bit CCD then you will need
to match your DSP accordingly. More bits usually gives a wider
dynamic range as well.
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Old July 14th, 2004, 04:53 AM   #10
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Rob I thought a CCD was analogue and it's the conversion process from A to D that converts this to the bits needed. I assumed that by 8bit DSP that inluded the conversion hardware - maybe I was to presumptuous.

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Old July 14th, 2004, 05:11 AM   #11
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Aaron: you are correct that it is analogue. I thought everyone was
talking about the DSP that does all signal processing like white
balancing, digital effects etc. I'm not sure if the A->D process is
done by something you call a "DSP". I'm not that far into CMOS/
CCD's inner working yet <g>

So if you guys where talking about that little device then I
withdraw my comments. They where totally targeted to the DSP
that does all the image processing lateron.
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Old July 14th, 2004, 05:18 AM   #12
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The original XL1 read the CCD as an analog signal, applied gain, gamma, white balance, AGC, and pedestal adjustment, to this analog signal, then converted it to a 9-bit (yes nine) signal for 9-bit DSP. The processed 9-bit signal was then converted to 8-bit for recording to tape.

I doubt that the XL2 is less than 9-bit, and it could be higher.
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Old July 14th, 2004, 06:33 AM   #13
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Quote:
In reality I think it is more marketing hype (smoke and mirrors) than a make it or break it mandatory feature. I look at 16 bit RAW and 8 bit JPEG files from 11 plus megapixel cameras. It is very difficult to ever see much of a difference. The processing power inside these video cameras is very limited and probably negates any advantage that the larger but files have over the smaller files.
There's actually a huge difference in these files when you start to do adjustments and manipulations. With RAW files I can still put awesome images out of pictures that I underexpose by up to two f-stops to maintain the highlights in high-contrast situations, and still get wonderful, practically noise-free images. JPEG's break up long before then.

The reason you see no difference is because both the 8-bit JPEG and the 16-bit RAW started from the same 12-bit DSP in the camera. So the picture in the camera is created at 12-bit precision, and then dithered down to 8-bits and compressed into JPEG.

Saying that you can't see the difference between an 8-bit and 16-bit RAW file is like saying you can't see the difference between a JPEG and a TIFF. The fact is you're not suppose to see the difference, but that doesn't remove the fact that extra information is NOT there, and therefore depending on how much you desire to tweak the image, you won't be able to since the information and bit-depth isn't there for the tweaking adjustments.

The idea behind a RAW file is to give you all the information that the camera started with before it made that 8-bit JPEG. That means the high color-depth, no sharpening, the ability to change color balance, etc. YOU get to be the camera after the fact, it's basically like having a digital negative.

Another good analogy would be like saying I have a chrome slide here and a contact print from the chrome slide. They both will probably look the same, but I can assure you that if you throw away the slide and decide to use the contact print as your scanning source, you're going to be in for a rude awakening.

Moral: Higher bit-depth DPS's are a good thing, but as mentioned before, Canon may be doing a good portion of the processing in analog rather than digital, and then digitizing those adjustments at 8-bits, which is a different story all together than simply digitizing a signal at 8-bits and doing all the processing on that informaton.
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Old July 14th, 2004, 08:26 AM   #14
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<< I doubt that the XL2 is less than 9-bit, and it could be higher. >>

See my CCD / DSP comparison chart at the bottom of this page.
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Old July 14th, 2004, 05:21 PM   #15
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Where did your table data come from?

The Canon documentation I've seen (DM-XL1A Service Manual, Jan 1998) clearly indicates it is 9-bit DSP (the DSP chip is an MN67343A2). Even the schematics show 9-data lines for R, B, and G. Further the A/D converters are 10-bit with the LSB discarded.
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