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Start From the Beginning: I/O

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Michael Cummins
Start From the Beginning: I/O
on Aug 11, 2009 at 1:37:39 am

I've found this is one of the weak points of my experience/knowledge base. What exactly are I/O devices used for and what benefits do they bring over recording to tape or flash storage?

I've also heard that most cameras (including the Canon XH-A1) that have digital outs or HDSDI outputs can bypass the HDV 4:2:0 compression and record to a 4:2:2 colorspace. So I figured I'd give it a whirl. I took my XH-A1, setup my greenscreen and talent, and did some tape recordings and then ran the HDSDI output directly into a Blackmagic Intensity Pro card, into FCP6. Imported the footage into AE and zoomed in to see if there was any difference in the color info between the white pixels of the talent's shirt and the greenscreen. No noticeable difference. Looked like the same 4 pixel blocks. I'm beginning to think I just don't understand I/O devices. It seems like one day they were suddenly all over the market, and everyone just assumes you know what they do.

I'm very humbled by all this.


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Richard Herd
Re: Start From the Beginning: I/O
on Aug 12, 2009 at 5:00:43 pm

Michael, I suggest you try the Apple Final Cut Forum (or some other editing related place on the Cow).

In very simple terms, an I/O device stands for Input/Ouput Device. Essentially, video is an electrical charge running through copper wire. You can store that electrical impulse on tape. The i/o allows you to hook up the computer. It turns the electrical signal into a "computer file."

While it's true HD-SDI is a lot of signal, before the signal can be saved to a computer, you have to tell the i/o device what kind of computer file it will be. That's where we get all these nasty words like: DVCPro HD, MJPEG, HDV, Apple Prores, Uncompressed. Closely related to this is the data rate.

The I/O device will work with your editing platform, so you can choose the codec and video file format you want to turn the signal into. (Does that make sense?!)

Hopefully, you get a chance to experiment, but in general, as you move away from HDV and toward uncompressed, your file sizes should get really large.

The people inventing this stuff are pure geniuses.


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Michael Cummins
Re: Start From the Beginning: I/O
on Aug 12, 2009 at 7:09:16 pm

I've always assumed that when I record to tape I'm recording that video signal with HDV compression, meaning I'm getting a 4:2:0 colorspace giving me color information only a quarter that of my luminance information. Does this HDV compression happen before it goes to tape or as the taped signal is being pushed to firewire? Could I use an I/O device connected to the XHA1 via Component Out and to the computer via firewire to bypass the HDV compression?


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Richard Herd
Re: Start From the Beginning: I/O
on Aug 12, 2009 at 8:07:57 pm

In the business and marketing forum, folks are always saying "I'm not a lawyer." In the cinematography forum, let me say: I'm not an electrical engineer :)

My understanding is that the HDV is processed before it goes to tape.

Regarding i/o via XHA1's component video...doesn't hurt to experiment. The manual says it's 1080i (D3) -- not certain what that means, since D-3 is very old school.

But for a studio-type workflow, you might consider HDSDI and work in Apple Prores. Camera > i/o > apple prorez > RAID drives.

Otherwise, for uncompressed HDSDI, an HDCAM deck and tapes are needed as well as some kind of offline/online workflow design...simply because of the amount of data. 1.5 Gbps is lot of information to store!










[Michael Cummins] "Could I use an I/O device connected to the XHA1 via Component Out and to the computer via firewire to bypass the HDV compression?"

[Michael Cummins] "Could I use an I/O device connected to the XHA1 via Component Out and to the computer via firewire to bypass the HDV compression?"



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Steven Bradford
Re: Start From the Beginning: I/O
on Aug 13, 2009 at 7:56:08 am

"and did some tape recordings "

This is your problem right here. The tape recording is HDV with it's limited colorspace and massive compression.

You want to capture live directly out of the HD-SDI ports to your blackmagic card while you are doing the shoot and leave the tape step completely out of the equation.

This is a very cost effective way to get great results. Otherwise you have to use very expensive tape decks to record that HD-SDI output.



Steven Bradford
http://www.3dstereomedia.com 3D company I've worked with since 1990
http://www.seanet.com/~bradford/ my personal home page, find my greenscreen page there.
http://www.seattlefilminstitute.com the school I teach at.


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Michael Cummins
Re: Start From the Beginning: I/O
on Aug 13, 2009 at 10:13:16 am

That might be the problem there, then. I've had several people yell me to use the HD SDI ports on the XHA1. Fact is, it doesn't have HD SDI, only a component out. Ran a test capturing live out of that component port to the blackmagic intesity pro and compared the results in AE. Still looked like 4:2:0 to me. This was a while back however and I can't remember if it was actually liv or just the vtr playback.


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John Fishback
Re: Start From the Beginning: I/O
on Aug 13, 2009 at 4:10:12 pm

Checkout the AJA KiPro. http://www.aja.com/products/ki-pro/ You feed it HD-SDI from any camera and it records ProRes (which is 10-bit) to either an included hard drive module or an ExpressCard. You can instantly edit that material with FCP.

John

MacPro 8-core 2.8GHz 8 GB RAM OS 10.5.5 QT7.5.5 Kona 3 Dual Cinema 23 ATI Radeon HD 3870, 24" TV-Logic Monitor, ATTO ExpressSAS R380 RAID Adapter, PDE enclosure with 8-drive 6TB RAID 5
FCS 2 (FCP 6.0.5, Comp 3.0.5, DVDSP 4.2.1, Color 1.0.3)

Pro Tools HD w SYNC IO, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec Monitors, PrimaLT ISDN


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Michael Cummins
Re: Start From the Beginning: I/O
on Aug 13, 2009 at 4:35:19 pm

But again, the XH-A1 doesn't have that SDI out; it only has component video out. Am I right in thinking the component out will not provide the same uncompressed signal as the SDI will if fed into an I/O device?


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Steven Bradford
Re: Start From the Beginning: I/O
on Aug 17, 2009 at 6:23:42 am

Actually, the component out is less compressed than the HD-SDI is, sort of. Since it is the direct analog out signal, before digitizing.

Which means the digitizing is happening in your capture card, not the camera. How are you digitizing?

Steven Bradford
http://www.3dstereomedia.com 3D company I've worked with since 1990
http://www.seanet.com/~bradford/ my personal home page, find my greenscreen page there.
http://www.seattlefilminstitute.com the school I teach at.


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