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Is a capture and playback device a passthrough or a pitstop?

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Frank Black
Is a capture and playback device a passthrough or a pitstop?
on Dec 23, 2011 at 4:05:25 am

Hi guys,

Some of you already know that I'm researching the depths of what a capture and playback device does WHAT but that I'm looking for "precise" answers in a language a 2nd grader -- me! -- would understand.

Anyway -- I've been going in circles for 2 and half weeks -- no lie. I have so much in notes -- in word, in notepad, on paper, in emails, IN MY HEAD -- that I need to be UNCOMPRESSED!

I feel like all the answers are in my notes (so many notes). But the dots aren't connecting, and part of the reason is that I'm getting opinions that seem to contradicting but in all probability are most like just not "sensitive" enough. The other part of the reason is that I'm a second grader!

The dots aren't connecting because there actually are "holes" in the lines at the beginning and end the dots are. These holes are cause by the two "parts" mentioned above. I'm stuck MAINLY on one thing! And I need an a "sensitive" answer!! -- not an industry answer! I need (and I say need respectfully) an answer sensitive the level of my mind's current capacity not to shut off the moment an unfamiliar term injects it with fear and doubt.

So here's where I'm stuck (and there's gotta be a simple way to FULLY explain this):

1. why do we say that a cap and play device transfers uncompressed when a camera compresses the data when recording it, and since the camera's decompression doesn't fully restore. a full answer if possible. i will remember your name forever. full meaning something like this (though this probably wrong): well, a camera compresses it, then a cap and play connects to it via sdi, a button is pressed and a camera begins playback, and another button is pressed, and the cap and play sucks the wind out of the sdi and the sdi vacuums the footage out of the camera's playback (yep, takes it right off the LCD), and then it takes the data, encodes it, and shoots it thru a thunderbolt pipe into a mac. oh ye and by the way -- the playback was uncompressed. we call it uncompressed because the playback just orders the lost data to come out where it has fled to and get back in line to form a full original file in all its megabytes, and the guys that aren't coming back -- well we won't even notice they're gone. and so and so. (sorry for getting carried away. thanks if you're still with me. help!)

2. when does the camera decompress -- in playback or when you press the magic "decompress" button.

3. how do aja Io, BM UltraStudio 3d, and MXO2 transfer? does it serve as a passthrough or a pitstop? does it say: hang on data! cant go to the mac yet! we must do something to you first. we must: encode? decode? x? y? z?...

4. do all of the three machines mentioned above capture from playback only?


thanks. i'm in debt to you already if you've read this far.


val


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Jeff Greenberg
Re: Is a capture and playback device a passthrough or a pitstop?
on Dec 27, 2011 at 1:51:39 pm

Frank or Val,

Sure, I'll take a crack at this!

1) I think the answer has nothing to do with hardware or pipelines for the most part.
There are two types of compression: Lossless and Lossy.

Lossless compression your familiar with - you've zipped a spreadsheet, right? When you send it to someone else, it's crucial that it arrives with all the data - otherwise we're in real trouble with our financial data!

Lossy compression is like the JPEG we have on our still cameras. It's possible to average some of the information and throw out some of the data that your ey just can't see. For example, at a distance, somebody is holding up a square of 4 lights–3 of them bright green one of them a darker green. We could, average all that to be just one group of 4 dots all just a little darker than the bright green. Less information, but your eye and brain can't really tell the difference. We've lost information, but we have a picture that your eye and brain pretty much interpret as what happened.

Where I think you're getting confused is this–most professional compression schemes used by editorial software are lossy in nature. To display an actual picture though, the last step decompresses this lossy nature back to the full picture (uncompressed).

2) In a DV world (for example), Data on your HD (compressed), goes down the FireWire cable (Compressed), hits the cameras chip during playback for display and the chip uncompresses the lossy information back to full raster.

3) If hardware can't keep up - say BM is requesting to playback uncompressed video @ 1080 and the drives aren't delivering data fast enough, your system 'drops' frames.

4) I don't understand this question.

Best,

Jeff G


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Frank Black
Re: Is a capture and playback device a passthrough or a pitstop?
on Dec 30, 2011 at 1:58:50 am

Hey Jeff,
Thanks a lot... To be honest, I've already been "uncompressed"... a lot of folks pitched in and I finally understand. The example you give of the four dots does paint a picture though. I'd like to know more from such an angle now. And about the dropped frames you mentioned...

What do you mean by this:

3) If hardware can't keep up - say BM is requesting to playback uncompressed video @ 1080 and the drives aren't delivering data fast enough, your system 'drops' frames.

Which drives are you talking about? In the camera? And what happens when the frames drop? How do they drop? How is the drop represented -- in layman's terms and in precise terms?

Thanks Jeff


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Jeff Greenberg
Re: Is a capture and playback device a passthrough or a pitstop?
on Dec 30, 2011 at 3:43:13 am

Frank,

I used a simple 'averaging' formula with the four dots, it's a valid method, but very destructive.

DCT (Discrete Cosine Transform) compression (used in MPEG, JPEG and DV formats) uses a mathematical formula to remove information without you visually knowing it's gone; it's lossy (once gone, in this case, it can never come back.)

The problem with this new data, is that any further operations (such as color correction) will expose how much lossage there is.

[Frank Black] "hat do you mean by this:

3) If hardware can't keep up - say BM is requesting to playback uncompressed video @ 1080 and the drives aren't delivering data fast enough, your system 'drops' frames.

Which drives are you talking about? In the camera? And what happens when the frames drop? How do they drop? How is the drop represented -- in layman's terms and in precise terms"


A camera that can't record the information is useless! So it's not a problem in the camera.

It's in the capture to the drives or the playback from the drives.

Let's say you're playing footage that is stored on the drives uncompressed. Arbitrarily, let's say the demand for uncompressed is 30 units per frame, or 900 units per second (30 frames x 1 frame of 30 units=900)

What if the drives can't push 900 units per second...but only 300. We'll get 1/3 of the information (remember, we're using chunks that we've called frames, and each is 30 units) So let's display every third frame and just skip the ones that we can't get in time.

Other factors: The drives may deliver 900 units, but we chose firewire which only delivers 400? Again, not enough data shows up and some data is ignored; since we're slicing them into 30 unit frames, we'll skip some to display an actual picture.

Factors that can 'fail' - drives, drive attachments (esata, firewire, USB), computer failure (CPU capability, Graphic card capability or Ram capability.)

-----

Oh, you asked for specific too? Go back and change units to megabits per second and then it's specific. (P.S. Yeah, I did some fudging, but it's forgivable.)


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Frank Black
Re: Is a capture and playback device a passthrough or a pitstop?
on Jan 2, 2012 at 3:36:58 am

Theres so much to know Jeff... Thanks man


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Gary Hazen
Re: Is a capture and playback device a passthrough or a pitstop?
on Dec 29, 2011 at 6:03:38 pm

[Frank Black] "1. why do we say that a cap and play device transfers uncompressed"


[Frank Black] " we call it uncompressed because..."

Who is this "we" that you speak of?

If you record something in the DV format it has been compressed at a ratio of 5:1. Your "We" might say that this video is uncompressed during playback, but this would be incorrect.


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Frank Black
Re: Is a capture and playback device a passthrough or a pitstop?
on Dec 30, 2011 at 2:00:57 am

Hey Gary,

... what? :)

It looks like you're making an important point, but I don't understand it.


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Gary Hazen
Re: Is a capture and playback device a passthrough or a pitstop?
on Jan 4, 2012 at 1:49:59 pm

I think I removed too much of the original quote. Here's the entire quote from the first post.

[Frank Black] "we call it uncompressed because the playback just orders the lost data to come out where it has fled to and get back in line to form a full original file in all its megabytes, and the guys that aren't coming back -- well we won't even notice they're gone. and so and so. "

You say "uncompressed" at the beginning of the quote and then describe compressed video at the end. In simple terms, the playback of compressed video doesn't make it uncompressed. All I'm saying is that the "we" you reference in your post is mucking up the language.

I can feed an SDI signal into my Avid and set the amount compression (ex: 1:1, 2:1, 10:1, etc.)
If I record some video at 10:1 and play it back I'm viewing video that was compressed at a ratio of 10:1. To say that I'm viewing "uncompressed video" would be incorrect.

[Frank Black] "why do we say that a cap and play device transfers uncompressed when a camera compresses the data when recording it, and since the camera's decompression doesn't fully restore."

With no reference to the actual hardware or the settings this statement is meaningless.

If you plug a Sony 1550 into a Sony HDCAM SR the recording and playback will be uncompressed.
If you record on a Sony PD150 (DVCAM) the signal will be compressed.

If you had both cameras on the set and checked playback you wouldn't say that the signal coming off the PD150 was uncompressed. Well, the "we" you reference might say that, but it would be an incorrect statement.


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Frank Black
Re: Is a capture and playback device a passthrough or a pitstop?
on Jan 8, 2012 at 11:11:02 pm

hey Gary, I hear what you're saying about the "we" -- as I started understanding more, I noticed that some of my questions may not be phrased right, and I understand what you mean by "hardware" but even without that the concept of uncompressed revealed itself here with people's responses. thanks though. i am trying to wrap my head around the ratios that you're speakin about though... thanks


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