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

[John Rofrano]

[Frank Black] “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.”

These devices only capture “live” from your camera and only playback from your PC! They connect to your camera via an SDI port that records directly from the sensor bypassing your camera’s recorder. This is why they get uncompressed video but you have to have your computer connected to your camera while you’re shooting in order to do the original recording. Once you record something to your camera, it has been compressed by the camera and you can never get the uncompressed version back.

I’m assuming that is the missing link in your understanding as the fact that you must have this device connected to your camera “while you record” in order for them to capture uncompressed video directly from the sensor AND you need a camera that allows access to the sensor via SDI so they won’t work with just any old camera.

Does this make more sense?

[Frank Black] “2. when does the camera decompress — in playback or when you press the magic “decompress” button.”

The camera compresses to record and decompresses on playback. I don’t know of any “decompress” button. if it’s playing, it’s decompressing and there’s nothing you can do to stop that. I think you got confused because you thought that by some magic these devices got uncompressed video from what your camera recorded. They do not. Once the video is compressed the loss is done.

[Frank Black] “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?…”

It’s a passthru and there is no stopping. Unless you have a computer with a RAID 0 array of at least 5 drives, you will not be able to sustain the transfer of uncompressed HD and it won’t work. Working with uncompressed HD is not for the faint of heart and IMHO not needed unless you are working on a blockbuster movie and plan to do film transfers for your final output.

[Frank Black] “4. do all of the three machines mentioned above capture from playback only?”

No, NONE of them do and this is the hole in your understanding. They capture from a live camera. They playback from your computer to a studio monitor. Your computer is the only thing doing playback with these devices so that you can watch the video you’ve captured to your computer on a broadcast quality studio monitor via SDI. They have NOTHING to do with your camera’s playback mechanism and, in fact, bypass it entirely. In this scenario, your camera serves only as a lens and a sensor. The transport and recorder of your camera is not used.

Feel free to ask more questions base don this new understanding.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Frank Black]

John, hi — sorry for the delayed response. Things just turned out this way, but I did get a chance to read your response almost immediately.

I gotta say, I posted this question in many forums here on the cow, and a lot of folks surprisingly took the time like you took the time. I felt so much appreciation. Anyways, you guys really painted a clear picture for me. I actually understand uncompressed. I also gotta say, John, there are a couple of points that you made here that seem to contradict what I now understand about uncompressed, but I can say with confidence that this is due to the way I phrased some of my questions. I can tell this based on your answers to them. Anyway, I understand uncompressed, and was also able to pick up a few extra gems from everyone’s input. Thanks a lot, John. You actually once helped me in a Vegas vs FCP question I posted. If you remember, it is probably still one of the longest posts in cow history. Thanks man. I do have more questions, but wanna formulate them better first. Thanks for offering. Cheers

Val

[Frank Black]

Hi John, if you can help me further, please do; I’m still researching I/O devices and am finding only definitions for many details.

I’m trying to find about 3G-SDI/HDMI, 4:4:4 RGB, and conversions (and genlock if possible).

So what if I have SDI? Tougher cables, etc… But so what? What stands out? Can it transfer better quality? Is that even possible?

(beat)

What is 4:4:4 RGB?

And if I’m converting between 4:4:4 and 4:2:2 to set up single-link HD-SDI output, why am I doing this? And how is it happening?

In up/down/cross conversions, do I choose one of the three (is there a button?). Can I choose NOT to choose one and to just let the data through? And how is it down? What happens to the footage? Do lines get added? And why would I need to do this? I know that in a broadcast setting, and I’m sure other settings as well, one may be given some SD footage and some HD and will need to kinda match them up. But why else?

Please find the time if you can. Thanks for before and thanks a lot anyway.

[John Rofrano]

[Frank Black] ” I also gotta say, John, there are a couple of points that you made here that seem to contradict what I now understand about uncompressed, but I can say with confidence that this is due to the way I phrased some of my questions.”

Such as…?

~jr

http://www.johnrofrano.com
http://www.vasst.com

[John Rofrano]

[Frank Black] “So what if I have SDI? Tougher cables, etc… But so what? What stands out? Can it transfer better quality? Is that even possible?”

It’s not about the cable (although they are better quality). It’s the components behind the cables that implement SDI which is the standard for high-end, uncompressed digital video formats. SDI is designed for uncompressed. I don’t know the technical details as I’m not a hardware engineer.

[Frank Black] “What is 4:4:4 RGB? “

It is a color space that uses 4 bits for each of Red, Green, and Blue signal as apposed to chroma subsampling like the 4:2:2 Y’CbCr scheme which uses 4 bits for luma/green and 2 bits each for chroma blue and red. Since our eyes are less sensitive to changes in color than they are in luminance, we can cheat and not record all of the color signal thus reducing bandwidth requirements.

[Frank Black] “And if I’m converting between 4:4:4 and 4:2:2 to set up single-link HD-SDI output, why am I doing this? And how is it happening? “

I assume you are doing it to reduce the bandwidth needed. You need dual-link SDI to carry 4:4:4 and only single-link to carry 4:2:2. So if you want to use single-link SDI you need to down convert.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Frank Black]

Hey John,

First, as per your “such as” question – you said, in response to my asking whether all I/O devices capture from playback, that “NONE of them do…. They capture from a live camera. They playback from your computer to a studio monitor.”….. You’re obviously right, but I understand now that even if the output is not live, and is a from a memory card, the capture IS from a playback. And obviously you know this. You even said so when you said “it’s decompressing and there’s nothing you can do to stop that.” You said this in regards to when the camera plays back the recorded file. So my question was probably not phrased right. By the way – when you said “it’s decompressing and there’s nothing you can do to stop that,” that really helped me understand “uncompressed.” So simple.

OK, now about my new questions – you said:

[J.R.] It’s not about the cable (although they are better quality)…… SDI is designed for uncompressed.

Although you said you don’t know the technical details behind this, would you know WHAT makes it designed for uncompressed. What does this mean? Won’t any cable transfer whatever is fed into it? Do some cables, perhaps, compress the data as they transfer it? Someone told me that firewire compresses because it can’t handle uncompressed. But someone else told me that firewire is a pipe and whatever goes into a pipe comes out the other end just the same. Perhaps it gets compressed without “throwing away” any 1s and 0s?

[J.R.] You need dual-link SDI to carry 4:4:4 and only single-link to carry 4:2:2. So if you want to use single-link SDI you need to down convert.

By dual-link SDI, do you mean two SDI outputs? And is this how some I/O device capture 4:4:4, because they have two SDI inputs? And these same I/O devices also have dual SDI outputs. So do they output only via a single SDI output because there’s nowhere to connect two SDI outputs to, leaving us with the only option of connecting 1, and thereby we have to convert?

Thanks John.

[John Rofrano]

[Frank Black] “You’re obviously right, but I understand now that even if the output is not live, and is a from a memory card, the capture IS from a playback.”

Yea, I don’t consider copying files from a memory card “capturing”. It’s just copying files from a memory card like you copy files between hard drives. There is no “playback” in that case. It’s just copying the recorded file bit-for-bit to a new device.

[Frank Black] “Although you said you don’t know the technical details behind this, would you know WHAT makes it designed for uncompressed. What does this mean? Won’t any cable transfer whatever is fed into it? Do some cables, perhaps, compress the data as they transfer it?”

OK, forget about the cable. SDI is a protocol. It’s about the data that is being transmitted through the cable, not the cable itself. The cable is just a wire. It doesn’t do anything on it’s own except flow electrons.

[Frank Black] “Someone told me that firewire compresses because it can’t handle uncompressed. But someone else told me that firewire is a pipe and whatever goes into a pipe comes out the other end just the same. Perhaps it gets compressed without “throwing away” any 1s and 0s?”

Firewire is also a protocol (i.e., a pipe). It doesn’t change the data in any way. It just flows bits unchanged over a wire in a certain way. This is why you can use it for video cameras, or audio devices, or hard drive connections.

[Frank Black] “By dual-link SDI, do you mean two SDI outputs? And is this how some I/O device capture 4:4:4, because they have two SDI inputs? And these same I/O devices also have dual SDI outputs.”

Yea, dual link HD-SDI is essentially of a pair of single HD-SDI links to allow greater bandwidth. I believe single HD-SDI allows about 1.5 gigabits per second and dual allows approximately 3 which is required for 4:4:4 video.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Frank Black]

Good stuff. I gotta say, though, from everything I’m learning, there IS playback for copying from the memory card. The cam has to play back in order to even be able to let the files out of the memory card.

[John Rofrano]

[Frank Black] “… from everything I’m learning, there IS playback for copying from the memory card. The cam has to play back in order to even be able to let the files out of the memory card.”

I’m not sure what camera that is, but every camera I’ve ever seen presents the memory card as a hard drive and you just copy the files from it to your computer. I’m not saying that you can’t play back the video in the camera and capture from some other camera output but why would you bother when you can just copy the original files directly from the card with no loss of quality?

Unless, of course, you are referring to a Mac computer which can’t handle native camera formats. That’s an entirely different story all together and is a limitation of the Mac software. This is the Vegas forum and Vegas copies the files directly from the camera’s memory card with no playback required and no loss of quality.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Frank Black]

Really?! You mean if I hook up an I/O device to a PC w/ Vegas, Vegas will capture through the I/O without playback? Or capture it otherwise without playback? Will it have to be from a camera that has a codec that Vegas will support? What codec?