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Tuesday, October 9, 2012

SPEED KILLS

Q I'm hoping to do some editing in the field so I can rough-cut some of my scenes. This way I can know what I have before I move on to my next day of shooting. While I'd like to have a full-blown edit suite in a van or a truck waiting for me to work, my budget is slim to nothing, and I'm trying to do it with the laptop I have. It really doesn't work. I started out trying to use the built-in drive and copy all of my files to it, but that didn't work. Then I tried an external drive, but it couldn't play back a sequence without stuttering. I'm sure there are lots of people doing what I'm trying to do, but how are they doing it?
L.L.
Via email

A I don't really know your complete setup, but since there may be others facing the same situation, I'll generalize. Because you mention stuttering in your sequence, I think you're experiencing dropped frames. The software is calling for frames to be sent to your screen, but they're late coming off the disc.

In the previous answer, I talked a lot about speed and theoretical data transfer rates, but I didn't talk about how long a drive might sustain that speed.

For editing, a system needs to reliably play back a stream of video at the right data rate. If your timeline includes dissolves and "real-time" special effects, your system might be required to play back two streams of video. Why two? For a dissolve, you need to play back the outgoing shot and the incoming shot.

By "reliably play back," I mean completely reliably. If you're going to present a frame of video every 1⁄30 of a second, that doesn't mean frame 29 comes in a little late; it has to be there on time. In fact, it really should be there ahead of time.

Sustained transfer rate isn't something people talk about a lot. It's more difficult to quantify. Going back to my comment about FireWire 800, by using 800 in the name, it was easy to say a drive was twice as fast as FireWire (400). But your problem with dropped frames is all about sustained speeds.

Via testing software, you can put a drive through its paces and get a reading of a certain data rate, but it will be either an average or a maximum speed. Neither really helps you with evaluation of sustained speeds. For example, a maximum reading may show 200 Mb/s, but while the maximum is 200 Mb/s, you don't know if sometimes it drops to 20 Mb/s. If you need 150 Mb/s, you'll have trouble.

An average reading won't work, either. Let's say you need 75 Mb/s, and the average speed you get is 100 Mb/s. You figure you have a little extra speed so that should cover things. But an average is an average. That average of 100 might come from 200, 150, 50 and 0! When you drop below what you need, you'll start dropping frames.

One tool I like to use is the AJA System Test, which is shipped with their KONA I/O systems. (You can install the software without having the device installed.) This software gives a typical interface showing average speeds, but it has a graph mode that shows you what's happening. Run it a few times to get samples so you can judge the consistency of the speed.

With your playback problem, there may be some other issues related to software installation, memory capacity, background processes, etc., but a lot of dropped-frames problems are caused by drives not getting the frames to the computer in time. The graph can really show what's going on.

How much speed do you need? A separate application called AJA DataCalc can show you both speed and storage requirements. Blackmagic Design also makes the Disk Speed Test app.

Send in your video production technical questions to This e-mail address is being protected from spambots. You need JavaScript enabled to view it or mail to Video Assist, HDVideoPro Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025.

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