I understand that I need a large, fast hard drive in my computer if I want to use it for nonlinear editing. How large is "large"?

Bert Kovaks

Magalia, CA

There is no one answer to your question, Bert, as several related variables determine the amount of space you need for your projects. Nevertheless, it is possible to give you some rough guidelines.

The variables are:


  1. The type of nonlinear system you plan to use.
  2. The compression ratio.
  3. The size of picture.
  4. The video and sound quality you find acceptable.

A few digitizers need only render those frames of video you have changed, in editing, from their original state. Examples would be frames containing special effects, transitions between scenes, titles and graphics. One of the advantages of these systems is their economy of storage space. In other words, they require only enough space to store your original footage plus enough for these re-rendered frames.

Most video compression boards, however, generate a new video file that contains all the frames of the finished movie. This means you need to have a good chunk of hard drive available after you have captured all of your footage. Often, this chunk should be almost as large as the space required to hold the footage itself. A good rule of thumb: estimate the space that will be needed for a given project by doubling the amount of space that will be taken by the footage you capture.

"Well," you might ask, "how much space do I need for the footage I want to capture?" Again, this is a matter of calculating a few related variables. The grid in Figure 1 is a good place to start. It shows some of the variables from a recent 14-minute project that was captured and rendered at full-screen (704×480 pixels at 64 million colors), full-motion (30 fps) resolution, with 8-bit mono natural (nat) sound and 16-bit stereo music.

Note that at an 11:1 compression ratio (with quality hovering around typical VHS quality), the data rate of the captured video was about 1.758 kilobytes per second. If we increased the compression ratio to 6:1 the data rate would increase to 3.516. At a certain ratio the data rate would be too high for the capture board, hard drive and CPU of a given computer to keep up with it. In other words, your particular system will determine just how low you can set the compression ratio (and therefore, how high you can set the quality).

But lets get back to the question of how much hard drive space was necessary for our project: at the 11:1 ratio, the footage, music, titles and graphic elements for our project took up 2.007 gigabytes. After our system rendered the final edit, the resulting file took up 1.64 gigabytes. Therefore, the whole 14-minute production, including raw footage and rendered transitions, took up 3.647 gigabytes of hard drive space. To be on the safe side, one would want to have at least another 100 MB of space left over for swap files. This project at this compression ratio, nevertheless, could fit comfortably on a 4GB drive.

Reduce the compression ratio (increase the video quality) to 3:1 and this 14-minute project would require in the neighborhood of 15 gigabytes.

































































Compression ratio/time/space ratio

For a typical 14-minute project with 8-bit nat sound and 16-bit music


Compression Ratio (X:1)

Data rate in kB/sec

Quality

Space for footage in GB

Space for finished edit in GB

Total space for project in GB

11

1.758

Medium

2.007

1.64

3.647

6

3.516

Higher

4

3

7

3

7.032

Highest

8

7

15

Space needed for each minute of a finished project

11

1.758

Medium

0.14

0.12

0.26

6

3.516

Higher

0.29

0.23

0.52

3

7.032

Highest

0.57

0.47

1.04

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