Remarkable tip! How to increase capture bandwidth & reduce

[tacosalad]

I didn't find this tip in the forum or "how to" guides, so I figured I'd better share it.

TIP: Set VirtualDub's "Chunk Size" to 4 MB or more.

By adjusting VirtualDub's Disk I/O settings, I have quadrupled my computer's capture bandwidth. Under the "Capture" menu, I opened the "Disk I/O..." dialog box and raised the Chunk Size to 4 MB. Instantly, my Bt878 could effortlessly capture at 720x480, with bandwidth to spare.

So my advice is, make the "chunk size" as big as conveniently possible. (There's probably a point of diminishing returns around 6 MB.) The number of "chunks in buffer" doesn't seem to be quite as critical. It works okay with just 2, but raising it to 4 seemed to reduce the risk of dropped frames during very long captures.

When the chunk size is very large like this, the hard drive becomes relatively placid while capturing; it writes in small bursts once or twice a second.

As an experiment, I started pushing the limits last night, configuring my card for 720x480 @ 30 fps, 24-bit RGB uncompressed. (These are wasteful settings just for testing.) That's over 20 MB/sec, but it still dropped only one frame in the first 30 seconds. By contrast, my system barely handled 4.5 MB/sec with the default disk settings. And yes, I did verify that the captured video was intact.

This trick might not yield such a dramatic improvement for everyone, but give it a try and report your results (or failures).

I was inspired to try this after reading in the "Hardware Bible" that IBM hard drives omit address marks and sector gaps to save space. I wondered if this meant cylinders had to be written all-at-once, rather than a sector-at-once. That's probably wrong, but I decided to test my IBM drive with much bigger chunk sizes, and that yielded a four-fold bandwidth increase! A 5400 RPM Western Digital drive showed a similar improvement.

Any comments?

-TacoSalad

[matt-o]

I am curious what is your system set-up like?
cpu.. etc?

[gtrz]

Does anyone know if setting my lite raid on my K7T Turbo-R
to the(video editing)setting instead of(desktop)would help
for dropped frames?Do I need to fdisk it that way after I
change it?

[buzz10]

Tacosalad,

There is a little blurb about chunk-sizes in Lukes Video Guide http://www.geocities.com/lukesvideo/index.html, under the Hi-Res VDub Capture section. He recommends 2mb x 4, but you can experiment to see what works best.