[mdlug] Raspberry Pinewood Derby

[Ingles, Raymond]

> From: Dan Pritts

> Make sure that whatever storage you are using isn't getting pegged.  It's quite possible you haven't overwhelmed the USB but rather the storage bandwidth.

Actually, that's probably a key factor. The USB bandwidth of the Pi is in the neighborhood of 30MB/sec. http://elinux.org/RPi_Performance#USB_bus

Since the storage is *also* connected by USB, that means half of that bandwidth is available. (Video coming in by USB, then going out to storage via USB.) So the most you'd expect to get by those calculation is about 15MB/sec sustained.

Using Yuyv422, that's 2 bytes/pixel. So at 320x240, 15MB/sec gives you ~102fps max.

Storing to a RAM disk, to free up USB bandwidth, would probably allow 120fps (~18MB/sec). Real-life transfer speeds have been over 20MB/sec. The problem with a RAM disk is making enough room. Three seconds of video is 54MB. Five seconds is 90MB. That's probable doable on a 256MB device not running much else.

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