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Re[2]: [OM] Digital camera lag

Subject: Re[2]: [OM] Digital camera lag
From: Dave Haynie <dhaynie@xxxxxxxxxx>
Date: Fri, 25 Dec 1998 22:34:22 -0500 (EST)
On Thu, 24 Dec 1998 08:58:41 -0800, GMA <gma@xxxxxxxxxx> jammed all night, and 
by sunrise was overheard remarking:

> So my question about the digital lag is: Do they all do it? 

The delay from "I pressed the shutter" to "photo caught in the CCD (or
CMOS) sensor? Far as I know, it's very common. It's not unavoidable, far
as I know, but it does seem to be rather acceptable to have a small
delay here, though I think more recent cameras are eliminating it,
perhaps because folks have complained, perhaps because they want to
offer a "sequence" mode. 

The more annoying lag is "I pressed the shutter" to "photo in NV storage,
ready for the next". This is true in all digital cameras, though some can
buffer it.


> Does anyone know? 

The details are usually based on the storage medium, the size of the
image, the compression, if any, and of course, how much you paid. The
CCD itself can run in realtime -- I don't know that all necessarily do,
but they certainly can -- virtually all video cameras (in the consumer/prosumer
price ranges at least) use CCDs exactly the same way. Some digital
models even offer progressive-scan CCDs like a digital camera. They
obviously do what they do in realtime.

The main difference is that the data needs to be processed, and most
companies aren't going to spring for hardware encoders. So there's a
tiny microcontroller used to "run" the camera's operating system, and
this same MPU converts the buffered raw data from CCD into a compressed
format, usually JPEG, though fractal and maybe wavelet compression are
other options from some makers. 

>From there, the data is written to something. Most digitals use solid
state "flash" ROM memory. This isn't especially fast memory, but chances
are, you can't afford a camera that would take much more than a second
for such storage. At least if the flash is erased before the storage is
needed. You might think cameras with a raw data capture mode would
capture faster, and maybe some do, but there's a real tradeoff, since
it's all that much more data to store. Cameras with multiple-shot
capability use some faster "main" memory for this (all have at least
enough main memory for one shot, plus pieces of the operating system
that need to live in main memory). That's 3.75MB for a 1280x1024 shot in
24-bit raw data -- just one.  Only 900K for 640x480, so you can see why
some of these multi-shots only run that option in low-res mode. 

> Do we care?  I briefly held the 600L in my hand, pointed it at
> something and pressed the shutter. The image locked on the LCD and a red
> LED blinked for about 3-4 secinds as the image was downloaded.  That of
> course is a different lag than what you guys saw.  This lag is for image
> download to permamnent storage from the RAM buffer.

And compression, which may actually be the longer of the two. A PC-Card
or other flash ROM device can run at 4MB/s or faster, in theory, but in
practice, it's likely to be slower. And in fact, the interface allows it
to be slower, so the camera can't necessarily know how long download to
flash is going to take. Sony's floppy thing is even worse, though of
course, the floppy speed is always the same, at least within motor/battery
tolerances. 

But JPEG encoding of a non-trivial image can be a noticable wait even on
a fast PC. The MPU in your digital camera is likely to be 1/10-1/25 of
that performance level, on a good day. 

--
Dave Haynie  | V.P. Technology, Met@box Infonet, AG |  http://www.metabox.de
Be Dev #2024 | NB851 Powered! | Amiga 2000, 3000, 4000, PIOS One



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