Date: Thu, 13 Aug 1998 13:20:06 -0500 (CDT)
From: Ethan Brodsky <brodskye at cae.wisc.edu>
Subject: Re: Sound card programming

On Thu, 13 Aug 1998, somebody wrote:
> Again I want to sample the line in or mic and get an unsigned integer
> back. I don't want or need 5 thousand samples in one second and I don't
> want wav or voc files. I want to sample maybe 1000 samples in one second
> tops. 16 bit only.

The problem is that what you want to do isn't directly supported by
SB cards.  Reading samples one at a time from the sound card is called
"Direct Mode" and is only supported for reading eight bit mono audio.
Creative Labs never implemented commands to do it for sixteen bit audio
or stereo.  It's also very CPU intensive, though you would have no 
problem reading at 1ksample/second.  
 
If I needed something like that, I would begin with a standard DMA
recording program such as sb16snd.  Cut out all the program to write
data to disk, since you don't need to do that.  Then write code that
reads the current position from the DMA controller.  You can use that
to get the address of the most recent sample.  You still need to keep
the circular buffer (though you could get away with a small one *),
but you can get samples whenever you want.

While writing this, I thought of another solution.  Instead of having
a large buffer and picking out the most recent sample, why not use
a one-sample buffer.  Program the DMA controller for autoinitialized
DMA on a buffer of one word.  You can program the DSP for a larger
block size (that way you won't get as many interrupts), or just read
from 2xF in the inner loop to keep the sound card going.  The most
recent sample should always be in that memory location.  The only
problem might be if you try to read in the middle of an update, but
I think that the DMA controller locks the bus for the full duration
of the memory write, so you should be ok.  If you want a bit of 
filtering to get rid of high-frequency noise, use a slightly larger
buffer and average all the samples in the buffer to get your value.

    Ethan Brodsky

----
<A HREF="http://www.pobox.com/~ebrodsky/"> Ethan Brodsky </A>
UW FutureCar Team Paradigm - 1998 FutureCar Challenge Winner