On 3/11/13 11:19 AM, Phil Burk wrote:
Regarding power-of-2 sized circular buffers, here is a handy way to
verify that a bufferSize parameter is actually a power-of-2:
int init_circular_buffer( int bufferSize, ... )
{
assert( (bufferSize & (bufferSize-1)) == 0 )
...
might be silly, but a way to force the caller to constrain it to a power
of 2 is:
init_circular_buffer( int logBufferSize, ... )
{
unsigned long bufferSize = 1L << logBufferSize;
unsigned long indexMask = bufferSize - 1;
...
On 3/11/13 2:59 AM, Nigel Redmon wrote:
Also a note that the modulo-by-AND indexing is built into some processors—the
56K family, at least, as Robert knows well…buffers are the next power of two
higher than the space needed, and the masking happens for free…
actually the 56K and other DSPs (like the SHArC) can do buffers of any
size below 32K. the 56K has a restriction that the base address of the
buffer must be an integer multiple of a power of 2 that is at least as
big as the bufferSize. the modulo arithmetic doesn't really happen for
free. choosing to use a DSP over a cheap ARM chip or something similar
has both advantages and disadvantages. and they have to put a bunch of
logic on the chip for the modulo. even the 563xx chip has that 32K
restriction, even though the address space increased to 16M. such a
shame. you have minutes of addressing space, but your modulo delay lines
are still limited to less than a second at any decent sampling rate.
but what you can do with C where you might have a bunch of different
delay lines (like in a Shroeder/Jot reverb), all running at the same
sampling rate, is create a *single* circular buffer that has length that
is a power of 2. then each little delay line can have a piece of that
buffer allocated, but all of the allocations move at the same rate. the
various delay line allocations are "stationary" relative to each other.
it can be compared to an analog tape delay like this. you have a fixed
amount of tape media but as many record and playback heads as your heart
desires. so instead of cutting a separate loop of tape (which has to be
of length equal to a power of two) and connect that up to a record and
playback head, you create one big loop of tape and put a record/playback
head pair for each delay on the tape loop at different locations.
that way you can efficiently allocate a delay line of 129 or 257 or 4097
samples long along with a bunch of others. only the whole big buffer
need be of length 2^p .
--
r b-j r...@audioimagination.com
"Imagination is more important than knowledge."
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