[sdiy] Digital delay memory

[Richie Burnett]

Tom and Eric, have you looked at the 23LC1024 SPI SRAM chip to supplement 
on-board delay memory?  (1MBit external static-RAM in 8-pin SOIC/TSSOP 
package for a couple of bucks.  A couple of seconds delay at 32kHz/16bit.)

I found that the hit for the SPI exchange wasn't too bad if you're doing 
your audio processing in blocks.  You can arrange DMA to "page in" or "page 
out" a block of external memory into the DMA RAM space in the background 
whilst doing other processing, so that it is there just ahead of when you 
want to use it.

It's good for things like digital delay memory.  Less so for reverb where 
you need lots of read/writes and the memory bandwidth over SPI becomes 
limiting.

Also had good success with old computer DRAM simms for *loooooooong* audio 
delay.  (Made a profanity delay for broadcast applications.)  All the 
RAS/CAS/refresh stuff isn't really that hard to implement for the old 
page-mode or EDO chips, and you could pick them up cheap on ebay.  It 
requires more I/O lines than SPI, but not as much as parallel SRAM.  Haven't 
tried interfacing to modern Synchronous DRAM though.  It looks a lot more 
complicated!?

-Richie,