[sdiy] Digital delay memory

Tom Wiltshire tom at electricdruid.net
Wed Apr 27 18:30:40 CEST 2016


Yes, I've got a few heading my way. That was my next step. In the meantime, I've been getting some of the wrinkles out with just the RAM that's on the chip.

And yes, I've got the processing in blocks. Figuring I'd need it for the SPI RAM, the current scheme uses an interrupt feeding data into an IN buffer and reading data from an OUT buffer. The main code then supplies new IN and OUT blocks from RAM. When I get an external RAM, all I have to do is replace my SendToRAM and FetchFromRAM routines with SPI-based ones.


On 27 Apr 2016, at 17:22, "Richie Burnett" <rburnett at richieburnett.co.uk> wrote:

> 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, 
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