[sdiy] FPGA versus audio DSP

[Eric Brombaugh]

Nicholas Gregorich wrote:
> Although I think the DIY spirit is great with the release of the new Spartan 3e development board, I am somewhat confused on the hype this has created.  This isn't the first FPGA development kit and even though its said to be very well priced its not dramatically cheaper than a last generation board, for example.
>   

This board is inexpensive and has a good complement of I/O, including 
user interfaces, memory and analog<->digital converters, as well as 
having the USB->JTAG capability required for programming the FPGA 
built-in. That hasn't been common on boards in this price range.

> What are the advantages of using an FPGA for audio rather than a DSP?  I realize the pico Blaze might have an appeal for lowering parts count for MIDI or other typical microcontroller functions, is there anything else I am missing?  Wouldn't DSP development tools for an audio specific DSP be much more useful?
>   

The big advantage is speed and parallelism. An FPGA is harder to set up 
initially because the designer has to specify how every bit and gate 
connect, and once that design is done it's not easy to change in real 
time. However, once the circuit is designed it can run very fast. A DSP 
is easy to reprogram and algorithms can be extended without too much 
trouble. The downside is that a DSP typically can only do a few things 
simultaneously (unless you get to very expensive vector processors, or 
SIMD machines). Usually there's only one ALU, one multiplier, two or 
three paths to memory and registers. An FPGA design done on the 
Spartan3E starter kit can do 20 multiplies simultaneously at a 200MHz 
rate and can access dozens of memories and thousands of registers at once.

DSPs and FPGAs do have a lot of overlap but they're not necessarily 
interchangeable. FPGAs would be overkill for a lot of audio 
applications. On the other hand, when you want to do 512 simultaneous 
voices at a decent sample rate, most inexpensive DSPs aren't going to 
get you there. A cheap FPGA will - you'll just have to work a bit harder.

Eric


Eric