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Curious about FPGAs

Hi. I'm curious about exactly what sort of things are done with FPGAs. Is the main reason for their use the additional speed over using a generic processor, due to clock rate and/or parallelism? Is it mainly to make cheaper low-run products? What sort of things can be done with them?

As an example, it would be possible to build a digital reverberation unit using a *REALLY BIG* FIR filter. If the reverb impulse response lasted for say 15 seconds requiring M samples to be buffered, then assuming that x[n] is the current sample and that suitable b[0..M] coefficients can be found, then the reverb could be calculated as:

reverb[n] = SUM {i=0 to M} x[n - (M - i - 1)] * b[i]

This could be done in fixed point arithmetic.

Given that 15 seconds of 44.1khz sound requires 15 * 44100 = 661500 past input samples to be cached along with the 661500 b[] coefficients, would this be feasible with current FPGAs?

Cheers,

Ross-c

Reply to
clemenr
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No. You are trying to do about 29 GMACs. The *computations* are possible in 100-200 hardware multipliers, but the memory bandwidth will kill you. (661500 coeffs won't fit in the on-chip ram, so you'll have to use external ram.) Assuming 16 bit coefficients, the coeff "bandwidth" is about 470 Gb/s. You can't do that (yet).

Ask this same question in comp.dsp, and you'll be told there are frequency domain methods (patented by Lake (now Dolby)) that greatly reduce the amount of computation required for reverb. You still might end up using an FPGA, of course.

Regards, Allan

Reply to
allanherriman

Generally a lot than a single processor is the answer. For DSP you can have many processing engines running in parallel. If you want to use microprocessors you can implement a soft IP microrprocessor like MicroBlaze many times within a single FPGA. Most modern FPGAs can have a reasonable amount sram memory available internally for storage, delay, etc. etc.

Cost can vary hugely. Xilinx for instance have low cost Spartan-3 families where you can be down to £2-3 for a chip in high volume or you can spend £2000+ for the largest high end performance Virtex-4.

As you look as if you are from a University or College you may be interested in out UAP program. We also have some free seminars coming up that might be of interest in January and February. Some basic details on our news/seminar webpages if either is of interest.

John Adair Enterpoint Ltd. - Home of Raggedstone1. The Low Cost Spartan-3 Development Board.

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Reply to
John Adair

Thanks for the replies. I wasn't suggesting using the reverb method described above in real life, it was just something with fairly extreme parameters to use as an example when asking questions about fpgas. I've recently changed jobs from a computer science department to an engineering department, and am interested in learning about the new tools available to me in my new environment. We do have hardware and I believe software for programming FPGAs here, and was hoping to try some experiments. If I end up doing something "real" with FPGAs, then it would be in the audio/musical domain. And looking at past postings on this group that appears to be a fairly common application domain of interest.

Cheers,

Ross-c

Reply to
clemenr

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