Author Topic: Tap Count for FIR filter design  (Read 28 times)

Tranquility Bass

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Tap Count for FIR filter design
« on: May 17, 2023, 05:46:26 PM »
When designing FIR filters it is important to note the limitations of the DSP in handling the filter. The tap count of the DSP is dependent on the clock rate of the DSP and the number of clocks needed to perform a multiply accumulate (MAC).

For the ADSP-21489 SHARC used on the UP the DSP is clocked at 392MHz since it is syncro-locked to the master clock of 49.152MHz and not an asynchronous 400MHz clock as used in most designs.

Since the SHARC can achieve two Multiply-Accumulates (MACS) in one clock cycle because of the SIMD capability the SHARC is capable of 786 MMACS. To get the tap count per sample clock divide 786432000 by the sample rate. For example:-

At 48 KHz Total Tap count = 16384 taps
At 96 KHz Total Tap Count = 8192 taps
At 192 KHz Total Tap Count = 4096 taps

These are the theoretical maximum tap counts for a given sample rate and then you need to divide that amongst the number of filters used. However Audioweaver has another trick up its sleeve to expand the tap capability by down sampling to a much lower sample rate which would be appropriate when building a filter to process low frequency information for a woofer etc. For example if you are only interested in processing bass then you could theoretically down sample from 192KHz to 3Khz which would expand the tap capability by 64 times from 4096 to 2621444 taps. Once you have done all of the processing at 3kHz then you up sample back to 192kHz !

If you need huge tap count at the native sample rate then the UPP is the way to go by offloading the DSP duties onto external PC hardware. See here for more info

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