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@ -9,7 +9,7 @@ |
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* for the library structures and wonderful Teensy products. |
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* |
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* There are enough different FIR filter Audio blocks to need a summary. Here goes: |
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*
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* |
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* AudioFilterFIR (Teensy Audio Library by PJRC) handles 16-bit integer data, |
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* and a maximum of 200 FIR coefficients, even only. (taps). For Teensy Audio. |
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* AudioFilterFIR_F32 (OpenAudio_ArduinoLibrary by Chip Audette) handles 32-bit floating point |
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@ -32,21 +32,21 @@ |
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* FIR filters suffer from needing considerable computation of the multiply-and-add |
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* sort. This limits the number of taps that can be used, but less so as time goes by. |
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* In particular, the Teensy 4.x, if it *did nothing but* FIR calculations, could |
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* use about 6000 taps inmonaural, which is a huge number. But, this also
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* use about 6000 taps inmonaural, which is a huge number. But, this also |
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* suggests that if the filtering task is an important function of a project, |
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* using, say 2000 taps is practical. |
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*
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* |
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* FIR filters can be (and are here) implemented to have symmetrical coefficients. This |
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* results in constant delay at all frequencies (linear phase). For some applications this can |
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* be an important feature. Sometimes it is suggested that the FIR should not be |
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* used because of the latency it creates. Note that if constant delay is needed, the FIR |
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* implementation does this with minimum latency. |
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*
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* |
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* For this block, AudioFilterFIRGeneral_F32, memory storage for the FIR |
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* coefficiients as well as working storage for the ARM FIR routine is provided |
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* by the calling .INO. This allows large FIR sizes without always tying up a |
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* big memory block. |
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*
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* |
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* This block normally calculates the FIR coefficients using a Fourier transform |
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* of the desired amplitude response and a Kaiser window. This flexability requires |
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* the calling .INO to provide an array of response levels, in relative dB, |
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@ -57,17 +57,17 @@ |
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* else dbA[i] = -140.0f; |
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* } |
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* firg1.FIRGeneralNew(&dbA[0], 300, &equalizeCoeffs[0], 50.0f, &workSpace[0]); |
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*
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* |
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* As an alternate to inputting the response function, the FIR coefficients can be |
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* entered directly using LoadCoeffs(nFIR, cf32f, *pStateArray). This is a very quick |
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* operation as only pointers to coefficients are involved. Several filters can be |
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* stored in arrays and switched quickly this way. If this is done, pStateArray[] |
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* as initially setup should be large enough for all filters. There will be "clicks" |
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* associated with filter changes and these may need to be muted.
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* associated with filter changes and these may need to be muted. |
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* |
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* How well the desired response is achieved depends on the number of FIR coefficients |
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* being used. As noted above, for some applications it may be desired to use |
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* large numbers of taps. The achieved response can be evaluated
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* large numbers of taps. The achieved response can be evaluated |
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* by the function getResponse(nPoints, pResponse) which fills an .INO-supplied array |
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* pResponse[nPoints] with the frequency response of the equalizer in dB. The nPoints |
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* are spread evenly between 0.0 and half of the sample frequency. |
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@ -99,11 +99,11 @@ |
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* frequency response. |
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* LoadCoeffs(nFIR, cf32f, *pStateArray); // To directly load FIR coefficients cf32f[]
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* getResponse(nFreq, *rdb); // To obtain the amplitude response in dB, rdb[]
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*
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* |
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* Status: Tested T3.6, T4.0 No known bugs |
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*
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* |
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* Examples: TestFIRGeneralLarge4.ino TestFIRGeneralLarge5.ino |
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*
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* |
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* Copyright (c) 2020 Bob Larkin |
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* Any snippets of code from PJRC or Chip Audette used here brings with it |
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* the associated license. |
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