Example Zoom FFT Implementation

zoom_fft.py

from pyqtgraph.Qt import QtGui, QtCore
import numpy as np
import pyqtgraph as pg
from scipy import signal
import math
import random
import cmath

def gen_tone_data( sampleRate, seconds ):
    samples = int(sampleRate * seconds)
    f1 = 12140.0
    f2 = 12145.1
    dt = 1/sampleRate
    pi2dt = 2.0*math.pi*dt;

    return [ 2*math.sin(pi2dt*f1*i)+1*math.sin(pi2dt*f2*i)+(random.random()-0.5) for i in range(samples)]

def myfft( signalBuf, sampleRate, nBins, offset ):
    outBins = (nBins//2) 
    binWidth = (sampleRate/2) / outBins
#    print( "sampleRate", sampleRate, "min", offset, "outbins", outBins, "binWidth", binWidth, "max", (outBins-1)*binWidth+offset )
    return [ [ offset + i * binWidth for i in range(outBins) ], [abs(x) for x in np.fft.fft(signalBuf,n=nBins)[0:outBins] ] ]

def shiftFrequency( samples, sampleRate, adjustBy ):
    dt = 1/sampleRate
    pi2dtcf = 2*math.pi*dt*adjustBy
    vals = [ cmath.exp(1j*pi2dtcf*i)*samples[i] for i in range(len(samples)) ]
    return vals

def zoom_fft( samples, sampleRate, nBins, fStart, fEnd ):
    bandwidth = (fEnd - fStart)
    decFactor = sampleRate//bandwidth//2
    shifted = shiftFrequency( samples, sampleRate, -fStart )
    resampled = signal.decimate(shifted,decFactor,ftype='fir')
    newSampleRate = sampleRate//decFactor
#    print("decFactor",decFactor,"newSampleRate",newSampleRate)
    vals = myfft( resampled, newSampleRate, nBins, fStart )    
    return vals

def update():
    pass

app = QtGui.QApplication([])

win = pg.GraphicsWindow(title="Basic plotting examples")
win.resize(1900,600)
win.setWindowTitle('pyqtgraph example: Plotting')

sampleRate = 48000
n = 512 
signalData = gen_tone_data(sampleRate, 0.5)

fftData = myfft(signalData, sampleRate, n, 0.0)
zoomFFTData = zoom_fft( signalData, sampleRate, n, 12130, 12160 )

signalGraph = win.addPlot(title="Signal Graph")
sigPlot = signalGraph.plot(pen='y')
sigPlot.setData( signalData )

fftOutput = win.addPlot(title="FFT Output")
fftPlot = fftOutput.plot(pen='y')
fftPlot.setData( *fftData )

zoomFFTOutput = win.addPlot(title="Zoom FFT Output")
zoomFFTPlot = zoomFFTOutput.plot(pen='y')
zoomFFTPlot.setData( *zoomFFTData )

timer = QtCore.QTimer()
timer.timeout.connect(update)
timer.start(1)

app.exec_()