fft beep detection with overlap

[Al Grant]

Hi All,

My code is setup to detect regular beeps in a real time buffered
stream of samples from a SDR. For efficiency reasons I perform beep
detection using fft - and if no beeps are detected no further
processing is done.

This works pretty well except for one edge case - where the beep is on
the edge of the fft, then it gets skipped.

I think I need to overlap the fft processing. My beeps are 0.018s long
with a usual sample rate of 2.048e6.

So this is my current code without overlapping:

    def process(self, samples: SamplesT):

        # look for the presence of a beep within the chunk and :
        # (1) if beep found calculate the offset
        # (2) if beep not found iterate the counters and move on

        start_time = time.time()
        fft_size = self.fft_size
        f = np.linspace(self.sample_rate/-2, self.sample_rate/2, fft_size)
        num_ffts = len(samples) // fft_size # // is an integer
division which rounds down
        fft_thresh = 0.1
        beep_freqs = []
        for i in range(num_ffts):
            fft =
np.abs(np.fft.fftshift(np.fft.fft(samples[i*fft_size:(i+1)*fft_size])))
/ fft_size
            if np.max(fft) > fft_thresh:
                beep_freqs.append(np.linspace(self.sample_rate/-2,
self.sample_rate/2, fft_size)[np.argmax(fft)])
        finish_time = time.time()

        # if not beeps increment and exit early
        if len(beep_freqs) == 0:
            self.stateful_index += (samples.size/100) + 14
            return

And this is my current attempt at overlapping:

    def process(self, samples: SamplesT):
        # look for the presence of a beep within the chunk and :
        # (1) if beep found calculate the offset
        # (2) if beep not found iterate the counters and move on

        fft_size = self.fft_size
        f = np.linspace(self.sample_rate/-2, self.sample_rate/2, fft_size)


        size = fft_size # 8704
        # step = self.sample_rate * self.beep_duration + 1000
        step = 7704
        samples_to_send_to_fft = [samples[i : i + size] for i in
range(0, len(samples)//fft_size, step)]

        num_ffts = len(samples_to_send_to_fft[0]) # // is an integer
division which rounds down
        fft_thresh = 0.1
        beep_freqs = []

        for i in range(num_ffts):

            # fft =
np.abs(np.fft.fftshift(np.fft.fft(samples[i*fft_size:(i+1)*fft_size])))
/ fft_size
            fft =
np.abs(np.fft.fftshift(np.fft.fft(samples_to_send_to_fft[0][i]))) /
fft_size
            if np.max(fft) > fft_thresh:
                beep_freqs.append(np.linspace(self.sample_rate/-2,
self.sample_rate/2, fft_size)[np.argmax(fft)])

        # if no beeps increment and exit early
        if len(beep_freqs) == 0:
            self.stateful_index += (samples.size/100)
            return

However it doesn't work at all. I think the way the array is
constructed with this overlapping is different structure?

My overlapping technique is as per here :
https://stackoverflow.com/questions/38163366/split-list-into-separate-but-overlapping-chunks

I am very interested if anyone can help with this please?

Thank you

Al