This transforms microphone input into MIDI data in real time and spits it over ALSA.

Midify.py

import pyaudio
import alsaseq
import numpy as np

CHUNKSIZE = 1024*2 # fixed chunk size

# initialize portaudio

p = pyaudio.PyAudio()
stream = p.open(format=pyaudio.paInt16, channels=1, rate=44100, input=True, frames_per_buffer=CHUNKSIZE)

# Variable to hold data about the previous note

previous = None

# Open a socket one can connect to via alsa

alsaseq.client("Midify", 0, 1, False)

# The contents of the previous chunk

olddata = np.array([])

try:

    while True:

        # Get the last two chunks into numpydata

        data = stream.read(CHUNKSIZE)
        newdata = np.fromstring(data, dtype=np.int16)
        numpydata = np.append(olddata, newdata)
        olddata = newdata

        # Print the last guessed note

        print(previous)

        if np.max(numpydata) < 500:
            if previous != None:

                # Midi meaning that the last note has ended

                alsaseq.output( (7, 0, 0, 253, (0, 0), (130, 0), (128, 0), (0, previous, 0, 0, 0)) )
                previous = None
        else:

            # Extract the maximal frequency using numpy-sorcery

            w = np.fft.fft(numpydata)
            freqs = np.fft.fftfreq(len(w))
            idx = np.argmax(np.abs(w))
            freq = freqs[idx]

            try:

                # Get the note number from the frequency

                note = int(round(12*np.log2(abs(freq*44100)/440)+49))

                if previous != note:

                    if previous != None:

                        # Tell the listener of our midi stream to silence the old note...

                        alsaseq.output( (7, 0, 0, 253, (0, 0), (130, 0), (128, 0), (0, previous, 0, 0, 0)) )

                    # ...and start playing the new one

                    alsaseq.output( (6, 0, 0, 253, (0, 0), (0, 0), (0, 0), (0, note, 100, 0, 0)) )
                    previous = note


            except OverflowError:
                pass

except KeyboardInterrupt:
    pass

# Cleanup

stream.stop_stream()
stream.close()
p.terminate()