Demodulation Radio Teletype (RTTY) in Python

This is a Python program to demodulate the radio teletype known as FSK modulation. This is the simplest example, and only the Terminal Unit part of the RTTY is implemented. The rest should be coded according to ITA2, for example.

Description of the source code

import wave
import numpy as np
fname='rtty3s.wav' # should be specify the filename.
smp= 8000          # Sampling Rate
FQm= smp/914.0     # Mark Frequency 914Hz
FQs= smp/1086.0    # Space Frequency 1086Hz
wind= 32           # windows size Integer
waveFile = wave.open(fname, 'r')

mq=[];mi=[];sq=[];si=[]
for j in range(waveFile.getnframes()):
      buf = waveFile.readframes(1)
      mq.append((buf[0]-128)*np.sin(np.pi*2.0/FQm*j))
      mi.append((buf[0]-128)*np.cos(np.pi*2.0/FQm*j))
      sq.append((buf[0]-128)*np.sin(np.pi*2.0/FQs*j))
      si.append((buf[0]-128)*np.cos(np.pi*2.0/FQs*j))
      mk = np.sqrt(sum(mq)**2 + sum(mi)**2)
      sp = np.sqrt(sum(sq)**2 + sum(si)**2)     
      print(mk,sp,int(mk>sp),sep=",")
      if j>wind:
            mq.pop(0);mi.pop(0);sq.pop(0);si.pop(0)
waveFile.close()

Sample sound file

should be convet to wave format.

The input audio file should have a sampling rate of 8000 Hz and a quantization bit rate of unsigned 8 bits.

https://en.wikipedia.org/wiki/File:RTTY.ogg

it is from wikipedia

Parameters

Some parameters in the source code need to be modified according to the audio file to be input.

fname='rtty3s.wav' # should be specify the filename.
smp= 8000          # Sampling Rate
FQm= smp/914.0     # Mark Frequency 914Hz
FQs= smp/1086.0    # Space Frequency 1086Hz

fname
should be specify the filename.
smp
Sampling Rate.
FQm
smp / Mark Frequency
FQs
smp / Space Frequency

How to specify the MARK & SPACE frequency

To find MARK & SPACE frequences, You can use any spectrum analyze tools on your PC. For example I use Sazanami Version 1.7.3 2020/10/22 .

MARK Frequency about 915Hz
SPACE Frequency is about 1085Hz

Usage

Please specify an appropriate audio file for the input. This program assumes 8KHz sampling, mono, 8bit quantization, and no sign.

python rtty8k.py > rtty.csv

Demodulation example

imaoca / RTTY