fm_radio.py

import subprocess
import os
import time
import sys

os.system("pkill rtl_fm")
FM_frequency = "103.3e6" #desired FM station
#FM_frequency = sys.argv[1] + "e6"
modulation_mode = "wbfm" #wideband FM
sample_rate = "200000" #lowpass/resample
output_rate = "44100"
gain  = "20"
while True:
    #rtl_fm commands
    new_frequency = input("Enter a new frequency (in MHz): ")
    if FM_frequency != new_frequency + "e6":
        os.system("pkill rtl_fm")
        FM_frequency = new_frequency + "e6"
    command = "rtl_fm -A lut -E deemp -f {} -M {} -s {} -r {} - | ffmpeg -nostats -loglevel quiet -r 200k -f s16le -ar {} -ac 1 -i - -f wav - | aplay -q -t wav > /dev/null 2>&1".format(FM_frequency, modulation_mode, sample_rate, output_rate, output_rate)
    time.sleep(3)
    #Some times it take a few secs to start the SDR. using this loop to help
    print ("playing:" + FM_frequency, "MHz")
    subprocess.Popen(command, shell=True, 
    stdout=subprocess.DEVNULL,
    stderr=subprocess.STDOUT)

The script you provided uses the subprocess.Popen() method to run the command, this method starts the command as a separate process and allows the script to continue running while the command is executing in the background.

It uses the os.system("pkill rtl_fm") command to kill any existing rtl_fm processes before starting a new one.

It uses the input() function to prompt the user to enter a new frequency and assigns the new frequency to the FM_frequency variable, then it compares the current frequency with the entered one, if they are different it kills the rtl_fm process and assigns the new frequency to the FM_frequency variable.

It uses the time.sleep(3) command to wait for 3 seconds before starting the SDR, this is to help with some timing issues that might occur when starting the SDR.

It also uses the subprocess.Popen.stdout=subprocess.DEVNULL, stderr=subprocess.STDOUT to redirect the output of the command to /dev/null so no output will be displayed on the screen.

this command is a pipeline of 3 different commands:

rtl_fm -A lut -E deemp -f {} -M {} -s {} -r {} - : This command is used to receive the FM signal using an RTL-SDR device.
-A lut : This option sets the automatic gain control algorithm to use a look-up table.
-E deemp : This option enables the de-emphasis filter that is used to remove the pre-emphasis that is applied to FM signals.
-f {} : This option sets the center frequency of the RTL-SDR device. The frequency value is passed as an argument to the command and it is stored in the variable FM_frequency.
-M {} : This option sets the modulation mode of the RTL-SDR device. The value is passed as an argument to the command and it is stored in the variable modulation_mode.
-s {} : This option sets the sample rate of the RTL-SDR device. The value is passed as an argument to the command and it is stored in the variable sample_rate.
-r {} : This option sets the output sample rate of the RTL-SDR device. The value is passed as an argument to the command and it is stored in the variable output_rate.

• : This option tells the rtl_fm command to output the received data to the standard output (stdout) so it can be piped to the next command in the pipeline.
  ffmpeg -nostats -loglevel quiet -r 200k -f s16le -ar {} -ac 1 -i - -f wav - : This command is used to process the received data and convert it to a WAV file.
  -nostats -loglevel quiet : These options are used to prevent ffmpeg from displaying any output on the console.

-r 200k : This option sets the input data rate to 200 kbps

• -f s16le : This option sets the input format to signed 16-bit little-endian samples
• -ar {} : This option sets the audio sample rate of the output, the value is passed as an argument to the command and it is stored in the variable output_rate.
• -ac 1 : This option sets the number of audio channels to 1 (mono)
• -i - : This option tells ffmpeg to read the input from the standard input (stdin), which is where the data is coming from the previous command in the pipeline.
• -f wav - : This option tells ffmpeg to output the processed data in WAV format to the standard output (stdout) so it can be piped to the next command in the pipeline.

aplay -q -t wav : This command is used to play the WAV file.
-q : This option tells aplay to operate in quiet mode, so it will not display any output on the console.
-t wav : This option tells aplay to assume the input file is in WAV format.

/dev/null 2>&1 : This redirect the output of the command to /dev/null so no output will be displayed on the screen.