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I am trying to receive data which is transmitted at 144.025 Mhz, using RTL-SDR with the below command and it works perfectly (even tested with local fm radio and the audio is good)..

rtl_fm -f 144.025e6 -s 200000 -r 48000 - | aplay -r 48000 -f S16_LE

But when i tried to decode the received signal using minimodem using the below command, i am getting some breaking audio and i am not able to decode the

rtl_fm -f 144.025e6 -s 200000 -r 48000 - | aplay -r 48000 -f S16_LE | minimodem --rx 1200

Later i notice, when i am playing any audio (like youtube) and if i run minimodem --rx 1200 in terminal, at once the audio is breaking or distorted with crackling sound..

I am using raspberry pi - stretch os. Can anyone please tell me where i am doing mistake and is there any other way to decode the data from command line (terminal).. Thanks in advance.

  • What is the sound card you are using ? max sample rate is 44100 Hertz on raspberry for the Analog output from here : less /proc/asound/card0/pcm0p/sub0/hw_params . – Ephemeral May 28 at 14:56
  • Thanks for the reply... I have even tried with lesser sample rate like 22050, 24000 too.. But still i am facing the same problem...Is there any other suggestions.. – user3661448 May 29 at 2:51
  • You can try: −−float-samples option for 32bits value ...Maybe ... or −a, −−auto-carrier , from here ? – Ephemeral May 29 at 7:24
  • this tutorial show msg.wav prepared with minimodem, the file use 1200Hz for the carrier : link – Ephemeral May 29 at 7:55
  • Hi, Ephemeral, Once again thanks a lot for your reply. I am transmitting a text data encoded with minimodem, and the audio (wav file) is transmitted through a radio. I am trying to receive the same through a RTL-SDR which is connected to a Pi. Using RTL-SDR with rtl_fm to receive the RF freq and aplay to process those received data as audio further piped to minimodem to decode as text. This has to be done in realtime which mean continuously the receiver will be on and once the signal receives it has to be decoded.. Please support me with your ideas... Thank you again... – user3661448 May 29 at 12:10

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