fix "RuntimeError: check topology failed on audio_alsa_sink(11) using ninputs=1, noutputs=0" on raspberry pi 1B

fm_radio.py

@@ -40,6 +40,9 @@ class fm_radio(gnuradio.gr.top_block):
 
         audio_sink = gnuradio.audio.sink(audio_sample_rate_hertz, "", False)
         self.connect((wbfm_demodulator, 0), (audio_sink, 0))
+        # for raspberry pi 1B:
+        # > RuntimeError: check topology failed on audio_alsa_sink(11) using ninputs=1, noutputs=0
+        self.connect((wbfm_demodulator, 0), (audio_sink, 1))
 
 
 def _main():