$ /home/pi/special/ffmpeg/ffmpeg -re -f alsa -acodec pcm_s32le -i mic_sv -acodec aac -y -f nut /tmp/temp_audio.nut
ffmpeg version git-2017-02-05-e57fd92 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.9.2 (Raspbian 4.9.2-10)
  configuration: --extra-cflags=-I/opt/vc/include/IL --enable-nonfree --enable-omx-rpi --logfile=CONFIG.TXT
  libavutil      55. 46.100 / 55. 46.100
  libavcodec     57. 75.100 / 57. 75.100
  libavformat    57. 66.101 / 57. 66.101
  libavdevice    57.  2.100 / 57.  2.100
  libavfilter     6. 73.100 /  6. 73.100
  libswscale      4.  3.101 /  4.  3.101
  libswresample   2.  4.100 /  2.  4.100
Guessed Channel Layout for Input Stream #0.0 : stereo
Input #0, alsa, from 'mic_sv':
  Duration: N/A, start: 1487309422.048265, bitrate: 3072 kb/s
    Stream #0:0: Audio: pcm_s32le, 48000 Hz, stereo, s32, 3072 kb/s
Output #0, nut, to '/tmp/temp_audio.nut':
  Metadata:
    encoder         : Lavf57.66.101
    Stream #0:0: Audio: aac (LC) ([255][0][0][0] / 0x00FF), 48000 Hz, stereo, fltp, 128 kb/s
    Metadata:
      encoder         : Lavc57.75.100 aac
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s32le (native) -> aac (native))
Press [q] to stop, [?] for help
[alsa @ 0x2ab7460] ALSA buffer xrun.
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=N/A speed=   0x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=N/A speed=   0x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=N/A speed=   0x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate= 363.0kbits/s speed=0.000467x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate= 363.0kbits/s speed=0.000376x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.3kbits/s speed=   1x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.3kbits/s speed=0.858x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.4kbits/s speed=0.753x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.4kbits/s speed=0.668x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.4kbits/s speed=0.602x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.4kbits/s speed=0.547x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.3kbits/s speed=   1x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.4kbits/s speed=0.924x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.4kbits/s speed=0.859x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.4kbits/s speed=0.801x
[alsa @ 0x2ab7460] ALSA buffer xrun.rate=   0.4kbits/s speed=0.752x