Convert audio files for CMU Sphinx 4 input

Question

I have a big batch of files I'd like to run recognition on using CMU Sphinx 4. Sphinx requires the following format: - 16 khz - 16 bit - mono - little-endian My files are something like 44100 khz, 32 bit stereo mp3 files. I tried using Tritonus, and then its updated version JavaZoom, to convert using code from bakuzen. However, `AudioSystem.getAudioInputStream(File)` throws an `UnsupportedAudioFileException`, and I haven't been able to figure out why, so I have moved on. Now I am trying ffmpeg. The command `ffmpeg -i input.mp3 -ac 1 -ab 16 -ar 16000 output.wav` seems like it should do the trick (except for little endian), but when I check the output with Audacity, it still labels it as "32-bit float". The command I found on this site also uses `-acodec pcm_s16le`, which from its name seems to be outputting 16 bit little endian; however, Audacity still tells me the output is `32 bit float`. Can anyone tell me how to convert audio files into the format required by CMU Sphinx 4?

Accepted Answer

Did you actually try the output from ffmpeg in CMU Sphinx 4? 32-bit float is probably your default sampling format in Audacity (Edit > Preferences > Quality). I'm guessing it converts any imported file to these settings, so it may not be reporting the parameters of the actual file, but perhaps the working file in Audacity.

Remove `-ab 16`. This would instruct the encoder to use 16 bits/s and ffmpeg will ignore it for pcm_s16le anyway. So your command will look like:

```
ffmpeg -i input.mp3 -acodec pcm_s16le -ac 1 -ar 16000 output.wav
```

To convert all mp3 files in a directory in Linux:

```
for f in *.mp3; do ffmpeg -i "$f" -acodec pcm_s16le -ac 1 -ar 16000 "${f%.mp3}.wav"; done
```

Or Windows:

```
for /r %i in (*) do ffmpeg -i %i -acodec pcm_s16le -ac 1 -ar 16000 %i.wav
```

In Windows Batch file:

```
for /r %%i in (*.mp3) do ffmpeg -i "%%i" -acodec pcm_s16le -ac 1 -ar 16000 "%i.wav"
```

You can see file information with `file`, `ffmpeg`, `ffprobe`, `mediainfo` among other utilities:

```
$ file hjl0bC.wav 
hjl0bC.wav: RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit, mono 16000 Hz

$ ffmpeg -i hjl0bC.wav
[...]
Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 16000 Hz, mono, s16, 256 kb/s
```

Convert audio files for CMU Sphinx 4 input

Solution

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