Audio Downsampler

Although the Tellme VoiceXML interpreter supports downsampling of high fidelity formats on the fly, it is more efficent to convert your audio files into a format that is optimal for use on the public switched telephone network (PSTN). You can use Tellme's audio conversion tool to downsample your master audio files (usually 44.1kHz) into the optimal format. The optimal format is the following:

  • mono
  • 8 kHz sample rate
  • u-law encoded
  • RIFF data
  • WAV file

The conversion tool supports RIFF wav (any sample rate) and Sphere wav (any sample rate) as input. The tool does not support MP3 or RealAudio formats.

To use the converter (convert.pl), you need the following:

  • A Perl interpreter. Microsoft Windows developers can download a Perl interpreter from ActiveState.
  • A decompression utility. Microsoft Windows developers can use WinZip. UNIX users typically have gunzip and tar at their disposal.
  • SoX. You will need to download this utility from the SoX homepage.

Microsoft Windows users should follow these steps to use the audio converter:

  • Download the compressed (.gz) tape archive (.tar) audioconversion.tar.gz.
  • Use WinZip or another decompression utility to decompress audioconversion.tar.gz
  • Copy convert.pl and sox.exe to a directory specified in your PATH environment variable.
  • Create two subdirectories - an input directory and and output directory. You'll use the input directory for the files you want to convert. The converter will place the converted files into the output directory.

Follow these steps to use the converter to downsample a set of audio files:

  1. Copy the audio files you wish to downsample into your input directory.
  2. Open a command shell (MS-DOS prompt) and run the converter using a command in the following format:
    perl convert.pl input-dir  output-dir

Your converted audio files will be found in your output directory.

The conversion tool provides the ability to adjust the peak amplitude and the rms amplitude. These features are describe in the following sections.

To change the volume of the sound, use the following command:

perl convert.pl indir  outdir peak value

The value argument is a number between 0.0 and 1.0.

If your audio produces lots of clicks and pops, increasing the volume might help. Use the following command:

perl convert.pl indir outdir rms value

The value argument is a number between 0.0 and 1.0. This argument could reduce sound quality. Be sure to listen to the resulting output files.

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