Audio Recordings Best Practices

Revolution uses 8-bit PCM mono uLaw with a sampling rate of 8 kHz for audio notifications. This format is bandwidth-limited to the 300 Hz – 8 kHz audio range and is designed for voice. It provides acceptable voice-quality results. This standard isn't ideally suited for quality music reproduction but will provide minimally acceptable results. Audio files uploaded to Revolution are transcoded to the 8 kHz format.

You can achieve the best recording results with careful mic placement in close proximity to the sound source. For voice, place the mic below or to the side of the speaker's mouth in order to avoid distortion due to plosives. You can also incorporate a pop-filter to avoid this distortion. We recommend you record your source at 44.1 kHz or 48 kHz sample rate to a 16- or 24-bit mono uncompressed WAV or AIFF file. A compressor/limiter and equalization processors can help you get very best audio quality.

After recording, you should save a copy of your recordings in the source format. Transcoding to 8 kHz audio degrades the audio quality to a large degree. By keeping a high-quality archive, you have the option of reusing the source material. We recommend using an audio software editing program such as Audacity (a very suitable freeware utility) to trim leading and trailing silence from the recording, to normalize the volume, and to apply and equalization to the source file. If you have stereo sources, we recommend converting these to mono prior to uploading to your server for use with Revolution. This enables you to monitor the audio for any stereo-to-mono phase artifacts.

There are unavoidable compression artifacts once the file is transcoded to 8-bit uLaw. When uploading an audio file to your server for use with Revolution, you have two options for handling the sample rate conversion. Depending on your audio software's capabilities, each option may yield the best results, so it is worthwhile to try each:

  • Upload the high-quality file to your server and let Revolution handle all aspects of the transcoding.
  • Transcode your audio file in advance. If your audio software editor has sample rate convertor and encoding capabilities, this option gives you some degree of control over the final results. Also, the quality of sample rate convertors (to go from high-quality 44.1 kHz or 48 kHz to 8 kHz) varies depending on the algorithm used, so you can compare Revolution with that of your own audio software.

Always use the best source recording and avoid any file format that converts one lossy format to another (such as MP3 to 8-bit μLaw) as this will definitely introduce additional artifacts. Also avoid the temptation to compensate for the limited 8 kHz bandwidth by over-emphasizing the higher frequencies in your audio source. This doesn't accomplish much, and your results can sound worse. That said, you can experiment with modest amounts of EQ.

Tip: Use an equalizer to roll off low frequencies (under 200 Hz) to help remove room background noise, emphasize the 2-3 kHz range to improve intelligibility, and notch out 1.2 kHz slightly to smooth out harsh sounding voices.