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# Audio Bandwidth

The bandwidth is a property of the audio signal and represents the frequency up to which the signal holds information. The bandwidth of a signal is often equal to, but never higher than, half the sampling rate (this principle is known as the Nyquist theorem). For example, an audio stream with a sampling rate of 32 kHz has a maximum bandwidth of 16 kHz. The bandwidth can also be lower than half the sampling rate.

You need to find the bandwidth of an audio stream to decide what sampling rate to choose. For each language, you have a choice of two language packs–one pack can process audio at 8 kHz and the other pack can process audio at 16 kHz. Audio streams sampled at 16 kHz contain more information than streams sampled at 8 kHz, therefore HPE recommends that you choose the higher sampling rate where possible. However, an 8 kHz language pack expects the audio bandwidth to be close to 4 kHz, whereas the 16 kHz language pack expects the audio bandwidth to be close to 8 kHz. If you discover that an audio stream has a bandwidth much lower than expected for the 16 kHz pack, HPE recommends that you downsample it to 8 kHz before processing. The bandwidth of an audio signal can be lower than half the sampling rate for many reasons, including low pass filtering and upsampling (increasing the sampling rate).

You can use waveform analysis tools, such as wavesurfer (http://sourceforge.net/projects/wavesurfer/) and Adobe Audition, to check the bandwidth of an audio stream.

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