Indeo^®Audio Software Technical Overview

About Indeo® Audio Software

Considerations in Compressing Audio With Progressive Download Publisher

In the original AVI clip, start with a clear, high-quality audio recording. Use 16-bit Pulse Code Modulation (PCM) format at a high sampling rate, such as 44.1 kHz (CD rate). The Progressive Download Publisher will not compress audio streams in any format other than 16-bit PCM. 

Data rate selection. The data rate is chosen indirectly in the audio compression options dialog by setting the sample rate and stereo/mono. The resulting data rate is displayed as you select the options. When choosing an audio data rate, the operating range is from 16 kilobits per second (kbps) to 176 kbps. Stereo always requires twice the data rate of mono.  Higher data rates yield higher audio quality but take longer to download and occupy more space on diskettes and CDs. Using the highest data rate, it would take over 6 seconds to download each second of compressed CD-rate stereo audio over an ideal 28.8 modem connection.

In general, when considering how to compress audio, look at the bandwidth available for download. Remember that stereo doubles the data rate and therefore the download time. For example, 44 kHz CD-quality audio in a single channel requires 88 kbps per second to download; in stereo, it requires 176 kbps. For most Indeo audio software tracks, AM-quality (11 kHz mono) is a good starting point. At 22 kbps it is easily downloaded in real time on a 28.8 modem with some bandwidth left over for video. 

Sample rate. The default sample rate is the same as the sample rate in the audio track of the input .AVI file. It may be reduced for faster download but the quality will be lower. The following is a general guidline for each sample rate: 

For CD quality compressed audio at 88 kbps use 44,100 Hz.
For FM quality compressed audio at 44 kbps use 22,050 Hz.
For AM quality compressed audio at 22 kbps use 11,025 Hz.
For telephone quality compressed audio at 16 kbps use 8,000 Hz.