UKOLN AHDS Audio For Low-Bandwidth Environments


Audio quality is surprisingly difficult to predict in a digital environment. Quality and file size can depend upon a range of factors, including vocal type, encoding method and file format. This document provides guidelines on the most effective method of handling audio.

Factors To Consider

When creating content for the Internet it is important to consider the hardware the target audience will be using. Although the number of users with a broadband connection is growing, the majority of Internet users utilise a dial-up connection to access the Internet, limiting them to a theoretical 56kbps (kilobytes per second). To cater for these users, it is useful to offer smaller files that can be downloaded faster.

The file size and quality of digital audio is dependent upon two factors:

  1. File format
  2. Type of audio

By understanding how these three factors contribute to the actual file size, it is possible to create digital audio that requires less bandwidth, but provides sufficient quality to be understood.

File Format

File format denotes the structure and capabilities of digital audio. When choosing an audio format for Internet distribution, a lossy format that encodes using a variable bit-rate is recommended. Streaming support is also useful for delivering audio data over a sustained period without the need for an initial download. These formats use mathematical calculations to remove superfluous data and compress it into a smaller file size. Several popular formats exist, many of which are household names. MP3 (MPEG Audio Layer III) is popular for Internet radio and non-commercial use. Larger organisations, such as the BBC, use Real Audio (RA) or Windows Media Audio (WMA), based upon its digital rights support. Table 1 shows a few of the options that are available.

Format Compression Streaming Bit-rate
MP3 Lossy Yes Variable
Mp3PRO Lossy Yes Variable
Ogg Vorbis Lossy Yes Variable
RealAudio Lossy Yes Variable
Windows Media Audio Lossy Yes Variable

Figure 1: File Formats Suitable For Low-Bandwidth Delivery

Once recorded audio is saved in a lossy format, it is wise to listen to the audio data to ensure it is audible and that essential information has been retained.

Finally, it is recommended that a variable bit-rate is used. For speech, this will usually vary between 8 and 32kbp as needed, adjusting the variable rate accordingly if incidental music occurs during a presentation.

Choosing An Appropriate Encoding Method

The audio quality required, in terms of bit-rate, to record audio data is influenced significantly by the type of audio that you wish to record: music or voice.

Assessing Quality Of Audio Data

The creation of audio data for low-bandwidth environments does not necessitate a significant loss in quality. The audio should remain audible in its compressed state. Specific checks may include the following questions:

Further Information