UKOLN AHDS Recording Digital Sound


The digitisation of digital audio can be a complex process. This document contains quality assurance techniques for producing effective audio content, taking into consideration the impact of sample rate, bit-rate and file format.

Sample Rates

Sample rate defines the number of samples that are recorded per second. It is measured in Hertz (cycles per second) or Kilohertz (thousand cycles per second). The following table describes four common benchmarks for audio quality. These offer gradually improving quality, at the expense of file size.

Table 1: Description of the various sample frequencies available
Samples per second Description
8kHz Telephone quality
11kHz At 8 bits, mono produces passable voice at a reasonable size.
22kHz 22k, half of the CD sampling rate. At 8 bits, mono, good for a mix of speech and music.
44.1kHz Standard audio CD sampling rate. A standard for 16-bit linear signed mono and stereo file formats.

The audio quality will improve as the number of samples per second increases. A higher sample rate enables a more accurate reconstruction of a complex sound wave to be created from the digital audio file. To record high quality audio a sample rate of 44.1kHz should be used.


Bit-rate indicates the amount of audio data being transferred at a given time. The bit-rate can be recorded in two ways - variable or constant. A variable bit-rate creates smaller files by removing inaudible sound. It is therefore suited to Internet distribution in which bandwidth is a consideration. A constant bit-rate, in comparison, records audio data at a set rate irrespective of the content. This produces a replica of an analogue recording, even reproducing potentially unnecessary sounds. As a result, file size is significantly larger than those encoded with variable bit-rates.

Table 2 indicates how a constant bit-rate affects the quality and file size of an audio file.

Table 2 Indication of audio quality expected with different bit-rates
Bit rate Quality MB/min
1411 CD quality 10.584
192 Good CD quality 1.440
128 Near CD quality 0.960
112 Near CD quality 0.840
64 FM quality 0.480
32 AM quality 0.240
16 Short-wave quality 0.120


Digital Audio Formats

The majority of audio formats use lossy compression to reduce file size by removing superfluous audio data. Master audio files should ideally be stored in a lossless format to preserve all audio data.

Table 3 Common Digital Audio Formats
Format Compression Streaming support Bit-rate Popularity
MPEG Audio Layer III (MP3) Lossy Yes Variable Common on all platforms
Mp3PRO (MP3) Lossy Yes Variable Limited support
Ogg Vorbis (OGG) Lossy Yes Variable Limited support
RealAudio (RA) Lossy Yes Variable Popular for streaming
Microsoft wave (WAV) Lossless Yes Constant Primarily for Windows
Windows Media (WMA) Lossy Yes Variable Primarily for Windows

Conversion between digital audio formats can be complex. If you are producing audio content for Internet distribution, a lossless-to-lossy (e.g. WAV to MP3) conversion will significantly reduce bandwidth usage. Only lossless-to-lossy conversion is advised. The conversion process of lossy-to-lossy will further degrade audio quality by removing additional data, producing unexpected results.

What Is The Best Solution?

Whether digitising analogue recordings or converting digital sound into another format, sample rate, bit rate and format compression will affect the resulting output. Quality assurance processes should compare the technical and subjective quality of the digital audio against the requirements of its intended purpose.

A simple suite of subjective criteria should be developed to check the quality of the digital audio. Specific checks may include the following questions:

Objective technical criteria should also be measured to ensure each digital audio file is of consistent or appropriate quality:

Further Information