Recording Digital Sound

Background

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

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.

 

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.

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:

• Can listeners understand voices in recording?
• Can listeners hear quiet sounds?
• Can listener hear loud sounds without distortion?
• Can the listener distinguish between digitised audio and original recording?

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

• Is there a documented workflow for creating the digital audio files?
• Is the file format and software used to compress the audio documented?
• Is the bit rate equal to or less than the available bandwidth?
• Does the sample and bit-rate of the digital audio match or exceed that of the original analogue recording (or is the loss of quality acceptable, see subjective tests above)?
• For accurate reproduction of an original analogue recording, is the digital audio master file stored in a lossless format?
• For accurate reproduction of the original sound is the sample rate at least twice that of the highest frequency sound?

Further Information

• MP3Pro Zone,
  <http://www.mp3prozone.com/>
• Measuring Audio Quality,
  <http://www.itworld.com/AppDev/1469/NWW1204revside4/>
• Ogg Vorbis,
  <http://www.vorbis.com/>
• PC Recording,
  <http://www.pcrecording.com/>
• Real Networks,
  <http://www.real.com/>
• Slicing and Dicing MP3 bit rates,
  <http://www.digitalprosound.com/Htm/WebAudio/2000/Oct/MP3bitrates.htm>
• Xorys' MP3 FAQ,
  <http://webhome.idirect.com/~nuzhathl/mp3-faq.html>