Understandling Audio and Sampling

Understanding Audio and Sampling Rates

Audio data is characterized by the following parameters:

* sampling rate (in samples per second, such as 8 k or 44.1 k

* number of channels (1 for mono, 2 for stereo, etc.)

* number of bits per sample (8 or 16)

Sound is stored as a sequence of samples taken from the audio signal using constant time intervals. A sample represents volume of the signal at the moment when it was measured. In uncompressed digital audio each sample requires one or more bytes of storage. Number of bits required depends on number of channels (mono, stereo) and sample format (8 or 16 bits, mu-Law, etc.). The number of bits per sample is usually either 8 or 16. Much like 8 and 16-bit color, 8 and 16-bit audio are limited to 256 or 32,000 frequencies. The length of this interval determines the sampling rate. Normally used sampling rates are between 8 kHz (telephone quality) and 48 kHz (DAT tapes). Sampling rates are usually quoted in Hz or kHz (kilohertz). Samples per second is also used. Sampling rates are measured per channel so a stereo audio file recorded at 8000 samples/sec would actually be 16000 samples in a second.

Sampling parameters affect quality of sound which can be reproduced from the recorded signal. The most fundamental parameter is sampling rate which limits the highest frequency than can be stored. Nyquist's Sampling Theorem states that the highest frequency that can be stored in sampled signal is at most 1/2 of the sampling frequency. For example, 8 kHz sampling rate permits recording of signal in which the highest frequency is less than 4 kHz. Higher frequency signals must be filtered out before feeding them to Digital Analog Converters (DACs).

There are several sampling rates at which one can record audio. Some recording hardware is restricted to particular rates and some playback hardware has direct support for others.

Samples/sec Description

8000

Exactly 8000 samples/sec is a telephony standard that goes together with U-LAW (and also A-LAW) encoding. Some systems use an slightly different rate; in particular, the NeXT workstation uses 8012.8210513.

11 k

Either 11025, a quarter of the CD sampling rate, or half the standard Mac sampling rate

16000

Used by, e.g. the G.722 compression standard.

18.9 k

XA standard.

22 k

Either 22050, half the CD sampling rate; the latter is precisely 22254.545454545454 but usually misquoted as 22000. (perhaps the most popular rate on the Mac).

32000

Used in digital radio, NICAM (Nearly-Instantaneous Companded Audio Multiplex [IBA/BREMA/BBC]) and other TV work, at least in the UK; also long play DAT and Japanese HDTV.

37.8 k

XA standard for higher quality.

44056

This peculiar rate is used by professional audio equipment to fit an integral number of samples in a video frame.

44100

The CD sampling rate. (DAT players recording digitally from CD also use this rate.)

48000

The DAT (Digital Audio Tape) sampling rate for commercial and domestic use.

Currently 44.1 kHz 16-bit sound is the standard sampling and bit depth for compact discs, DAT recording, and other digital recorders such as ADATs, computers, etc. This standard was created over two decades ago and may eventually change to a higher frequency rate (possibly up to 48 kHz which is currently available or even higher).

44.1 kHz audio can be used for any project from online video to CD-ROM interactives. One has to be aware of the file size of 44.1/16 bit audio and how this may effect downloading, CD interactive load times, etc.).

Typically, one minute of stereo 44.1 kHz audio track occupies 10 MBs of disk space. Although 44.1 kHz offers the best quality sound, it may be unreasonable to download tens of megabytes through a 28.8 Kbs modem for a quick sound byte. One minute of 44 kHz audio could take 2 hours download time. It is important to know how audio will be used in the final product and how the final product will be viewed.

22 kHz audio is a good alternative for web and CD-ROM applications because of its relatively good sound quality and small file size. One minute of 22 kHz 16-bit audio occupies only 5 MBs HD space, and at 8-bit the sample would cut the size in half to 2.5 MBs.