Bit Depth - help on choosing the appropriate bit depth to work with
Sample rate is the number of samples of audio carried per second, measured in Hz or kHz (one kHz being 1 000 Hz). For example, 44 100 samples per second can be expressed as either 44 100 Hz, or 44.1 kHz.
Bandwidth is the difference between the highest and lowest frequencies carried in an audio stream. The sample rate of playback or recording determines the maximum audio frequency that can be reproduced, as shown below.
For audio work, bandwidth is normally about 20 Hz less than the highest recorded frequency, so for practical purposes they can be treated as the same thing. Hence the terms are used interchangeably here. The term bandwidth may be applied to the frequency content of an audio signal stream, or the frequency ability of audio hardware or software.
Which Sample Rate To Use
44.1 kHz / 20 kHz
44.1 kHz is the sampling rate of audio CDs giving a 20 kHz maximum frequency. 20 kHz is the highest frequency generally audible by humans, so making 44.1 kHz the logical choice for most audio material. High quality tape decks using metal tape, and medium quality LP equipment can reproduce 20 kHz (higher for top quality LP equipment, though some of this is harmonic distortion inherent in the medium). Note that the upper limit of human hearing falls rapidly with age. While people in their teens can hear 20 kHz, many older people cannot hear above 14.5kHz.
Reduced Bandwidth Recording
Audio may be recorded at below 20kHz bandwidth for a few reasons:
- To reduce file size
- To reduce CPU usage
- Because the source material itself is of less than 20kHz bandwidth.
A lower sampling rate can also be used to remove the highest frequency hiss present in a noisy signal. While in theory there is no loss of quality as long as the bandwidth of the sample rate stays above the audio signal bandwidth, in practice one often does not know exactly what the signal bandwidth is. So for most purposes, a better option is to use noise gating for hiss reduction, which has much more effect and is less likely to compromise the recorded signal. The Audacity 'noise removal' filter is a multi-channel noise gate.
32 kHz / 14.5 kHz
- 14.5 kHz is above the frequency limit of many medium quality sources, such as ferric cassette tape. On good tape decks, chrome tape can reproduce 18 kHz.
14.5 kHz cutoff also has a slight effect on speech, so 32k will give good quality speech recording, though not the best.
14.5 kHz is a little below FM radio bandwidth, so FM radio can be recorded with only a little quality loss.
32 kHz sample rate is thus good for:
- cassette recordings from ferric stock
- All other audio where smaller files than 44.1 kHz are desired with only slight compromise on sound quality.
22.05 kHz, 10 kHz
22.05 kHz (often lazily called "22 kHz") has been a reasonably popular sample rate for low bit rate MP3s such as 64 kbps in years past. Audio quality is significantly affected, with higher frequency content missing. With the general rise in the availability of large file storage space and faster data links, 22k is now of more limited use.
- For speech recording where perceived quality is unimportant, but clarity must be maintained.
- AM radio
- To squeeze mp3 music onto floppy disk & very small mp3 players.
11.025 kHz / 5 kHz
11.025 kHz (often lazily called "11 kHz") gives 5 kHz maximum recorded frequency. Very poor sound quality.
8 kHz / 3.6 kHz
3.6 kHz matches the bandwidth of telephone sytems. Microcassette dictaphones have also used a similar bandwidth. 8 kHz sample rate is thus suitable for:
- Telephone speech
- Microcassette recording
- To squash long speeches into small file sizes
Sound quality is terrible, with speech as clear as a mud bath. Any further drop in sample rate makes intelligibility a challenge.
Recording and playing with a bandwidth higher than 20kHz gives no audible benefit (assuming replay is not speed altered). It can be used:
- to maintain full bandwidth when the recording will be slowed down afterwards
- for analysis of signals above 20 kHz
- for some oscilloscope waveform recording
- to record sig gen waveforms up to 40 kHz
- for recording some wildlife
- and other ultrasonic applications.
When recording ultrasonic input it should be borne in mind that not all audio sources can output signals above 20 kHz. To capture ultrasound, all items in the chain of audio must support the full frequency range to be recorded.
Some sound cards support sampling at up to 96kHz, some don't.
When recording a signal of known limited bandwidth, you can minimise file size by using a recording sample rate of 2.2x signal bandwidth. This is seldom necessary.