2. Audio Basics

Audio can be quite complex to explain, but this information will help you understand the basic details of digital audio that you should keep in mind when handling your source. Audio is sound that consists of waves of pressure that are intercepted by ears and interpreted by the brain. Microphones are used to capture sound waves and convert them to electrical pulses that are then converted by a sound card as digital information. Puse-code modulation (PCM) is the most common way analog to digital devices encode audio digitally and the quality of the audio is determined by the sampling rate and bit depth. An example of digitally stored sound that you may be familiar with is an audio CD, which is 16-bit PCM audio with a sampling rate of 44100Hz.

Sampling Rate

The audio sampling rate determines the number of samples that are stored per second. Samples are location points stored from the original sound wave that are used to recreate the form of the wave during playback. Higher sampling rates allow for more points along the original wave form to be stored. The sampling rate is measured in Hertz and the most common rates you will encounter are 44100Hz and 48000Hz.

Bit Depth

The audio bit depth controls the way in which a sample is stored by limiting the number of possible values available. Samples are rounded up or down to the nearest available bit value. The rounding of a sample value is a variation or distortion from the original audio, but with higher bit depth it becomes less noticeable to the listener. This makes a higher bit depthmore ideal because it means the stored values will be closer to the original.

Resampling

The resampling of audio is when new values or samples, at the frequency of the new sampling rate, are recorded in relation to the recreated sound wave and those new values are rounded and stored based on the specified bit depth. It is not ideal to resample audio as any change in the sampling rate will cause distortion in the audio. While it is generally best to avoid this, there are different sampling rate standards that may force audio to be resampled to be supported by certain playback methods.

Audio Compression

Audio is often compressed to save space when storing or transferring. When compressing there are two methods to choose from, lossless or lossy, and it is important to know the difference between the two. Lossless audio, such as FLAC, is identical to the uncompressed version, once it is decompressed, and typically achieves a much smaller size than its uncompressed counterpart because it avoids storing redundant information. Lossy audio, such as MP3 and AAC, takes reduction of file size further by not only avoiding redundant information, but by also discarding sounds that are perceived as less important. When selecting audio, it is more ideal to select uncompressed or lossless audio as it will have less distortion.

Term Reference Guide:
Compression – Encoding data so that the information stored is less than the original.
Lossless – Compression method that avoids restoring redundant data to reduce the file size.
Lossy – Compression method that discards less noticeable information to reduce the file size.
Waveform – The shape of a signal such as sound waves.
Samples – Values that represent points along a waveform.
Sampling Rate – The frequency of samples captured or recorded.
Audio Bit Depth – The number of values a sample can be stored as.