## Digital Audio Technology 1

### September 21, 2009

Some useful concepts to help you to understand the technology behind working with digital audio.

Pulse Code Modulation

Pulse-code modulation (PCM) is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a numeric (usually binary) code

Sampling

In signal processing, sampling is the reduction of a continuous signal to a discrete signal.

A common example is the conversion of a sound wave (a continuous-time signal) to a sequence of samples (a discrete-time signal).

Sample rate

When it is necessary to capture audio covering the entire 20–20,000 Hz range of human hearing, such as when recording music or many types of acoustic events, audio waveforms are typically sampled at 44.1 kHz (CD), 48 kHz (professional audio), or 96kHz. The approximately double-rate requirement is a consequence of the Nyquist theorem.

Quantization

Quantization can be thought of as mapping a signal with a continuous set of sample values to a set of discrete values.

Or,  its a way of approximating a very large range of values by a relatively small (“finite”) set of (“values which can still take on continuous range”) discrete symbols or integer values.  For example binary code in a computer.

For digital audio both of these steps (sampling and quantizing) are performed in analog-to-digital converters (ADC) with the quantization level specified in bits. This is known as resolution or bit depth of the sample.

The resolution of the converter indicates the number of discrete values it can produce over the range of analog values. The values are usually stored electronically in binary form, so the resolution is usually expressed in bits. In consequence, the number of discrete values available, or “levels”, is usually a power of two.

For example, an ADC with a resolution of 8 bits can encode an analog input to one in 256 different levels, since 28 = 256. The values can represent the ranges from 0 to 255.

Quantization introduces digital noise!  This is in addition to the noise that exists in all analog electrical circuits.

The Signal to Noise Ratio (SNR) is a ratio of the signal power to the noise power corrupting the signal.

There is a relationship between quantization noise and SNR that has direct effect on the qulity of the audio sample.