Review of some basic concepts related to digital audio processes.

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.

Analog Box Synthesizer

September 25, 2008


Analog Box is a freeware, circuit based, modular software synthesizer. You put sound objects on the screen, connect them together, listen and adjust.

Visit Andy’s ABox site

Fourier Series Applet

September 25, 2008


Falstad’s applet gives a practical demonstration of application of harmonic analysis

Wikipedia article on Fourier Series

FM Synthesis

September 24, 2008


Additive Synthesis

September 24, 2008


Subtractive Synthesis

September 24, 2008


History of Sound Synthesis

September 24, 2008


Timelines

Sound synthesis has a long history

Timeline; History of the synthesizer

120 years of electronic music timeline

 Some interesting examples from the early 20th century

The Telharmonium

 Wikipedia article

Magic Music from the Telharmonium

Lee de Forest

de Forest developed his Audion valve based on the principles of beat frequency or heterodyning oscillators. This three element vacuum tube started the practical development of amplified audio, additive synthesis and radio broadcasting. 

de Forest’s Audion valve

His Audion piano

Sounds from the Ether

In addition to de Forest a number of other engineers noted the musical potential of beat frequency or heterodyning oscillators.   Amongst them was Leon (or Lev) Sergeivitch Termen a Russian Cellist and electronic engineer who developed the “Theremin” or the “Aetherophone”.  A unique electronic instrument which creates a sound that has become embedded in our culture, often associated with sci-fi movies from the 50’s and 60’s.

Short article on the development of the Theramin

Wikipedia article

Theremin world

Leon Theremin teaches Paul Lansky how to play Glinka’s Skylark,
the same tune he taught Lenin to play. Moscow, Feb. 1991. Theremin
is 95 years old here.

The Singing Arc

 First discovered by Physicist William Duddell a musical instrument based on the antique carbon-arc lamps that were used for lighting in the early 20th century.

Article from 120 years of Electronic Music

Wikipedia article on plasma speakers