Dither is a type of intentional variation (noise) which is added to a digital audio signal to avoid distortion caused by quantization errors.
Dithering in audio
Dither is often used when an analog signal is being quantized into a finite number of digital levels. It is also often used when a digital signal is being quantized into a fewer number of bits per sample corresponding to a fewer number of digital levels.
Dither randomizes the errors caused by quantization. Without dither, quantization can produce intermodulation distortion. This distortion is produced by an intermodulation between the audio and sampling frequencies. Dither prevents this distortion by randomizing the energy produced by the quantization errors.
When dither is properly applied, the quantization error signal is random noise. When dither is not applied, the error signal is distortion.
Dither gives a digital system infinite amplitude resolution between the finite digital quantization levels. With dither, audio signals that have a peak-to-peak amplitude of less than 1 bit, can be digitized, transmitted by the digital system, and converted back to analog. Dither gives a digital system a continuous amplitude resolution so that it behaves just like an analog system.
A digital audio device achieves bit-transparency if it passes digital audio data without affecting the data in any way. This device may be hardware, software, or a combination of both.
Testing for Bit Transparency
To determine if a device is bit-transparent, it can be tested by sending a pseudo-random bit sequence through it and monitoring the digital output. We typically generate the pseudo-random sequence using an Audio Precision digital signal generator. The Audio Precision includes a digital analyzer that is programmed to detect the pseudo-random sequence produced by the generator. The analyzer detects any differences between the generated sequence and the received sequence. The number of differences is tallied by a counter. A digital channel that is bit transparent will show no differences between the transmitted and received pseudo-random sequences.