Abbreviation for quaternary dispersion supported transmission. See DST.


Quadrature Amplitude Modulation (QAM):

A coding technique that uses many discrete digital levels to transmit data with minimum bandwidth. QAM256 uses 256 discrete levels to transmit digitized video.


Quadrature Phase-shift Keying (QPSK):

Phase-shift keying uses four different phase angles out of phase by 90°. Also called quadriphase or quaternary phase-shift keying.


Quality of Service (QoS):

1. The performance specification of a communications channel or system which may be quantitatively indicated by channel or system performance parameters such as signal-to-noise ratio, bit error rate, message throughput rate, and call blocking probability. 2. A subjective rating of telephone communications quality in which listeners judge transmissions by qualifiers such as excellent, good, fair, poor, or unsatisfactory.



The process of converting the voltage level of a signal into digital data before or after the signal has been sampled.


Quantizing Error

Inaccuracies in the digital representation of an analog signal. These errors occur because of limitations in the resolution of the digitizing process.


Quantizing Noise

Noise which results from the quantization process. In serial digital video, a granular type of noise that occurs only in the presence of a signal.


Quantum Efficiency

In a photodiode, the ratio of primary carriers (electron-hole pairs) created to incident photons. A quantum efficiency of 70% means seven out of ten incident photons create a carrier.


Quaternary Signal

A digital signal having four significant conditions.