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QAM:
Quadrature amplitude modulation. A downstream digital modulation technique
that conforms to the International Telecommunications Union (ITU)
standard ITU-T J. 83 Annex B which calls for 64 and 256 quadrature
amplitude modulation (QAM) with concatenated trellis coded modulation,
plus enhancements such as variable interleaving depth for low latency in
delay sensitive applications such as data and voice. Using 64 QAM, a
cable channel that today carries one analog video channel could carry 27
Mbps of information, or enough for multiple video programs. Using 256
QAM, the standard 6 MHz cable channel would carry 40 Mbps. See also: The
Engineering & Transmission chapter.
QPSK:
Quadrature phase shift keying. QPSK is a digital frequency modulation
technique used for sending data over coaxial cable networks. Since it's
both easy to implement and fairly resistant to noise, QPSK is used
primarily for sending data from the cable subscriber upstream to the
Internet.
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Quantization:
The process of sampling an analog waveform to convert its voltage levels
into digital data.
Quantizing:
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:
The noise (deviation of a signal from its original or correct value)
which results from the quantization process. In serial digital video, a
granular type of noise that occurs only in the presence of a signal.
QuickTime:
Apple Computer's system-level software architecture supporting
time-based media, giving a seamless integration of video, sound,
and animation. For Macintosh and Windows computers.
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