Otázka.:
What is AC3?
Odpověď:
Dolby AC-3 is a flexible audio data compression technology capable of
encoding a range of audio channel formats into a low rate bit stream. Channel
formats range from monophonic to 5.1 channels, and may include a number of associated
audio services.
Although AC-3 algorithms are independent of the number of channels coded, current
implementations have standardized on the SMPTE-recommended 5.1 channel arrangement:
five full bandwidth channels representing Left, Center, Right, Left-Surround,
and Right-Surround; plus a limited bandwidth Low-Frequency Effects (LFE) Subwoofer
channel. AC-3 conveys this channel arrangement with a high degree of transparency
at data rates as low as 320 Kbps.
5.1-channel audio consists of five discrete, full range main channels (Left,
Center, Right,Left Surround, and Right Surround) plus an optional band-limited
Low-Frequency Effects (LFE) channel.
In contrast to the main channels, the LFE channel delivers bass-only
information (<120 Hz) and has no direct effect on the perceived directionality
of the reproduced soundtrack. Its purpose is to supplement the overall bass
content of the program or to ease the burden on the other channels. The LFE
channel was originally devised for 70 mm movie productions to deliver a separate
bass signal to one or more additional subwoofers placed behind the movie screen.
This allowed deep bass effects to be added to movie soundtracks without having
to upgrade the existing speakers and amplifiers in the three main screen channels.
LFE does not equal subwoofer. Dolby Digital programs may include a bass-only
LFE channel, but this channel does not correspond directly to a subwoofer output.
It is possible for a program to contain an LFE channel, but a decoder may provide
no subwoofer output because all of the bass information in the program, including
the LFE channel, can be reproduced by the main speakers. The opposite is also
true: it is possible for a program to not contain an LFE channel, yet a decoder
may provide a subwoofer output because some or all of the main speakers are
unable to reproduce the bass information in the program. The difference between
the LFE channel and the subwoofer output is that the LFE channel is used to
carry additional bass information in the Dolby Digital program while the subwoofer
output represents how some or all of the bass information will be reproduced.
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