Orban opticodec 7200, Handbook

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OPTICODEC E 7
The Codec Technology
The "ISO-MPEG Audio Layer 2 and Layer 3“ compression
procedures developed by the Fraunhofer Institute and the
Institut für Rundfunktechnik allow audio signals (even large
amounts of data) to be reduced in real time and transferred
without any subjective loss of quality. The digitised signals
received in this form are compressed (encoded) to save
on transmission bandwidth, time and cost.
CODEC is a word coined from the verbs "enCOde“ and
"DECode“ and stands for a new data transfer technology
via ISDN or satellite.
The principle of codec technology for audio data reduction
is based on the frequency-dependent sensitivity of the hu-
man ear. According to its objective auditory properties and
subjective hearing habits, the ear ignores certain sounds
and concentrates on the most essential ones: the message.
This contrasts with purely electronic techniques which hear
everything, even the non-essential noise.
The codec technology takes advantage of the difference
between the ear and electronic measuring device when
transferring data. By masking all meaningless noise, even
the minutest, a reduction ratio is achieved, which is ne-
cessary to transport large amounts of data in real time via
ISDN, for example. The data is instantly decompressed and
subjected to A/B comparison and then the ear at the other
end of the line hears only what it is intended to hear –
no more and no less.
Some typical examples of data reduction rates achieved
with ISO-MPEG1 can be seen in the following table:
Algorithm Bitrate
(kbps)
Audio
mode
Reductions
ratio
in Layer 1 384 Stereo 1:4
in Layer 2 192...256 Stereo 1:6...1:8
in Layer 3 112...128 Stereo 1:10...1:12