Liberty USERS GUIDE
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applications, the LIBERTY located in the studio or control center is set up as the Master unit and the
LIBERTY at the remote location is set up as the Slave unit.
The easiest way to do this is with the Quick Function keys F2 - #. By pressing these two keys, one after the
other, it will sequence through the different modes of operation. The first sequence sets the LIBERTY in
Master mode. Pressing the keys again puts the unit in Slave mode. Pressing the key sequence again sets the
unit into Toolbox mode and the final sequence puts the unit back into Local mode. Alternatively, using the
MENU NAVIGATION KNOB and selecting the Advanced Setup menu can set up the unit.
When a unit is set up as a Master, the Input gain controls operate the audio gain at the remote LIBERTY.
The input level for the local LIBERTY remains at the setting last used when in Local mode. To adjust the
local input levels, LIBERTY has to be set back into Local mode.
When a unit is set to Slave mode, the input level is set by the local Input controls until the LIBERTY is
connected to a Master unit. When the connection is made, the Slave LIBERTY obtains the gain setting from
the Master unit and it then stores the setting in memory. If the Master LIBERTY is then switched to Local,
as can be done with the COMMANDER unit, the Slave unit maintains the level last received from the
Master unit.
PEAK LEVEL METER
The LCD provided on all LIBERTYS shows the peak audio levels being transmitted.
THEORY OF OPERATION
Coding Algorithms
Over the past two decades, there have been great improvements in the way that digital audio data can be
condensed while retaining much of the quality of the original signal. Many different algorithms have been
developed, including well-known ones like MPEG layer 2 and 3. These algorithms require a reasonably high
bit rate, typically 64kbps or higher, for high quality wide band mono real time operation. Many ISDN
codecs use MPEG layer 2 and 3 for mono communications at 64kbps and 128 kbps however the delay can
become a problem for real time operation. Some codecs use MPEG-2 layer 2 and 3 to overcome bandwidth
limitations at low bit rates.
While the MPEG algorithms may be suitable for ISDN operation, they are quite inadequate for the low bit
rates available for POTS operation. Some manufacturers have tried to use MPEG for low bit rate POTS
operation but have found the results less than satisfactory. As a typical POTS line will achieve bit rates of
28,800 or less, an algorithm capable of far greater performance than MPEG is required.
Fortunately, significant advances in the development of coding algorithms have made the design of digital
audio codecs like the LIBERTY possible. The proprietary coding algorithm used by LIBERTY achieves
compression factors of the order of twenty times or more. This enables 15 kHz bandwidth high quality
audio to be transported at a bit rate as low as 24,000bps. Reduced quality and bandwidth can be obtained at
bit rates down to 9600bps. In addition, low coding delays of 100 milliseconds provide the kind of
performance required for real time operation.
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