Liberty USERS GUIDE
25
Intelligent Gain Control (IGC)
The LIBERTY uses an ingenious system to ensure that the signal being broadcast is not too high;
the audio signal is passed through a digital volume control (Digi-Pot) and then to an Analogue to Digital
converter (ADC). From the ADC the digital audio is fed to the Digital Signal Processor (DSP). The DSP
monitors the level of the audio and if this is too high, it sends a control signal to the digi-pot to reduce the
input signal level. This ensures that the input level never exceeds the maximum level of the ADC. When the
IGC is active, the LCD display shows the IGC with
In1
or
In2
or
In1&2
, depending on which input(s) were
too high. When the control knob for the Input is reduced to its correct level, the IGC returns to a null
display.
DSP Based Compressor Limiter
When the audio signals for both inputs one and two are summed together in the DSP, the combined level can
be too high for the algorithm to encode it. Consequently, a compressor limiter is needed to ensure that the
maximum level is never exceeded. It also ensures that momentary peaks on either channel do not exceed the
maximum level as well. The response of the Compressor Limiter is extremely fast so it is almost impossible
to overload the algorithm.
The combination of the LIBERTY IGC and the Compressor Limiter insures that the signal that you are
sending always sounds good, even if you don't have the input levels set just right. It prevents splats and
distortion due to excessive levels ever spoiling your program.
Modem Negotiation and Line Quality
LIBERTY sends and receives high-speed digital information over a standard telephone line via an internal
modem. The modems communicate over the line to establish the best possible conditions for data
transmission and the highest reliable data rate. This also corresponds to best possible audio quality.
Initially, the modems “train” themselves to the line to measure and compensate for variations in frequency
response and to cancel out any echo that may be present. The modems then establish the highest possible
speed for data transfer using a process of “negotiation” with each Data Pump.
When the modems have established the connection, control is passed to the LIBERTY CPU and the LCD
will display the Line quality (LQ). Generally, for satisfactory operation, LQ needs to be greater than 30%.
LQ of 70% or more is excellent. One of the unique features of the LIBERTY is to show the LQ for both the
Local (L) and the Remote (R) units. This valuable information enables the user to manage the link more
effectively.
Refer to Menu
03 Modem Setup
in the Detailed Configuration section for more information on retraining
and renegotiation the bit rate.
MASTER SLAVE OPERATION
A unique feature of the LIBERTY is the remote control capability of the unit for audio input level control.
All LIBERTYS can be set up so that the input level can be controlled from a remote site, such as a studio or
control center. This enables the LIBERTY to be used by non-technical people or to be unmanned. All the
levels can be monitored and adjusted from the control center by using a master LIBERTY. In most
============
92496688
B: 33600 LQ: L 70% R 66%
Ctl: Local IGC: Ln1
[RENEG] RTRAIN MENU
If Line Quality is below 30%, then select
[RENEG] to renegotiate the connection
and lower the bit rate.
Intelligent Gain Control operative (see
above)
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