15
AUDIO LEVELS ALIGNMENT
This section describes how to determine and set the audio levels.
Audio Levels Overview:
To ensure the best audio quality, the unit must be configured to match the audio levels used by the radio. The unit
uses programmable gain amplifiers to accomplish this. Determining the gain settings for these amplifiers is an
involved process, so Midian simplified this process by developing an algorithm that requires the technician to
make only four voltage measurements. From these four measurements, all of the many internal settings are
determined.
Still, getting the best audio quality will likely require a bit of trial and error. The unit only has control of audio
voltage levels, not input and output impedances. These impedances can dramatically influence the levels.
The Four Voltage Measurements:
An oscilloscope and a communications test set/service monitor are required for the measurements. It is
recommended that the measurements be recorded in units of mV peak-to-peak. Each measurement must be
taken with system modulation at either 60% or 100%, but Midian recommends using 60%
A method for controlling transmit modulation is required for accurate measurements in the TX mode. A small
speaker held in place near the microphone by a rubber band can serve this purpose in most cases. Use a sine-
wave generator to inject a 1000 Hz tone into the speaker. Adjust the output of the sine wave generator so that the
transmitter produces 60% of rated modulation while PTT is pressed. Note that if the audio source (such as a
speaker) is moved even slightly, the TX modulation may change significantly. Care must be taken to avoid
changing the TX modulation while taking the measurements.
The first two measurements must be taken using a radio that has not been modified. The 2
nd
two measurements
require that the unit is installed and power is applied to the radio/unit. These measurements must be taken within
15 seconds of powering on the unit. This is because the unit may enter power saving mode after that time.
Measurements made while the unit is in power saving mode will not be valid. The unit ships with the power save
feature enabled by default. The power save feature can be disabled via the programming software so that it will
not interfere with taking measurements, if desired. Please note that the levels provided to the option board are
different between narrow band and wide band.
1.
TX Input:
This procedure is to determine the audio level that the unit will see at the TX audio pickup point
after it is installed. The unit must be installed and powered-on while making this measurement. Use the TX
Alignment Set-Up procedure and measure the audio level at TP1 on the unit.
2.
RX Input:
This procedure is to determine the audio level that the unit will see at the RX audio pickup point
after it is installed. The unit must be installed and powered-on while making this measurement. Use the TX
Alignment Set-Up procedure and measure the audio level at TP2 on the unit.
3. In the programming software under audio levels set the TX In to the same level as measured in step 1 and for
a preliminary adjustment set the TX Out for the same level. Set the RX In to the same level as measured in
step 2 and for a preliminary adjustment set the RX Out for the same level. Program the unit.
4.
RX Output:
This procedure is to determine the audio level that would normally appear at the RX audio
insertion point in an unmodified radio. Using the same RX Alignment Set-Up procedure verify the audio level
at the speaker is still at the same level measured initially in the RX Alignment Set-Up procedure. If not adjust
the RX Out level accordingly.
5.
TX Output:
The goal of this procedure is to determine the audio level that would normally appear at the TX
audio insertion point in an unmodified radio. Using the same TX Alignment Set-Up procedure verify the
modulation is still at 60%, if not adjust the TX Out level accordingly.
