Midian VS-1000, Manual

Page: 7 / 20

HARDWARE ALIGNMENT
This section describes to determine and set the audio levels.
Audio Levels Overview:
To ensure the best audio quality, the scrambler must be configured to match the audio levels used by the radio.
The scrambler 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 scrambler 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%. Most transmitters will not actually modulate at 100%, so
60% is recommended for TX measurements.
These following measurements should be made before installing the scrambler. Also, the measurement
procedures assume that the audio pickup and insertion points have already been determined by the installer.
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.
In most wired-in applications, a two-terminal component such as a capacitor must be removed from the radio to
break the audio path. However, the first two measurements must be taken using a radio that has not been
modified. The 2
nd
two measurements require that power be applied to the scrambler. These measurements must
be taken within 15 seconds of powering the scrambler on. This is because the scrambler may enter power saving
mode after that time. Measurements made while the scrambler 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 KL-3
programming software so that it will not interfere with taking measurements, if desired.
1.
RX Output: The goal of this procedure is to determine the audio level that would normally appear at the RX
audio insertion point in an unmodified radio. Modulate a 1000 Hz tone at either 60% or 100%. Measure the
voltage level appearing on the output side of the component that will be removed to break the RX audio path.
2.
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. Provide the radio an audio source generating a 1000 Hz tone and
key the radio. Adjust the audio source such that the modulation is at 60%. Measure the voltage level
appearing on the output side of the component that will be removed to break the
TX audio path.
7