User guide

MS-DMT Guide v1.04
Released 05-30-2014
64
SETTING UP TX AUDIO
Select the same sound device for use as the Input as the Output device using Line In vs. Mic in if
possible.
Leave the Modulation level at the default 75% to start, only use it if the sound panel and other
controls do not provide enough range in setting your radio ALC to zero.
Do not use the radio MIC port for TX audio if your radio provides an Auxiliary input which are
preferred to Mic inputs as they are less sensitive to RF and never get accidentally readjusted.
NOTE: Make sure that your MIC audio is not hot during data modem transmissions.
The ALC setting is significant in reducing distortion by maintaining linearity and thus improving
performance. The TX Audio level for MS-DMT must NOT drive the radio into ALC. ALC can drive
signals too high into the power amplifier, distorting the signal and possibly the modulating
signal. This effectively limits the maximum SNR measured at the demodulator and may cause bit
errors. For those with radios without ALC meters, no ALC typically equates to about 30-35% of
your SSB voice RF output for data, however some Amateur grade and most Commercial and
Military radios will often provide much more than 35%. Also be sure to power your radio from a
clean, stable power source and if running off batteries, be sure that the voltage does not drop
too low otherwise distorted audio and other issues may develop.
It is recommended, if possible with your sound device interface controls, that the sound panel
be used to set the TX level to max and then make use of an interfacing level control to adjust for
proper TX level to allow for easy manual adjustment when changing frequency as typical
Amateur Radio and Commercial Radio transceivers do not provide a constant RF output across
2-28Mhz. For ALE follow on use of the software with am ALE hardware radio, the proceeding
applies. When used with MARS-ALE instead of its internal MS110A capability, the process can be
the same where you setup per the MARS-ALE docs with one level setting used for all channels
and derive whatever output you get across 2-28Mhz. Alternately MARS-ALE can be setup per
the docs to automatically adjust the sound control panel level for each frequency, in which case
your interface control would be set close to maximum to achieve a constant RF output for each
channel, this still requires no ALC for PSK and whatever ALC achieved for ALE FSK.