User guide

MS-DMT Guide v1.04

Released 05-30-2014

LOOP BACK TEST

There are a number of purposes of the loop back test:

 Verify that the operating system latency issues have been mitigated.

 Check the effects of sound device distortion, noise and jitter.

 Determine a relative starting point for RX and TX sound device level settings.

 Check the effects of cable wiring as to noise when the radio cables are jumpered for

loop back testing.

Neither Sound Device Sample Rate Error or Stability enter into the loop back test as the same

sound device is being used for TX and RX which equates to a 0ppm error. How your system

performs during the loop back testing is an indication of how your system would perform using

radios were you to have a 0ppm sample clock error.

The Loop Back test requires a jumper from the sound device Line Out to Line In or Mic In if

required. The full benefit of the test is achieved when the actual cabling between the sound

device and HF radio are jumpered for loop back verses just using a separate jumper. That being

said, if there are issues when testing using the full cable set, then performing the tests with a

separate jumper can be used instead of the cables to determine if system cables are an issue. It

is usually with RF that issues with cables are the most prevalent.

Performing a loop back test requires two instances of the software running on the same PC. On

both instances uncheck “Save on Send” and “Save on EOM”. On the instance that will send the

message, select the 4800bps uncoded data rate by clicking the “Inc” button in the lower

Transmit panel displays “4800 BPS” in the MODE window.

As this is a loop back test, first send some short messages first and adjust TX level and RX level

so that the ERROR window in the receiving instance is at its lowest reading which should be less

than 0.25Hz. Too much TX or RX audio or too little will cause the ERROR value to increase.

The loopbacktest.txt file in the full install distribution is used for the loop back test message. The

use of the 4800bps uncoded data rate will easily reveal issues on the system being as 4800bps

has no FEC and no Interleaving. Open the file with Notepad and copy the message and past it