To make the measurement, Audacity played the QRM and test signal tracks simultaneously through a Virtual Audio Cable to deliver the combined audio to the decoders. For each test session I used Audacity with track 1 loaded with the two-station QRM recording and track 2 loaded with the appropriate propagation distorted file. These two recordings were then level-adjusted and combined into a single file, again using Audacity.Īs with last month’s tests, the RTTY software was used with its default settings. For the LF interferer, I repeated the process with the RTTY software tuned 250Hz below the wanted signal. I then used copy/paste to dump a long string of RYs into the transmit buffer and recorded the output using the Audacity audio editor. To make the QRM recordings I used RTTY software set to transmit and tuned 250Hz above the test frequency. For many of the comparisons, the test signal was significantly below the QRM stations. For the sake of clarity, the wanted signal is shown in its strongest state where it is just 10dB below the QRM signals. I’ve shown a spectrogram and waterfall display of one of the test signals in Fig. The audio tones of these two interfering stations were spaced 250Hz either side of the wanted station. I started with the same single signal test files as last month but created a second file that had a recording of two strong RTTY stations sending continuous RYs. The aim, therefore, was to create a repeatable test file that could be applied to multiple decoders. The most obvious problem with contest operation is strong signals located very close to the wanted DX station. So, this month I wanted to expand the tests to more realistically simulate the conditions found during a RTTY contest. However, last month’s tests were focused on a single transmission with no QRM. Notably, MMTTY and others that use the MMTTY engine generally fare much better with flutter fading than FLDIGI. The results were interesting and confirmed my suspicions that you must choose your decoder to match the conditions. Those tests centred around the use of RF path simulation tools to synthesise a variety of common propagation conditions.

Last month I spent some time making comparative tests between popular RTTY software. This month Mike Richards G4WNC has more RTTY comparisons plus news on the LimeSDR.

Rtty decoders