Friday 2 December 2011

digimodes and modulation

I have just come off 20m PSK around 14.070 MHz having had my QSO blasted away by a station overmodulating so badly the PSK31 signal covered in the region of 250Hz, faint intermod products just discernable outside that range.
I appreciate that it is not easy to check your own signal visibly, but I have read the following guidelines which may be of interest to those wondering (worrying) about their own signal quality.
Steps are simple:
  1. Ensure any audio processing is OFF
  2. Right-click the audio icon on the system tray to bring up the Playback devices (Windows 7, maybe Vista also). Set the level on the device used to provide audio input to the rig at 60 - 70%
  3. If rig has a menu setting to adjust audio in from accessory socket set midway value.
  4. If interface has an audio setting control then initially set midway.
  5. Fire up the rig, set to transmit on a clear frequency and adjust volume either from the Interface control, or the PC output to interface level slider so that the ALC just starts to kick-in.
  6. Back off slightly so the ALC is dormant.
  7. If the ALC won't back off, or never registers then adjust the other controls accordingly.
  8. When conditions seem OK then make a QSO and ask for honest reports
If a signal is bad I do not give the standard RST(Q) of 599. The last digit is for Quality (Tone) and if it is not free of intermodulation products I reduce this to 7 or even 5. I have seen several references to reporting guidelines on the web, and recommend this one from RSQ-INFO

I have noticed that as the 'tones' generated by the digimode software go lower in frequency, then the audio power tends to make the ALC kick in, conversely the higher the frequency the less audio, and less transmit power is generated. I usually check around a tone of 1500Hz, midband with frequency set at the PSK 'calling'. This will depend on the linearity of the soundcard I think.

I have two rigs which require differing input levels, I adjust from the Playback devices level control as I know the settings I need for each rig now, the rig with the menu driven volume level TS570D is too coarse to equalise both rigs. Settings are 61 for Kenwood, and 69 for the Icom. These will be particular to me though as rigs, sound cards and interface combinations will vary.

I have asked several stations to check my output periodically to ensure that something has not gone awry.

Hope this helps folks

UPDATE 2 December 2011
After several attempts to get the station to correct their overmodulation I have decided to post the spectrum display from the DM780 Superbrowser to highlight the worst signal I have seen on the bands
even able to resolve the ghost transmissions.

4th February 2012 EPC WW contest:
nice bunch of overmods here - or could be "Space Invaders"
Posted by at

2 comments:

  1. Casey Bahr8 September 2011 at 18:03

    Rather than rely on ALC my usual MO is to set the output low from the PC then bring it up while tuning to see the maximum output, then back off the audio to about the 50-60% point.

    On my usual laptop this is pretty easily done with the dedicated volume control on the keyboard, but otherwise I have to rely on that tiny Windoze graphical control. I wonder if someone has written a better interface for controlling PC audio settings that is more fine-grained?

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  2. Charlie Cartmel8 September 2011 at 18:33

    Casey
    regarding volume control, I would also use the keyboard controls with a single audio output, however I have a dedicated card which is not keyboard controlled, hence my method as outlined. DM780 has a built in soundcard management tab and very fine control on output is possible through that method, I fact I use this now especially when changing rigs. My current problem is on the audio output of my two rigs which are very different so audio into the PC has to be changed regularly, and DM780 does not have a tab for input.

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