21 September 2012

Poor men's WSPR generator with 4x shift..

Using a GW3UEP class D amplifier driven by 4x the frequency has it's restrictions. Using WSPR requires a different approach.
There is a simple solution for it. You must increase the shift 4 times. This can be done in various ways. The PIC solution with a separate LF drive or 2MHz oscillator. A LF multiplier VCO. The latter I tried but the settling time was somewhat unstable, so I left that idea for what it was.

The message which is transmitted is in most cases always the same, so why not just the SSB modulator with a WAV or MP3?

But how do you create such a file with the 4x shift?
Well as follows:
Install Audicity (http://audacity.sourceforge.net/)
- Within this program there is a recording facility in the top row. Start WSPR, choose MF, dial 501,500 and in the TXbox 500,500 should be entered. So a 500Hz tone will be generated when you use "Tune".
Activate "stereo mix"for the soundcard used in WSPR and select this soundcard for the recording in Audacity.
Don't overdrive the recording, just the half which is necessary is best to become a good result.
After the recording you must tidy up the pre and after silence so you will have a complete sequence of 1min50sec.
Now the trick...
- Choose in Audacity "effects" "Change Pitch". The pop-up is showing all kinds of possibilties but just look at the box "Percentage Change". Enter 200%. Click OK. Audicity will now create a sound sample with 1,5kHz and 4X the shift.
- For a test you can play the file from Audicity by selecting the sound card which drives your trx. Manual start the sequence on the even minute.

In order to finish the goods, export the file in WAV or MP3.
Now you can use that file in a timer, Windows scheduler or within SpecLab.
Both I was not very successfull but made a workaround :-)

I added 8 mins of silence to the file. Audacity has this utility "generate" and one of the options is "silence".
Make sure the total file is now 8mins or a multiple of 2mins.
Audicity can arrange this for you within the msec!

I exported this file and played in a mp3 player with the file repeating itself.
When you start manually the sequence it will be kept in sync for several hours within the few seconds WSPR can accept.
For this purpose I used STP player a no nonsense low overhead player. Just Google it most certainly it will be the first hit.

Update: Mainwhile I found a "simple" systemscheduler program from Splinterware, were you can specify on the minute when the file should be played. STP can also work on the commandline mode. So the "out-of-sync" problem is over. Times can be scheduled at intervals you wish. I will either put some pics or a PDF with "how to" later.  Tip: the STP player likes a playlist (m3u) to get started. The systemscheduler itself has a utility called "PlayWAV.exe" so the STP player is not directly needed when you are satisfied with adressing the Windows selected soundcard.

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