Making WAV Sound Files

How to make audio files for MEPT from images using Reverse FFT

Murray Greenman ZL1BPU
November 2014


In order to use an SSB transmitter as a MEPT transmitter, you will generally need a sound file which contains the intended pattern. We are now fortunate to have two methods with which to generate these patterns. There are also programs which will transmit .MP3 and .WMA files as well as .WAV files.

Compressed audio files (such as .MP3) are much more compact that .WAV files, and you can generate these from a .WAV source, using either a file conversion program, or an audio editor such as Audigy.

ZL2AFP Method

You can now use the ZL2AFP MEPT Controller to generate .WAV files from a visual pattern which you create on screen. In that sense it is a WYSIWYG process. Furthermore, you can also generate pattern segments at controlled lower power levels (in 6 dB steps).

The process is very simple, and is explained in depth.

Introduction to WYSIWYG MEPT Controller

Chirppix Method

This more traditional method of making sound files for MEPT was the most complicated part of the whole MEPT business, but fortunately you needed only do it occasionally. It can be straightforward if you follow the instructions below. This method of making audio pattern files relies on a very clever program written by Marcus DF6NM, called Chirppix. This program uses a reverse FFT technique to turn an image file into sound! While not originally developed for this application, the author (ZL1BPU) has developed settings which allow Chirppix to generate excellent clean patterns with no unwanted artifacts. Chirppix used in this way generates beautifully shaped single-tone elements, with raised cosine shaping, and so no key clicks.

DF6NM's Chirppix DOS executable by Marcus DF6NM.

Since the message starts life as an image (actually an image almost identical to how the message will appear on ARGO, as you will see), just about anything you can draw in a long thin strip can be transmitted! This certainly includes OOK Morse, FSK Morse, differentially keyed Morse (often erroniously called DFCW), CASTLE, Sequential multi-tone Hellschreiber (S/MT-Hell) and simple graphics. By changing the colour of the image (from whilte to grey) you can also accurately adjust the power level of the transmission in 6dB steps!

There is a simple procedure for using Chirppix to generate your .WAV files, which is outlined in the help file which accompanies the WAVMEPT program.

Here's an example. This is the 'source' .BMP image:

Source pattern file example (x2 and rotated)

Chirppix is then used to make a WAV file. If you make a .WAV file from the example 2T_dual.bmp, this should be the result:

  2T_dual.wav, a WAVMEPT ready .WAV file example

This is what the transmission of this file looks like on ARGO in 3 second slow mode:

ARGO screen-shot of the example

Compare this picture with the original graphic just above! This is truly a WYSIWYG (what-you-see-is-what-you-get) application!

Note: The five 'power stripes' clearly show at different levels of brightness. The first two are hard to tell apart due to AGC action in ARGO, but if your transmitter is suitably linear, they will be in accurate 6dB steps. The 'power stripes' are very useful in assessing signal strength on grabbers and other spectrograms.

Copyright M. Greenman 2014. All rights reserved.