Hi all, As mentioned in my introductory posting on this list, I am attempting to develop a simulator that will run on Windows, using DSP algorithms that will emulate the audio effect of AM broadcasting when it is subjected to multipath effects, such as sky wave mixing with ground wave,
I want to be able to emulated the sound of, for example, Radio Luxemburg as received in the UK when they used the sky wave antenna and 1.3 MW, and synchronous AM broadcasters such as used by the BBC (Radio 1 247m in the e60's 70's etc. You may well ask why one would want to do such a thing! Well, two reasons, firstly , just to see if it can be done (always a good reason) and secondly, believe it or not, for listening to some music especially `oldies', I actually prefer the sound of AM it probably has a lot to do with the song sounding like I remembered hearing it on AM Radio originally! The characteristic AM fading sound is caused by partial or full cancellation of the modulation envelope due to one of the two (or maybe more) copies of the signal having a longer path, one could be travelling via ground wave, the other being reflected by the `E' layer of the Ionosphere adding typically 80-150 km extra path length. The height of the `E' layer is constantly changing, thereby changing the path length. This effectively creates a swept notch filter effect through the modulation envelope. As has been noted in a recent post, the audible effect of this is much more noticeable if the demodulator is an envelope detector as in a standard domestic radio. What I plan to do is write a program which would amplitude modulate a signal, split it two ways, and have a variable delay on one of these paths perhaps of the order of several wavelengths) Both the time delay and amplitude of this second path would be slowly varied (by separate pseudo- random low frequency to imitate the effect of the varying height and reflectivity of the `E' layer) I have recently re- read Andy Talbot G4JNT's `Command' book on DSP for the amateur, and was particularly interested in the last chapter, which I believe was written by Alberto, I4PHD, about interfacing with a windows sound card. As mentioned previously, my C++ skills are pretty rudimentary, and I might be able to copy and paste bits of code from other sources, but a blank sheet of paper no way! I plan to use a few tens of kilohertz as the carrier frequency to minimise the processing overhead I am concerned though as to whether the variation of delay could be made smooth enough so that the `steps' were not audible I need a DSP AM modulator, a variable delay, and an Envelope detector (I notice reference to one in CIAO radio in an earlier post. The effects of the audio filtering, and audio compression could be met by readily available plug- ins for the likes of WINAMP etc. I would also like to have the option of outputting it at typical soundcard IF frequencies such (and indeed zero Hz) so that it could be used with various software radio programs as well Rather than try to re-invent the wheel, I would be most grateful for thoughts and comments and hints as to how the delay should be implemented, the choice of modulating frequency and the whereabouts of any public domain DSP code for any of the bits! By the way, this is not a commercial project, just a hobby one, and I would envisage making it freeware and open source if possible. 73's Andy EI3HG
