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KK7CW and all,
Thanks for the fine writeup concerning this. I agree that limiting AM audio to 10 kHz. is a good idea. Another reason for this is many AM radios won’t produce audio above this frequency and the transmitter power is just wasted at high audio frequencies.
A properly tuned high Q antenna actually reduces splatter, but your approach is much better…control the audio.
In addition to high modulating frequencies, the surest way to cause splatter on AM is to overmodulate the transmitter. This causes clipping of the audio peaks on the envelope producing strong harmonics which are multiples of the audio frequency and they appear added to and subtracted from the carrier frequency. For example, a 1 kHz. sine wave audio tone at >0 to 100 % modulation produces a 2 kHz. bandwidth. Over 100 % modulation which clips the top of the envelope or the nadir of the envelope will produce strong sidebands out to about the 7th audio harmonic which gives a bandwidth of 14 kHz. Audio programming up to 5 kHz. with clipping will yield a bandwith of 70 kHz. instead of the usual 10 kHz. So, in addition to limiting the audio bandwidth, the modulation level has to be set to avoid clipping.
I believe you have a good approach with audio processing. I don’t have an audio processor so I use an equalizer program set to cutoff at 10 kHz. for my AM work. Such an equalizer program could also be set to give preemphasis but I have not done so. For those who want to try this approach, the corner frequency corresponding to 75 uS. is 2120 Hz. where the boost is 3 dB with a positive slope of 6 dB/octave above this.**EDIT. This is based on a 1 zero response. When I reviewed the NRSC-1 document I noticed that their response curve for AM has a slope of about 3dB/octave.***
I use a SSTRAN AMT-3000 which has built in modulation limiting but it still has to be adjusted properly. I use an oscilloscope for this. If one does not have a scope, listen to the audio on a receiver and make sure it is not distorted. Increase the modulation until distortion is heard and then reduce it until the signal is clean. This is not perfect but it is close enough for part15 work.