Asymmetrical Modulation AM Samples
I had a discussion with an engineer who claimed the only way to achive clean audio on AM was to have symmetrical modulation. So I
decided to do my own test using my part 15 AM transmitter. It doesn't matter to the radio how big the transmitter is so the part 15 transmitter will work
just fine. And a part 15 station does not need to comply with the 125% positive peak regulation that broadcast stations do. The positive peak modulation
was allowed to go as high as it wanted at times hitting 129%+ for 98% negative.
The transmitter used was a modified Ramsey AM-25 with my own modulator/final. Power input was 65 mw at a frequency of 1290 Khz using a legal, 3 meter, non
loaded antenna. Modulation was monitored with a TFT model 713 modulation monitor calibrated against an oscilloscope. The recordings were made using a AWIA
model TX706 Walkman style radio as the receiver.
Equalized audio was fed to a Knat Hindes Mic Maze, Texar Prism, a Mosley TAL - 320 limiter and finally an Inovonics model 222 to get things on the standard
NRSC pre-emphasis curve. A Radio Design Labs line amplifier feeds the audio input of my homebrew modulator in the Ramsey transmitter.
Left - Negative Modulation Right - Postive Modulation
Variable peak flasher set to 129% measuring positive peaks.
AM Sample 1
AM Sample 2
Audio was also loud and clean on a Sony SRF 42, a Sony ICF M410V, a Realistic STA-111, the radio in my Honda CRV or any radio I tried.
There is no reason that clean, asymmetrical modulation can't be achieved provided your equipment capable of doing it and is properly set up. Not all AM
transmitters can cleanly modulate 125% positive. Can your transmitter handle the audio you're trying to run through it? It's a matter of correctly
adjusting and leveling everything to work as a system. Just cranking up the audio processing wont work. You may need to begin at the antenna and work your
way all the way back to the microphone. If you have trouble and you haven't done this you don't know what you have and you're flying blind. If clean,
asymetrical modulation can be done with a part 15 transmitter and old audio processing equipment why can't it be done with a modern transmitter and state
of the art audio processing gear??
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