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The CBS 4000 series Volumax employed a 180-degree phase inverter that was not too graceful on moderate amounts of speech asymmetry. I remember we had one particular newsman that would drive that thing crazy with his very harmonically rich voice. That limiter had a three-way switch on it that selected 100, 125 and 300 percent positive peaks. At some point, I don’t think there was any limit on the positive direction modulation peaks. In that period not too many, if any transmitters had the ability to modulate much past 100% without serious modification of the power supply, modulator or power amplifier. Also during that time, RF load symmetry was not much of a consideration or even a decent impedance match on the carrier frequency. A lot of what was seen as positive modulation was distortion. The introduction of clipper pre-distortion to compensate for “tilt” or reactive “bounce” in the transmitter was a big improvement; in the newer rig’s, it’s no longer an issue.
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As Walt mentioned, the ability of an AM transmitter to pass a square wave without tilt or ringing was/ is important. Then, the antenna system needs to pass the all the frequencies in the modulation bandwidth without too much phase or level shift. During that era clipping was the method of final peak control after gain reduction from AGC and limiters. The clipping made square waves that established the maximum peak level. If the peak level of the square wave was changed by the transmitter or antenna, modulation/loudness would have to be reduced to prevent overmudulation.
The Volumax had a diode to clip the audio. It was inside epoxy and called the "polarity sensor". Until the 125 percent rule was in place, the polarity sensor only clipped one side of the audio. Properly phased with the transmitter, the positive peaks from asymmetrical audio would be unclipped and could go as high as they wanted to (if the transmitter and antenna were capable of it). After the 125 rule was effective, a new polarity sensor was available that clipped both sides, one 25 percent higher.
Stations with transmitters capable of passing a decent square wave and good wide antenna systems could sound great and modulate hard.
Now, I believe all new AM transmitters pass square waves reasonably well. Antennas are what they are, some excellent, some not so good.
I know many here know all this very well, I mention it as background info for those who are not from the era.
The Volumax had a diode to clip the audio. It was inside epoxy and called the "polarity sensor". Until the 125 percent rule was in place, the polarity sensor only clipped one side of the audio. Properly phased with the transmitter, the positive peaks from asymmetrical audio would be unclipped and could go as high as they wanted to (if the transmitter and antenna were capable of it). After the 125 rule was effective, a new polarity sensor was available that clipped both sides, one 25 percent higher.
Stations with transmitters capable of passing a decent square wave and good wide antenna systems could sound great and modulate hard.
Now, I believe all new AM transmitters pass square waves reasonably well. Antennas are what they are, some excellent, some not so good.
I know many here know all this very well, I mention it as background info for those who are not from the era.
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