Welcome to WWDX...
There's a lot to be said about Asymmetry and "Hi-Fi" - you can't give one 20-20,000Hz resolution - there is simply not enough bandwidth to handle that - Heck even the FM stations use a 19kHz Pilot signal that has to be filtered out, thru the radio - but it is needed to tell the system that there is more info on the other side of this frequency that contains one channel ONLY - that can be used to remove the combined channels in the initial listener frequency (L+R) and leave the difference. (L-R) - so L is "removed" from one to leave R) Else without that processing the FM station would only be heard with L+R as Mono.
With the above in mind that is only FM, as a Frequency Change at an Audio Rate. Carrier stays steady - information moves in around it as shifts in frequency of that carrier but not it's power level.
Amplitude Modulation is similar but the Carrier is pre-set level of power and is NOT able to change frequency - but it's "envelope" can but with Audio - there is only so much spectrum to have information made available. With AM it's even less...and with CB even lesser (SIC) so for the Lessest (SIGH) we make of the effects of Hi Fi and expectations - we can at least achieve information applied to the carrier that rises and falls at a specific rate of change as a POWER LEVEL - the audio information then imposed on it is what appears as asymmetry - it's not - it is the carrier power that does this effect.
That is one way to approach this - others as you have read and seen their efforts - make use of a carrier bias power but change the rate in which that power is made available at any given moment thru the use of a Diode and Resistor and call it NPC, or Negative Peak Compression.
To understand that you need to know of how a regulator works and how the feedback principle of the regulation process works. The Resistor and Diode in the NPC mod, changes the way the "sense" and feedback works in the AM Regulator circuit. It affects how the power given to the Final and Driver as a DC Bias - carrier - gets replenished - which in turn - shows up as negative going power levels - falling levels of power - which the audio that needs to be impressed upon it - uses.
In a typical AM Regulator system - a mid-point is determined and a DC value is derived from it - using either a resistor divider system, or a tapped pre-set voltage regulation system that supplies the power to make the Set-point - or AM power (trimmer pot in a radio) work to make the "steady state" of DC a level of carrier in which to make the Audio power arriving from the Mic Amp section to mix with and develop and envelope of power from.
Note the use of the above to keep it within limits - requires a feedback principle - else if you try to impose too much audio - without feedback - you don't have any means to detect and correctly adjust carrier for the extra audio power needed to be placed in the sidebands - that's where the regulator needs to "sample" the output to adjust for more input of audio power and reserve for application of those audio peaks that would otherwise distort and clip in the efforts the regulator is trying to send to be transmitted along with the carrier - for if it is not kept in check - excessive bandwidth and or audio power applied, causes another effect of; Phase modulation - where the carrier is now mixing with a bunch of frequencies and cannot locate or find stability in any sort way because there is little to no presence of DC bias to keep carrier (Zero point) present.
A way to fix that...
So in a way, NPC is a way of helping to keep bias from dropping too low and audio running into a problem of not having enough carrier to help it generate that envelope.
But then, there is this - a typical design using 8 V DC regulated as your means to set mid-point...
From a PC-122 - basic building block shown...
NPC is two parts soldered in series (one lead of one to one lead of another) and installed across the output sense section of the above circuit.
But you lose 1/2 your audio in NPC environments. How, by the loss of the "negative" or down-ward slope or lower voltage than pre-set "carrier AM POWER" - it (Any reserve) drains off into the Output of the Regulator to the Final and Driver - once the AC value of audio as a frequency is converted to this Bias + Audio waveform that gets sent to the Transmitters Final and Driver - it never gets returned - you have to wait a given amount of time for it to replenish from the REGULATION's own supply source. (Your Diode and Resistor - the Diode is that one way valve that keeps power flowing only one way - in this case - out - the Resistor slows down the rate of discharge to a given level of time-slope so you can get some negative "dip" but not much).
Ok, I've gotta stop here for now, too much else is going on but I wish you the best - I am and will be out here, just needs of the family are pretty high on my priority list right now.
