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The NRSC standard was intended to improve the frequency response of AM broadcasts by pre-emphasizing the higher audio frequencies at the transmitter to complement the shape of the r-f bandwidth of receivers meeting the assumptions of the standard.
But almost no commercial receivers were ever built to meet that standard; they have considerably narrower r-f bandwidth — which make them cheaper, and less likely to receive adjacent channel interference. Using NRSC at the transmitter is practically useless for these narrowband receivers, because they “hear” almost nothing in the spectrum where NRSC pre-emphasis occurs.
Asymmetric modulation of an AM transmitter can be useful to limit negative-going modulation peaks to ~99% while letting positive peaks go to ~125%, which gives a marginal improvement in the amplitude of the audio recovered in the receiver (while adding some distortion). Some AM transmitters are not capable of doing that, though. It depends on their design.
An AM transmitter cannot be modulated more than 100% on negative-going audio peaks without creating high distortion in the receiver, and interference on adjacent channels.
Commercial AM stations use audio processing that raises the average value of the audio waveform with respect to the peak value. The average value determines how loud that audio sounds at the receiver, other things equal. Such processing also adds distortion.