2 ms·
Of the methods listed on Wikipedia only ISB is a true AM. All the others use phase modulation for the difference signal, as an easy way to achieve compatibility
by bonzini 2y ago
Of the methods listed on Wikipedia only ISB is a true AM. All the others use phase modulation for the difference signal, as an easy way to achieve compatibility with mono AM receivers; and PM is basically the integral of FM.
Wikipedia says that there was basically one station doing ISB stereo; which I guess is close enough to "nobody did it", but not quite "it was never done".