some transcriptions were recorded with a vertically modulated "hill and dale" groove. this was found to allow deeper bass (because turntable rumble was laterally modulated in early radio station turntables) and also an extension of the high-end frequency response. neither of these was necessarily a great advantage in practice because of the limitations of am broadcasting. today we can enjoy the benefits of those higher-fidelity recordings, even if the original radio audiences could not.