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How the longer playng time is acheived
To achieve a longer playing time there are a number of factors that need to come into play.
The recorded velocity is reduced, this is done by reducing the output level to the mastering cutter, on the recording lathe, making the recording sound quieter. This results a lower lateral movement of the stylus.
The remaining factor that comes into play is that the groove pitch is closer.
There is one side effect of this excersise, in relation to the physics of doing the above, the gtoove is slightly narrower. This was not realised at the time