There is a common belief that when cutting vinyl dub plates and masters the lower frequencies must be in mono, but this is only partly true. Its all a matter of amplitude. Its possible to cut very wide stereo at any frequency, but there is a limit to the amplitude of the wave. Below is a close up photo of a 600Hz sine wave cut in mono at 45rpm, magnified x40 (fig.1).
Fig. 1 (600Hz sine wave - mono)
The next photo is of the same frequency but this time one of the channels has been inverted so its in opposite polarity to the other, creating a sine wave with maximum stereo width. This means instead of moving from side to side, the cutting stylus moves up and down. In this case the vertical motion of the cutting stylus was not too great, and the groove never becomes too shallow.
Fig. 2 (600Hz sine wave - opposing polarity)
In the next photo, a wave of similar loudness wave cut, but this time the frequency has been halved to 300Hz. In this photo one can clearly see that groove at its narrowest point, is much narrower than the 600Hz groove, and is at the limit of what a stylus can reliably track.
Fig. 3 (300Hz sine wave - opposing polarity)
But why should this be the case? Why should a lower frequency need to have a larger amplitude? Its because the loudness is related to the linear velocity of the wave on the record. A lower frequency means a lower linear velocity, so the amplitude must be greater to make up the difference. Greater amplitude means greater groove excursion, and so for example cutting the same loudness at 150Hz would mean even greater amplitudes, and therefore even bigger waves in and out of the surface of the disc.
For this reason I use a 6dB per octave roll-off from 400Hz downwards on the stereo width of the recordings I make. This ensures that as the frequency gets lower, and the waves need greater amplitude, the stereo width gets narrower and the chance of the vertical modulations being too large to cut is reduced in accordance (see fig. 4).
Fig. 4 (Sonnox Filter used in M/S matrix on Side channel)
For more details on how to achieve this yourself before submitting material to the studio please see the blog entry on Stereo Processing.
