In article , Serge Auckland
wrote:

Some of the RCA discs have the Dynagroove system, which, if I remember
correctly, involved pre-distorting the recording to compensate for the
tracing distortion of a 0.7thou spherical stylus. The system wasn't
taken up by other labels, and anyway, 0.5thou sphericals soon were
available as were ellipticals, rendering the system unnecessary.

IIUC The above is correct. Although I think they were a bit vague about how
the 'correction' was done in practice, so what they did may not have
actually corrected the spherical distortion with realistic cases. Possible
just make some sine test tones seem better in specific cases.

Playing the LPs with my cartridges, all of which have line-contact
styli, I would have expected that the treble would have sounded more
distorted than a non-Dynagroove LP, but in fact, these seem to be quite
clean at the top, certainly no worse than others.

Does anyone know the detail of how the Dynagroove system worked, and
what effect it would have on being played with a modern non-spherical
stylus.

It will change the resulting distortion. The problem is that there are a
number of 'competing' distortion mechanisms involved in the LP recording
and replay, so it is difficult to say how much effect this will have. You
could model the differential between the contact radii, but there may be
other - bigger - contributions that swamp this difference.

I suspect that for the above reasons 'dynagroove' was seen as a waste of
effort by most LP manufacturers.

There is perhaps a parallel here with the way it is possible to 'tweak' the
IF system of an FM tuner to get lower distortion for some test signals.
This has on occasion allowed a tuner to get magazine measured THD values
below what is actually possible for an 'ideal' FM tuner. However it does so
by arranging for some cases to be 'better' at the expense of other being
'worse'. Not much help when music isn't just a 300 Hz L+R 30% modulation.
:-)

Slainte,

Jim

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