Fairchild 670 - Limiting in Stereo Disk Recording

LIMITING IN STFREO DISK RECORDING
The cutting of STEREO DISKS has uncovered a number of new problems
heretofore unknown. The normally modulated STEREO groove requires
approximately twice the space of lateral grooves for similar modula-
tion, yet the available space is no greater. In addition, the two
STEREO channels, depending on their phase relationship, may result
in either lateral or vertical modulation, or a combination of both.
It stands to reason that some peaks may result in purely vertical
modulation on the disk, others purely lateral. If this were allowed
to happen continually, the result would be only 15 minutes on the
STEREO LP side or else it would be necessary to reduce the recorded
level radically. Otherwise, judicious use of variable pitch and
variable depth would have to be exercised. The fact is, because of
the smaller playback tip radii and the response of the playback
pickups to vertical as well as lateral motions, the signal-to-noise
ratio on STEREO DISKS is already reduced over the lateral counterpart.
Besides, radical use of automatic variable depth will further increase
processing problems already severe in STEREO DISKS.
Another problem exists because of the difficulty of tracking large
vertical amplitudes. Most commercially available STEREO pickups have
considerably less vertical compliance and consequently are less capable
of tracking large vertical modulations. Also, we should not forget
that the tip radius on the STEREO playback stylus is still 0.7 mil,
which necessitates a minimum of 1 mil groove width, or poor tracking
might result.
The one apparent solution to these problems is to break the STEREO
signal down to its respective vertical and lateral components. Limit
the vertical and lateral canponents independently corresponding to
the available groove space and depth, then recombine these components
to regain the original STEREO signal. This can be done with the
FAIRCHILD MODEL 670 LIMITER and many thousands of STEREO masters
have been cut successfully with the help of this FAIRCHILD wit.
The limiting of the vertical and lateral components instead of the
left and right channels has additional merits. Such limiting will
retain the spatial distribution of instruments and soloists as orig-
inally recorded without producing any annoying image drift. Of course
some program material of the ping-pong type requires independent
limiting of each channel, and this is also available in the MODEL
670 LIMITER.