[net.audio] VTA IS important

kimr (02/01/83)

This is a summary of a paper presented at the Audio Engineering Society 
72nd convention (preprint #1940) entitled "Vertical Modulation Angles of
Commercial Stereo Phonograph Records" by Alan Woodard of Shure.  

A test tape was cut at 6 mastering facilities and the vertical modulation 
angles of the resulting records (angle at which the record was cut) were 
measured.  These angles varied between 15 and 24 degrees.  It was discovered
that VMA is determined only by the characteristics of the various cutters 
used, and does not vary with modulation level or frequency.

Then a "random sample of commercial phonograph cartridges" were measured
for vertical tracking angle, and they ranged from 22 to 35 degrees.

A analysis of the frequency intermodulation distortion (FIM) resulting
from the mismatch of vertical angles was made.  The conclusion:

  "For a recording with a VMA of 15 degrees played back with a cartridge
   which has a VTA of 25 degrees, the FIM distortion generated reaches 4.5%
   at the inner grooves for the relatively mild modulation level of 5 cm/sec.
   These tolerances represent a high degree of compromise.  A 15 degree VTA
   error, a possibility indicated by the survey, would generate almost 8%
   FIM distortion for the same recording.  In contrast, a 3 degree error
   results in a 1.4% distortion at the inner radius and represents a 
   significant reduction."