The columbia compatible stereophonic record

A "compatible" stereophonic record is one which will reproduce full stereophonic sound when played on a stereophonic phonograph and will be indistinguishable from an LP record when played on a monaural phonograph. Such a record may be obtained by recording the sum signal S=L+R as lateral modulation and the difference signal D=L-R as vertical modulation and suitably modifying D so that the tracking possibilities of monaural pickups are not exceeded.