Here's some patents from my collection. Some of them discuss FM carrier beat distortion and interference.
US 3985978 Method and apparatus for control of FM beat distortion
CD-4
US 4204091 Cancellation of interference distortions caused by intermodulation between FM signals on adjacent channels
Victor Company of Japan, CD-4
US 3936619 Four-channel stereophonic reproducing system for reproducing discrete CD-4 four-channel stereo disc
Matsushita Electric Industrial Co., muting and noise reduction, CD-4
US 3943303 Abnormal sound eliminating system in record disc reproducing apparatus
Victor Company of Japan, CD-4, var LPF
US 4152550 Apparatus for reproducing a multichannel record disc
Victor Company of Japan, CD-4, harmonic filter
US 3843850 FOUR CHANNEL RECORD REPRODUCING APPARATUS WITH MUTING OF SPEAKERS NOT USED FOR STEREO OR MONAURAL RECORDS
Victor Company of Japan, CD-4
US 3944755 Composite Filter circuit
Victor Company of Japan, CD-4
US 3991283 Multichannel record disc reproducing system
Victor Company of Japan, CD-4
US 4001518 Discrete four-channel disc reproding system
Matsushita Electric Industrial Co., CD-4
US 4075425 Multi-directional sound signal reproducing system
Nippon Columbia Kabushikikaisha, CD-4
US 3631345 MONITOR CIRCUIT
transistor switch controlled lamp
US 4198609 Phase tracked loop frequency demodulator
The CD-4 FM carrier is in the range between 20 to 45 kHz. Compare this to FM radio and the TV FM sound carrier (ie. old NTCS) where the deviation for FM radio is +/- 75 kHz and TV sound is +/- 25 kHz.
http://books.google.com/books?id=D9...wAw#v=onepage&q=tv sound fm deviation&f=false
http://en.wikipedia.org/wiki/NTSC