Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

MULTIPATH DISTORTION 295Figures 1, 3, 9, etc. If these holes are so deep that the limiter in theconventional receiver cannot hold the output constant, amplitude modulationwill be impressed on the discriminator.respond to these variations inIf the discriminator canamplitude, the output will be distortedat the part of the cycle which corresponds to the hole in the resultantcarrier.If the resultant-carrier amplitude drops below the noise level,the distorted part of the cycle will have noise superimposed on it.Cases have been observed in which the output was undistorted at allparts of the cycle except at one point, and there the output was allnoise.Each time the two nearly-equal carriers go out of phase, there is avery rapid phase shift which causes a peak or dip in the output correspondingto this phase modulation. The worst distortion occurs whenthe two signals are of nearly equal voltage, and when the two radiofrequencycarriers are out of phase when the modulation is removed.The distortion increases with the audio frequency because the higherthe audio frequency, the further the retarded wave lags behind, interms of an audio cycle. An increase in the maximum frequency deviationincreases the number of times the two signals go in and out ofphase in each audio cycle, and this also increases the distortion. Thisincrease of the distortion as the deviation is increased is in contrast tothe reduction of ordinary noise as the bandwidth of the transmissionis increased.The amplitude of each harmonic can be calculated by the formulasgiven. From these values, the effect of a low-pass audio filter can bedetermined. The low frequencies are not distorted very much, and thehigh frequencies will have their distortion reduced by the audio selectivity.This means that the medium frequencies will be the ones whichare distorted the most and they are the ones most needed for intelligibility.For shorter path differences the time delay is less, and thedistortion is reduced. If the two signals are approximately in phase atthe undeviated position, they cannot go entirely out of phase for pathdifferences less than about one mile with the deviations used today.However, if they are out of phase at the undeviated position, distortionwill be produced even for small time delays.For small deviations andlow audio frequencies the signals do not get far out of phase; thusthere is not much distortion.The distortion due to multipath transmission of frequency-modulatedwaves contains many high-order harmonics and is,therefore, differentfrom amplitude-modulation distortion caused by the same process.In amplitude modulation the harmonics are all of low order andare not so disagreeable.Frequency-modulation distortion often sounds