Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

DUPLEX SOUND AND FACSIMILE 397lower deviation. The ability of the facsimile equipment to operate ona much less favorable signal-to-noise ratio then becomes increasinglyimportant. The facsimile equipment used in these tests was designedto operate under conditions heretofore considered too severe for goodreproduction, and the finalresults obtained in the multiplex operationshould therefore be considered on the basis that all possible favoringhas been given to the aural channel. When used with a facsimile systemrequiring a higher signal-to-noise ratio, the facsimile would ingeneral become lessreliable and reproduction poorer.Practical difficulties were encountered in the field tests describedlater that limited the frequency deviations used for the aural portionof the multiplexing to 60 kilocycles. The facsimile service was addedat 15 kilocycles deviation, thus sharing the maximum deviation of 75kilocycles between the two services. Many things contributed to thislimitation of aural deviation and each will be described in turn.Before going to the tests and their results a description of the facsimilesignals used in these tests will be given. A general discussionof the frequency-modulation theory for multitone use is also in orderif the proper interpretation of the results is to be made.2. The Facsimile SignalsIn the usual broadcast facsimile services contemplated or now inuse, copy areas of from four to twenty square inches per minute aretransmitted. A good average at present would be 10 square inches perminute, the figure chosen for these tests. This was transmitted at 120scanning lines per inch, using a picture signal frequency range of fromzero to 1200 cycles. This signal can be transmitted in a variety ofways, the usual one being to carry the signals as an amplitude modulationof an audio tone. However, previous facsimile work has shownthat the use of a frequency-modulated sub-carrier for the facsimilesignals has many advantages over the amplitude-modulated subcarrier.1 The sub-carrier chosen in this case was 18 to 24 kilocycles,the picture signals occupying the band of 20 to 24 kilocycles and thefrequency being shifted down to 18 kilocycles for a short time onceper scanning line to indicate phasing. The spectrum then is as follows:18 kilocycles—phase signal20 kilocycles—white picture signal22 kilocycles—50/50 gray picture signal24 kilocycles—black picture signalThe use of a phase signal to hold synchronism of the recorder isusually insufficient if very fine detail of recording is expected, and a1Mathes and Whitaker, "Radio Facsimile by Sub-Carrier FrequencyModulation," RCA Review, Oct. 1939.