556 BELL SYSTEM TECHNICAL JOURNAL 



as the original but displaced to a new position on the frequency 

 scale determined by the carrier frequency. The interval which must 

 be allowed between these displaced messages in carrier operation is 

 determined by the selectivity of the filters which are available for 

 their separation. The imperfection of practical filters tends to make 

 the message-frequency-range which may be transmitted less than the 

 line-frequency-range which the messages occupy. The time for which 

 the line is used to transmit a given amount of information is the same 

 as the duration of the message conveying it. Thus the sum of the 

 products of frequency-range by time for the messages is always equal 

 to or less than the corresponding sum of the products of line-frequency- 

 range by time. 



In case the line-range available is less than the message-range, 

 as would be the case in attempting to transmit speech over a submarine 

 telegraph cable, it is still possible, if enough lines are available, to 

 accomplish the transmission. The message wave may, by suitable 

 filters, be separated into a plurality of waves each made up of those 

 components of the original which lie in a portion of the message- 

 range which is no wider than the line-range. Each of these portions 

 of the message may then, by modulation, be transferred down to the 

 frequency-range of the line and each transmitted over a separate line. 

 A reversal of the process at the receiving end restores the original 

 message. 



While it is theoretically possible, if enough messages and lines are 

 available, to fit the message-ranges to the line-ranges by modulation 

 and subdivision of message-frequency-ranges, it is not always practical. 

 It is sometimes more desirable to utilize the second method of trans- 

 formation already referred to. This consists in making a record of 

 the symbol sequence and reproducing it at a diff^erent speed in order 

 to secure the wave used in transmission. The tape used in sending 

 telegraph messages may be used in this manner. Here the symbol 

 sequence represents a series of selections of secondary symbols. These 

 selections are made at a rate at which it is convenient for the operator 

 to manipulate the keys of the tape-punching machine. The electric 

 wave impressed on the line by the holes in the tape represents a corre- 

 sponding sequence of primary symbols. The rate at which these are 

 applied to the line is determined by the velocity of the tape in repro- 

 duction. Since for a given number of different primary symbols the 

 frequency-range required is proportional to the rate of making selec- 

 tions, it is obvious that the frequency-range of the message as repro- 

 duced from the tape may be made to fit whatever line-frequency- 

 range is available, at least so far as width of the range is concerned. 



