FUNDAMENTALS OF TELETYPEWRITERS 625 



larger variation may be tolerated without causing errors except when 

 it occurs simultaneously with an abnormally large line distortion. 



In contrast with this, in a synchronous system such as employed in 

 the usual "Multiplex Printer System," it is necessary that the speeds of 

 distributors be very accurately maintained or errors in transmission 

 will result. This is accomplished by controlling the driving motors 

 from very accurate timing sources such as tuning forks, then testing 

 the speed of the receiving distributor two or more times every revolu- 

 tion from the signals transmitted over the line from the sending 

 distributor, and automatically correcting this speed as required. If it 

 were not for this last mentioned correction in order to receive signals 

 correctly on a two-channel multiplex system for a period of 15 minutes 

 at a speed of 60 words per minute per channel, the receiving distributor 

 speed would have to be held accurate within ± .002 per cent. In 

 other words, in 15 minutes the receiving distributor would make 

 about 5400 revolutions (60 words per minute X 6 revolutions per 

 word X 15 minutes). At the end of the 15 minutes, if it were one- 

 tenth of a revolution ahead or behind its correct position, a No. 3 pulse, 

 for example, would be received on a No. 4 or No. 2 segment resulting 

 in an error. Thus the limit would be that the receiving distributor 

 could not run fast or slow by as much as one-tenth revolution in 5400 

 revolutions or roughly 2 parts in 100,000. 



Tolerance for Distorted Line Signals 



During transmission over long lines, telegraph signals may become 

 badly distorted; that is, the dots and dashes may be considerably 

 shortened or lengthened from their correct values. It is essential that 

 the receiving distributors be capable of receiving and interpreting 

 these signals without error. To accomplish this the receiving distri- 

 butors are arranged so that they are sensitive for the reception of the 

 selecting impulses only for a very short time at the middle of each 

 impulse. The exact location of this sensitive period with relation to 

 the incoming signals is adjustable in each receiving distributor so that 

 it may have maximum tolerance for receiving distorted signals. This 

 situation is illustrated in the diagram, Fig. 5, which shows a receiving 

 distributor with the received signals under various conditions devel- 

 oped, and the sensitive points for the reception of the impulses indicated 

 by small arrows. 



It will be noted from this chart that the signals may be very badly 

 distorted (theoretically up to nearly 50 per cent of a pulse length at 

 either the front or rear end of a current pulse), without causing im- 

 perfect reception. 



