524 BELL SYSTEM TECHNICAL JOURNAL 



signal. The gain of the restoring device would be guided by this 

 auxiUary information. Hence, the device used to pass the information 

 along is called a "pilot channel." Various types of pilot channels are 

 listed in Part 4 as secondary characteristics of the control. 



When the control ratio is between and 1 the compression is in- 

 complete. A wave compressed in this manner has the property of 

 being able to cause re-expansion at the receiving end since the output 

 amplitude bears a definite relation to the original, assuming constant 

 transmission over the intermediate circuit. 



In the field of expandors having a control ratio between one and 

 infinity the signal modification is opposite to that of compressors. 

 Thus a convenient method is available for restoring the original wave 

 shape by using an expandor having a control ratio which is the re- 

 ciprocal of that of the compressor at the sending end. 



Effects of Control Ratio 



The control ratio is useful in determining the effectiveness of a 

 device in improving transmission in the presence of noise in the med- 

 ium. When noise alone is acting on the device, the noise determines 

 the action in a manner similar to speech. When both noise and speech 

 are present, the action is determined by the sum of the two. Thus, 

 room noise applied with the speech will be compressed or expanded 

 exactly as if it were part of the speech. In the case of a compressor 

 used at the sending end of a noisy circuit, an input range of say 60 db 

 might be compressed to 20 db, by using a control ratio of 1/3 over the 

 entire input range. At a point where the strongest signals are un- 

 changed, the weaker signals would then be 40 db stronger than when 

 the compressor was omitted. The improvement of the signal and 

 applied noise with respect to noise in the medium thus depends on the 

 difference in ranges at the input and output which depends on the 

 control ratio. 



A large part of the usefulness of an expandor is in changing the 

 apparent ratio of speech to the noise heard in the absence of speech, 

 since the noise is generally weaker than speech and is made even less 

 compared to speech by expansion. This is in spite of the fact that at 

 any instant the signal-to-noise ratio is the same at the output as at the 

 input. When the noise is comparable with the speech in amplitude, 

 or when the noise is so weak as to be negligible without a controller, 

 there can be no improvement in the noise conditions in using these 

 devices. Between these two limits, the noise improvement rises to a 

 maximum value also determined by the control ratio, and the time 

 actions and range to be discussed. 



