512 



BELL SYSTEM TECHNICAL JOURNAL 



Just as the line level range of an on-off arrangement may be extended by 

 the use of a level compensator, so it seems reasonable to expect that a similar 

 improvement could be obtained by using level compensators on the linear 

 on-off and single-sideband arrangements. However, when a level compen- 

 sator is applied to such arrangements, it must be a slow acting device in order 

 not to cause characteristic distortion. When large and rapid level changes 

 are frequent, as may occur on a radio circuit due to fading, an arrangement 

 including a fast acting device like a limiter is preferred, such as a two-source, 

 one-source two-band, or frequency-shift arrangement. 



The effect of level variations on the distortion of the narrow-band two- 

 source arrangement and of the wide-band frequency-shift arrangement with 

 and without low cut-off low-pass filter is shown in Fig. 14. At 60 w.p.m. the 

 wide-band frequency-shift arrangement with low cut-off low-pass filter could 



48 



LU 40 



5 



-48 



■40 



40 



48 



? -24 -16 -8 8 16 24 3 



RECEIVING LEVEL IN DECIBELS (FROM NORMAL) 



Fig. 14 — Distortion vs. receiving level characteristics of narrow- and AAide-band 

 arrangements at 60 w.p.m. (23 d.p.s.). 



tolerate the nearly same level change as the normal-band frequency-shift ar- 

 rangement if 20 per cent distortion is the limit. When the low-pass filter of the 

 wide-band frequency-shift arrangement had a high cut-off, the tolerance was 

 somewhat greater due to the steeper wave front of the detected signals which 

 rendered them less suceptible to bias caused by slight unbalance in the 

 detector and d-c amplifier at levels below the cut-off of the limiter. The 

 narrow-band two-source arrangement tolerated slightly less level change 

 than the normal-band two-source arrangement with limiter, for the same 

 reason. 



Distortion at 120 Words per Minute 



When the speed was doubled the effects of level change on distortion were 

 found to be as shown by Figs. 15 and 16, except that tests on the narrow- 

 band two-source arrangement were omitted because of high distortion at 



