100 BELL SYSTEM TECHNICAL JOURNAL 



Choice of Peak vs. R-M-S Types 

 General 



The first important decision to be made and one which would affect 

 the entire character of the development was whether the new volume 

 indicator should be of the r-m-s or of the peak-reading type. These 

 two types of instrument represent two schools of thought. The peak- 

 reading instrument is favored for general use by many European 

 engineers and is specified by the Federal Communications Commission 

 for use as modulation monitors in this country. The r-m-s type has, 

 however, been commonly employed in this country on broadcast pro- 

 gram networks and for general telephone use. In view of the impor- 

 tance of the decision and the difference of opinion that has existed, the 

 data on which the choice was made are given below in considerable 

 detail. 



In accord with common practice, the terms "r-m-s" and "peak- 

 reading" are used rather loosely throughout this paper. The essential 

 features of an r-m-s instrument are some kind of rectifier or detector 

 and a d.-c. milliammeter. The latter is not especially fast, generally 

 requiring tenths of a second to reach substantially full deflection. 

 Obviously, if a sufficiently slow wave is applied, say one whose fre- 

 quency is one or two cycles per second, the instrument can follow it 

 and the true peaks of the wave will be indicated, but when much higher 

 frequency waves are applied, such as the complex speech or program 

 waves, the instrument is too slow to indicate the instantaneous peaks 

 but averages or integrates whole syllables or words. As shown by 

 tests and practical experience, it is of secondary importance whether 

 the detector actually has an r-m-s (or square law) characteristic, or 

 has a linear or some intermediate characteristic. 



A peak-reading instrument capable of truly indicating the sharpest 

 peak which might occur in a high quality program wave would have 

 to respond to impulses lasting only a very small fraction of a milli- 

 second. Cathode-ray oscilloscopes or gas tube trigger circuits are 

 capable of doing this, and therefore might be used as peak-reading 

 volume indicators. However, the so-called peak-reading volume indi- 

 cators used in practice, designed to give a visual indication on an 

 instrument, are far from having the above speed although they are 

 much faster than the r-m-s instruments. They generally respond to 

 impulses whose duration is measurable in hundredths or thousandths 

 of a second. They therefore truly indicate the peaks of sine-wave 

 voltage whose frequency does not exceed, say, 50 to 100 cycles per 

 second. They are similar to the r-m-s instruments in that they are 



