MULTICHANNEL AMPLIFIERS BELOW OVERLOAD 599 



turbance may not be much greater than that of one alone. We shall 

 introduce here the concept of "plural S.T.M.F." Suppose v products 

 of x-type are superimposed and comparison of the resulting noise with 

 one fundamental talker shows that the difference is Lxv db. If inter- 

 fering effects add as mean power we should expect L^, to be equal 

 to Lx — 10 logio V. Hence it seems logical to write 



Sxv = Lxv + 10 logic V - Hx, (7.1) 



where Sxv is the "plural S.T.M.F." to be used when v products are 

 superimposed in order that power addition of products may be valid. 

 Combining (5.1) and (7.1), 



Lxy — Lx = Sxv — 5x — 10 logio V, (7.2) 



which shows that the correction to be subtracted from power addition is 



Pxv Sxr Sx' \i.Oj 



The value of pxv is best determined by experiment. Superposition of 

 a large number of products without using an excessive number of 

 talkers can be accomplished by making phonograph records of indi- 

 vidual products and combining their outputs in subsequent recordings. 

 The average total modulation of x-type in a channel is found by 

 multiplying the average value for one product by the average number 

 of products, and subtracting the quantity pxv, which may be called the 

 "plural S.T.M.F. correction," thus 



Vx = VxVop + .115(X. - Vx)a^ + 10 logio Vxkr"^'^ - Px.. (7.4) 



where Vx is the volume averaged on a power basis of the x-type 

 modulation in the ^-channel referred to the volume of one x-type 

 product from 0-vu talkers. We next wish to express Vx in terms of 

 db above reference noise. 



Let Ta represent the "noise" produced by a 0-vu talker in db above 

 reference noise. This is an experimentally determinable quantity and 

 is about 82 db. Let Tx represent the noise from an x-type product 

 from 0-vu talkers. Then Lx, the quantity appearing in (5.1), is 

 given by 



Lx^ Ta- r,. (7.5) 



The average total noise produced by all x-type products in db above 

 reference noise is given by 



W^= F, + r, = Vx+T.-Sx- Hx. (7.6) 



