BACKGROUND NOISE IN BROADCASTING 



345 



by inserting a capacity in series with the voice frequency circuit of the 

 speaker, but even when greatly reduced the bumping was detectable 

 and was more important than any flutter which may have been present. 



The Square Law Rectifier 

 Observations similar to those just discussed were made with a square 

 law detector. In Fig. 7, the ordinates represent the carrier ratio neces- 



— — ^ — . — 1 



30 40 50 60 70 80 

 BEAT FREQUENCY IN CYCLES PER SECOND 



Fig. 7 — Carrier ratio for perceptible flutter with a square law detector. Noise 

 equivalent to 9.5 per cent modulation. 



sary to reduce the flutter to a just detectable value, while the abscissae 

 represent the beat frequency. The noise is 10 db down from an equiva- 

 lent 30 per cent modulation. The curve is in striking contrast to that 

 of Fig. 5. At very low frequencies a carrier ratio of 28 db is required 

 when a square law detector is employed, while if the receiving set 

 embodies a linear detector a ratio of 1.5 db is sufficient. The right- 

 hand portions of the curves are fairly similar, since the carrier ratio is 

 here dependent upon the audibility of the beat note and not upon 

 flutter effects. The observations of which Fig. 7 is a record were made 

 in the small sound-proof room. In Fig. 8 are shown two curves made 

 in the open laboratory. In the upper curve the noise output was ap- 

 proximately 20 db down from that due to a 30 per cent modulated 

 signal, while in the lower curve it was approximately 30 db down. 



The theory which has been outlined indicates that in the case of the 

 square law detector the flutter eff^ects should be practically independent 

 of noise level, and the curves shown in the last three figures bear out 

 this prediction quite positively. Even more definite confirmation is 

 furnished by the upper curve of Fig. 6, which shows the result of ob- 

 servations taken with a fixed beat frequency of 3 cycles. The two 

 curves of this figure show the great superiority of the linear rectifier 

 over the square law in receiving non-isochronous transmissions in the 

 presence of a noise background. 



