STANDARD VOLUME INDICATOR AND REFERENCE LEVEL 131 



(A) Fundamental Requirements 



1. Type of Rectifier 



The volume indicator must employ a full-wave rectifier. 



2. Scales 



The face of the instrument shall have one of the two scale cards 

 shown in Figs. 16 and 17. Both cards shall have a "vu" scale and a 

 "percentage voltage" scale. The reference point at which it is in- 

 tended normally to read the instrument is located at about 71 per cent 

 of the full scale arc. This point is marked on the vu scale and 

 deviations from this point are marked in vu to + 3 and to — 20. The 

 same point is marked 100 on the other scale which is graduated pro- 

 portionately to voltage from to 100. 



3. Dynamic Characteristics 



If a 1000-cycle voltage of such amplitude as to give a steady reading 

 of 100 on the voltage scale is suddenly applied, the pointer should 

 reach 99 in 0.3 second and should then overswing the 100 point by at 

 least 1.0 and not more than 1.5 per cent. 



4. Response vs. Frequency 



The sensitivity of the volume-indicator instrument shall not depart 

 from that at 1000 cycles by more than 0.2 decibel between 35 and 10,000 

 cycles per second nor more than 0.5 decibel between 25 and 16,000 

 cycles per second. 



5. Calibration 



The reading of the volume indicator (complete assembly as shown 

 schematically in Fig. 18) shall be vu when it is connected to a 600-ohm 

 resistance in which is flowing one milliwatt of sine-wave power at 

 1000 cycles per second, or n vu when the calibrating power is n decibels 

 above one milliwatt. 



(B) Specific Requirements 



1. General Type 



The volume indicator employs a d.-c. instrument with a non-corrosive 

 full-wave copper-oxide rectifier mounted within its case. 



2. Impedance 



The impedance of the volume indicator arranged for bridging across 

 a line is about 7500 ohms when measured with a sinusoidal voltage 

 sufficient to deflect the pointer to the vu or 100 mark on the scale. 

 Of this impedance 3900 ohms is in the meter and about 3600 ohms 

 must be supplied externally to the meter. 



