STANDARD VOLUME INDICATOR AND REFERENCE LEVEL 99 



by the various manufacturers and have come into service in the plants 

 of the different companies. These instruments had different calibra- 

 tions and characteristics with little correlation between their readings. 



A further divergence occurred, regarding the philosophy of the cali- 

 bration of the original type of volume indicator. One view recognized 

 no correlation between the point at which the galvanometer was 

 normally read on peaks (the 30 division point on the scale, Fig. 12) 

 and the power of six milliwatts used for calibration. When calibrating 

 the instrument on six milliwatts of sine wave energy in- 500 ohms, the 

 galvanometer would read 22 divisions with the associated sensitivity 

 switch on step zero. There was not intended to be any correlation 

 between this calibrating power and reference volume. Nevertheless, 

 many people were led by this technique of calibration to refer to the 

 volume indicator as a 6-milliwatt instrument. This idea was furthered 

 by the fact that the vacuum tube to whose speech-carrying capacity 

 the reference volume was originally referred, has a nominal full load 

 capacity on sine waves of 60 milliwatts. The reference volume being 

 defined as 10 db below the maximum output of this tube, it was 

 natural to try to relate this reference volume to the corresponding 

 figure of 6 milliwatts for sine waves. 



The second view was based on the experimental fact that when the 

 potentiometer controlling the sensitivity was set at "0 db," a sine 

 wave potential of 2.5 volts (r-m-s) applied to the volume indicator 

 caused a deflection to mid-scale (scale reading of 30 divisions). This 

 was equivalent to 12.5 milliwatts in a 500-ohm circuit, and the sup- 

 porters of this view therefore referred to the volume indicator as a 

 12.5-milliwatt instrument. 



Thus the same volume indicator, having the same sensitivity and 

 giving the same readings of volume level, was variously referred to as 

 a 6-milliwatt and a 12.5-milliwatt device. This increased the difficulty 

 of coordination between the plants of the different companies which 

 are interconnected in rendering broadcast service. 



Some degree of standardization of the technique of reading volume 

 levels had already been made within different organizations both here 

 and abroad. The importance of the present development lies not 

 only in the particular merits of the proposed standards, but also in 

 the fact that they have been jointly developed and adopted by three 

 of the larger users of volume indicators, and have been approved by 

 many others. Thus there is good prospect that the needed standard- 

 ization is about to be realized, and that all will shortly use the same 

 instruments, the same reference levels, the same terminology, and the 

 same nominal value of circuit impedance. 



