In these days when we are concerning ourselves with such technological topics as digital compression, this writer recently came across a document written in 1932 entitled "Report on the Investigation of the Light-Beam Type of Volume Indicator." It is interesting to note that this report was written during the time frame in which representatives from CBS, NBC and AT&T were holding a series of meetings and investigations aimed at the standardization of an audio program-level indicator, and that, back then, there was not a standardized device to measure audio program level.
The document referenced was Engineering Report No. 82, made by a member of the Engineering Development Group of a broadcasting network that was only about six years old at the time, and which had recently moved into new headquarters in the newly developed area of midtown Manhattan, establishing what would later be called "Broadcast Row."
The meter consisted of a galvanometer that reacted to the audio voltage impressed on it by moving a small mirror that reflected light from a small bulb onto a ground-glass screen, instead of moving a conventional needle pointer.
The scale of this indicator was three inches long. The report recommended that " … future expansion of dynamic ranges in broadcasting should be considered. It may therefore be well to prolong the indicator scale to 1-1/2 times its present length." If the report’s author had only known what would really happen to dynamic ranges in broadcasting in the future, he might have recommended shortening the scale!
MASK SHAPES
The shape of the mask through which the light shone produced a diamond-shaped spot on the meter face. This was not particularly to the investigators’ liking, and a number of different mask shapes were tried. The investigators found a triangle with concave sides (resembling an arrowhead) to be the most clear and distinct indicator. The white ground-glass scale had black figures, but the report recommended that two colors be used – yellow or amber for the accepted volume range and red for the upper limit.
The report states that the normal operating range was between 5 and 30, and that the red zone should extend from 33 to 60, while the yellow or amber zone should extend from 33 down to zero. It would be very interesting to see this scale in action, as judging from its description it was quite different from any that we are used to seeing today.
The scale was horizontal, but the recommendation was made to arrange this or any other volume scale vertically. The recommendation was made because the operator could then associate an upward scale movement with an increase in volume, and because very small level changes could be discerned using a vertical scale, as its movement would be at right angles to other movements in the studio. With few exceptions, these reasonable-sounding suggestions never caught on for mechanical meters.
VERTICAL ORIENTATION
Vertical scales did, however, catch on with the relatively recent deployment of audio level meters that use LED and plasma displays. A principal reason for the vertical orientation of such meters today is of course to facilitate the inclusion of the large number of meters required on a large mixing console as is typically used today.
The report states that the galvanometer resistance of 460 ohms seems rather high for a meter of this type, and that a low meter resistance is desirable because when one or more meters are placed in series with the plate of a tube, the grid voltage/plate current relationship must remain constant if accuracy is to be maintained. This is not a big consideration today.
It was also found that a large difference in ballistics existed between the light-beam meter and two other meters to which it was compared, and a suggestion was made that retardation, if desired, should be accomplished electrically rather than mechanically. It was further opined that perhaps the ability to vary inertia at will might be a good feature to incorporate.
BALLISTIC CHARACTERISTICS
The damping in this meter was stated to be extraordinarily high, and this was viewed as a benefit, as the then-current needle-type instruments had large amounts of overshoot. We know that when the standard volume indicator (also known as the vu meter) was standardized, all its ballistic characteristics were defined in the standard.
This report from 1932 gives some insight into what issues were concerning engineers in the still-young business of radio broadcasting at that time. It also serves to illustrate some of the background that led to standardization of the measurement devices used in broadcasting.
The innovative light-beam audio volume level meter did not establish itself in the industry. This same display principle did, however, become established in precision laboratory balances, such as those used by your author in college chemistry lab.
