Metering in Transmission Systems
Today there are basically two kinds of metering, average and peak. The common VU meter is an example of an averaging meter and as such has nominal rise time and fall-back times of about 0.3 second.
The meter’s rise time is the time taken for a steady-state input signal to the meter to reach 63 percent of its final deflection; the fall-back time is the time taken for the steady-state signal to return from full deflection to 37 percent deflection. Rise and fall-back times are known collectively as the ballistics of the meter.
The original VU meters were passive devices and as such, had ballistic characteristics of a spring-loaded coil with inertia immersed in a magnetic field. Since it is basically an average-reading device, the VU meter has met with continuing success in broadcast work, inasmuch as its readings correspond to the perceived loudness of speech signals.
From their inception, peak program meters (PPM) have been electronic devices an as such can be made to respond very quickly. Typically, a PPM has a rise time of about 10 milliseconds and a fall-back time of about 4 seconds. The rapid rise time permits accurate reading of signals of very short duration, while the slow fall-back time gives the operating engineer adequate time to observe the signal’s value.
Figure 11 shows views of the VU meter (A) and the PPM meter (B). Rise time ballistics of the two types of meters are shown at C.
Relative calibration points on the meter faces for four kinds of meters are shown in Figure 12. If both VU and PPM meters are calibrated as shown in Figure 15-12, normal speech program will read maximum values of about +2 or +3 VU, while on the PPM the corresponding readings would be between markers 4 and 6 on the face of the meter, due to the more rapid rise time of the PPM relative to the VU meter.
GAIN STRUCTURE
As we have seen, normal speech has a peak factor of about 12 dB. Music on the other hand can have peak factors that are in the range of 16 to 20 dB, depending on the nature of the material.
Highly compressed music signals, such as are common in modern pop and rock music may have peak factors no greater than about 4 dB; however, classical music may present numerous operating levels, each requiring recalibration as the program progresses.
Many times during outdoor classical music events at summer festivals the sound reinforcement system is carefully adjusted manually, usually by an operating engineer working with an assistant producer with score in hand.
Figure 13 shows a typical example of how this is done. The engineer must be aware of how loud the orchestra will play and how these loudness peaks will translate through the music reinforcement system.
The aim is to contain the peaks within an agreed upon level at selected positions in the large audience area. Such levels as these are often established so as not to produce any disturbance at monitoring points in nearby residential areas.
At the same time, both engineer and producer know that low-level music passages may get lost in the ever-present noise level of large audiences, traffic, overflights and the like. Operating level shifts of the order of 12 dB are very common, and when smoothly executed may be barely noticeable as such.
