Audio Levels and Metering

BY KENNETH HUNOLD

It has been said that television without sound is a stand-by slide. While that statement may be an exaggeration, sound is an important part of the TV experience. Audio has become ubiquitous, and everyone seems to think they're an expert on the subject. There are almost as many opinions on how to meter and monitor audio signals as there are audio engineers. Audio people criticize TV audio products for their audio omissions, while video people bemoan the multitude of standards used on the audio side of the house. At the risk of offending or contradicting engineers on either side of the continent, a few suggestions about audio levels and metering follow.

THE HISTORY OF STANDARDS

First, let's look at how early standards developed. Audio transmission (in broadcasting) evolved from the telephone industry. Balanced audio transmission and the 60052 impedance were developed by the long-distance telephone industry and were often incorporated into broadcast facility designs. Transmission-Line Theory was cited to require impedance matching for maximum power transmission from source to load. The unit of measure was the Watt, and audio signals were measured by comparing them with a reference power level of 1mW into 60052. The ratio of any power to the reference power was given in decibels. The difference between the measured power and the reference power of 1mW was assigned the unit dBm. A common operating reference level was 8dB above 1mW into 60052, or +BdBm.

Today, power-matched audio transmission has been replaced by a voltage based interfaced system where source impedances are kept low. Most loads bridge the source rather than terminate it, and they do not draw significant power from the source. The term dBu describes this new measurement and is defined as decibels unterminated. To maintain some continuity with the older dBm notation, OdBu was defined as the voltage across a 60052 load when it is dissipating 1mW = 0.77459V (often rounded to 0.775V).

In North America and Japan, the +BdBu reference level has largely been replaced by +4dBu. Often, as facilities change from power-matched transmission to voltage-based transmission or when they convert to digital audio distribution, the reference level is simultaneously changed to +4dBu. The new level allows sufficient headroom (approximately 20dB) with popular IC devices using power supplies of - 15V.

METERING DEVICES AND STANDARDS

Metering, both for calibration and operation (gain riding), is an area in which multiple standards coexist. The traditional method of metering/monitoring audio signals was the VU meter. Learning how to "ride" audio level is an acquired skill. It is sometimes difficult to interpret the movement of a VU meter, and instructing operators how to set program levels based on its readings can give greatly varying results. The peak program meter (PPM) was developed to offer a better indication of program loudness and to aid in more uniform loudness settings. But old ways die hard. If there is a VU meter somewhere in the room, many operators will, almost subconsciously, glance at it occasionally as a sort of reality check during a session or show.

With the proliferation of solid-state metering devices, the ballistics, or movements, are often not obvious and may not represent a known standard. To that end, you may have to look at the configuration of the meters and make some choices. If the meter in question has only a few segments above the lineup level (the level you set your reference tone to), assume that the display has W-type ballistics. Such a meter would not let you see the peak level of the signal, but it would keep the display from coaxing out on audio peaks. Conversely, if the meter has much room above the lineup level, it could be easier to see the peak levels of the program. However, electronic meters can turn on and off again too quickly. In this case, a peak-hold feature can be used to hold the highest level for a second or two to ensure that operators do not miss it. Some electronic meters allow the scale resolution to be increased for greater precision when setting levels. Then, after level setting is completed, the scale resolution can be reduced to allow a greater range of audio levels to be indicated.

No metering technology can guarantee proper levels under all conditions. The best solution is the intelligent interpretation of the meter display by a quality processor-the human brain. Unfortunately, even this analog computer can suffer from bad "programming."