![]() ![]() Both scales (see Figure 1) cover the same span of signal-strength change. VU Meter Scalesīecause they were developed for two main uses (1) broadcasting and (2) recording, test equipment and telephone applications VU meters are available in A- and B-scale form. VU meters are often available at flea markets, swap meets and surplus outlets, such as Fair Radio Sales (address in Table 42 of Chapter 35 in the 1993 ARRL Handbook). The B scale emphasizes percentage of transmitter modulation. The A scale is of most use for audio-level adjustments because it emphasizes its decibel calibration. This results in a total instrument impedance of 7.5 kΩ a value that, when bridged across a 600Ω line, causes a level reduction of only 0.4 dB.įigure 1 - Both VU-meter scales span the same range of signal-strength change. (The rectifier is required to transform ac voltages into dc voltages because the basic meter movement responds only to dc.) The meter is intended to be used in series with a 3.6-kΩ resistor for connection across 600-Ω audio lines (the standard impedance for phone lines, and long-distance audio transmission in the broadcasting and recording industries). (2) VU Meter CharacteristicsĪ true VU meter consists of a 200-♚, 3.9 kΩ-coil d' Arsonval movement equipped with a copper-oxide full-wave bridge rectifier. But not every audio meter marked VU meets the specifications for a true VU meter. You've probably seen audio level meters marked VU in consumer audio, recording and public-address equipment. Nevertheless, by making the meter indicate a value somewhere in between average and peak power, it was possible to provide a close correlation between the meter indication and loudness. ![]() ![]() ![]() Although meters can be made to register the average power or the instantaneous power peaks of the signal, there is no simple relationship between these two qualities of a complex speech waveform and its loudness. The goal was a meter that would indicate complex speech waveforms in a way that would correspond closely with a listener's subjective impression of the signal's perceived loudness. That's where the VU meter comes in.įirst adopted in 1939 as an industry standard, the VU meter resulted from a joint effort by the Bell Telephone Laboratories, the Columbia Broadcasting System and the National Broadcasting Company to develop an audio level meter with standardized ballistics. Using a meter to adjust a phone patch for proper phone-line drive therefore requires a meter capable of providing acceptably accurate readings on voice signals at powers in this range. Telephone company specifications call for driving a phone line with no more than -9 dBm (about 0.13 mW) of audio. Some meters take longer to respond to level changes, but settle down to a steady reading relatively rapidly, with little overshoot. Different meters respond to sudden level changes differently: Some change indication almost instantly, but greatly overshoot the proper indication, coming to rest only after a frustratingly long period of diminishing overshoot/ undershoot cycles. Connected to such an audio source, a meter keeps moving and never settles down to a steady reading.Ī meter's ballistics - how its mechanism acts when in motion - play a major role in how it indicates changing speech and music levels. Measuring speech-audio levels is comparatively tricky, however, because speech varies continuously in frequency and amplitude. You just hook your meter to the tone source, wait for its pointer to settle down at a constant indication, and take the reading. You can easily measure the level of a continuous audio tone with just about any meter capable of accurate indication if the signal's strength and frequency stay the same. This article fills that gap by discussing how to adjust a patch for speech with the help of a VU (volume unit) meter. Recently, as a fellow amateur and I installed and adjusted a telephone patch, we discovered that current Amateur Radio references do not describe a method for adjusting phone patches for proper telephone-line drive on speech signals. Here's how to use the right tool for this audio-measurement job. Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - Using a VU meter for phone-patch adjustmentįiddling with a patch's phone-line output until it "sounds okay" just isn't good enough. ![]()
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