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Fiddling with a patch's phone-line output until it "sounds okay" just isn't good enough. Here's how to use the right tool for this audio-measurement job.
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. This article fills that gap by discussing how to adjust a patch for speech with the help of a VU (volume unit) meter.(1)
VU Meter Basics
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. You just hook your meter to the tone source, wait for its pointer to settle down at a constant indication, and take the reading.
Measuring speech-audio levels is comparatively tricky, however, because speech varies continuously in frequency and amplitude. Connected to such an audio source, a meter keeps moving and never settles down to a steady reading.
A meter's ballistics - how its mechanism acts when in motion - play a major role in how it indicates changing speech and music levels. 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. Some meters take longer to respond to level changes, but settle down to a steady reading relatively rapidly, with little overshoot.
Telephone company specifications call for driving a phone line with no more than -9 dBm (about 0.13 mW) of audio. 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. That's where the VU meter comes in.
First 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. 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.
This was not easy to do. 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. 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.
You've probably seen audio level meters marked VU in consumer audio, recording and public-address equipment. But not every audio meter marked VU meets the specifications for a true VU meter.(2)
VU Meter Characteristics
A true VU meter consists of a 200-µA, 3.9 kΩ-coil d' Arsonval movement equipped with a copper-oxide full-wave bridge rectifier. (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). 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.
Figure 1 - Both VU-meter scales span the same range of signal-strength change. The A scale is of most use for audio-level adjustments because it emphasizes its decibel calibration. The B scale emphasizes percentage of transmitter modulation. 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).
VU Meter Scales
Because 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. Both scales (see Figure 1) cover the same span of signal-strength change. The meter's 0 to 100 scale indicates percentage of transmitter modulation, its -20 to +3 dB scale indicates decibels relative to 0 VU (0 dB). (One volume unit equals a 1-dB level change in a complex waveform.)
The A scale emphasizes the -20 to +3 dB calibration and is preferred for recording and test-equipment applications; the B scale, which emphasizes the 0 to 100% calibration, is preferred for broadcast applications. Other than this difference in appearance, A- and B-scale VU meters are electrically and ballistically identical and can be used interchangeably.
Sensitivity
Connected in series with a 3.6-kΩ resistor, a VU meter indicates 0 VU when driven by a sinusoidal, 1-kHz, 1.228-V RMS (+4 dBm) signal. Without its 3.6 kΩ series resistor, the basic VU meter movement indicates 0 VU for a standard power level of 1 mW in 600 Ω (0.775 V RMS across 600 Ω).
With the series resistor in place (which brings the instrument's total resistance up to the 7.5-kΩ value called for by the VU meter standard), the meter indicates -4 dB with 1.228 V RMS applied. This 4-dB reduction is of no consequence if we remember to mentally add 4 dB to the pointer indication, and is a satisfactory arrangement for measuring speech levels in the 0 to +4 dBm range.
Ballistics
When a steady 1-kHz sine wave is suddenly applied to a VU meter, it indicates the signal's true value to 99% within 0.3 second and overswings the true value by not more than 1.5%. The required outboard 3.6 kΩ resistor plays an important part in maintaining these ballistics: The meter responds more slowly if less external resistance is used.
Making a VU Meter Cover the Range We Want
The level to which a speech circuit should be adjusted depends on two conflicting requirements. The level should be high enough to provide an acceptable signal-to-noise ratio at the receiving end, but not more than the telephone system allows. This means that the signal should be substantially stronger than line hum and noise, but not high enough to cause crosstalk or overload. The line level of -9 dBm, maximum, complies with these requirements.
If we want our meter to indicate 0 VU when connected to a line operating at -9 dBm, amplification is necessary. Without amplification, the meter will indicate -13 dBm, equal to -9 dBm plus the 4-dB loss contributed by the meter's outboard 3.6 kΩ resistor. (Yes, a VU meter's scale already covers -13 dBm, but in a part of the scale where resolution is poor.) We therefore need 13 dB of amplification to make our meter indicate 0 VU at our desired signal level of -9 dBm. Figure 2 shows a circuit that can do this.
Figure 2 - How to wire a general-purpose op amp to provide the 13 dB of gain necessary for a VU-meter indication of 0 VU when a -9 dBm signal is present. (See text.) M1 is a standard (3.9-KΩ coil, 200-µA full scale) VU meter. There is nothing critical about constructing this circuit; the op amp can be a 741 or 1/4 of a 324, for instance. Set the OFFSET ADJ trimmer so the meter reads zero with no signal present.
So far, I've talked about a VU meter with a 3.6 kΩ series resistor, but the 3.6 kΩ value assumes that the meter is bridged across a 600 Ω line terminated with a 600 Ω load - 300 Ω with both values paralleled - to ultimately load the meter with 3.6 kΩ + 300 Ω, or 3.9 kΩ. Such is not the case in Figure 2 because the output impedance of an operational amplifier is a few tens of ohms at most. The meter series resistor in Figure 2, therefore provides all of the 3.9 kΩ resistance necessary to ensure the meter's proper ballistic response.
In addition to providing 13 dB of gain, the preamplifier circuit must lightly load both sides of the telephone line, neither of which is grounded. This is accomplishedby taking advantage of the op amp's differential input feature. Sharp-eyed readers will note that the resistive loading on the lines is unequal, but this is of no consequence because both resistances are so much higher than 600 Ω that their difference contributes insignificant imbalance.
Connect the meter amplifier's input to the phone line at your phone patch with your phone off the hook or while the patch is disconnected from the phone line. Treat the phone-line wires as if they carry high voltage which they do, especially when ringing signals arrive (nearly 100 V RMS at the customer end of a phone line).
Using the Meter
Now comes the most difficult part: learning how to read and interpret the meter indication. The correct way to read the meter is to note the average indication of the three highest peaks during a 10-second period while disregarding occasional extreme peaks. This is not as formidable as it sounds. Just recite the following two sentences twice in succession in a normal speaking voice while observing the meter pointer's peak swings: "Joe took father's shoe bench out" and "She was waiting at my lawn." These two sentences contain all of the fundamental sounds of the English language and, when spoken twice, take about 10 seconds to say. (Try it!)
Once you're sure of how to read your VU meter, you can readily adjust your patch's output for -9 dBm of phone-line drive. That's all there is to it. Your phone patch is now optimally adjusted.
Notes
- Pages 28-13 through 28-16 of the 1993 ARRL Handbook describe how to use an audio tone in calibrating a nonstandard VU meter to indicate 0 dBm at 600 Ω. (For suggestions on how to modify the response of nonstandard VU meters that respond too quickly to level changes, see G. Schleicher, "Measuring phone-patch levels accurately," QST, Feb 1972, pp 24-26.) The Handbooks phone-patch coverage, which I recommend as an overview of phone-patch and phone-line interface topics, includes a statement that applies here: "You are advised to obtain a copy of FCC Part 68 from the Government Printing Office, and read Sub-Part D before connecting any home-made device to the telephone line."
- ANSI/IEEE 152-1953, abbreviated title: Volume Measurements of Electrical Speech and Program Waves.
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