(This column originally appeared in the December 1977 issue of Contemporary Keyboard magazine.)
My last few columns have dealt with the properties of specific types of amplifiers and sound modifiers. In the next two columns, I'll conclude our rap on synthesizer accessories (for the time being) with a discussion of levels and impedance. It is my firmly held opinion that all musicians, not just synthesists, should place the same importance on understanding these terms that they place on mastery of a particular scale fingering. If you don't know what level and impedance have to do with stringing audio equipment together, you will get by, but only with some time wasting and some less-than-optimum sound.
Audio signals appear in electronic equipment as variations of voltage, or electrical force. The level of a signal is a measure of the extent of a voltage fluctuation. There are three commonly used ways of measuring signal level. Two ways are shown in Fig. 1.
RMS volts is a measure of the effective average fluctuation, while peak-to-peak (P-P) is a measure of the extremes within which the voltage varies. The level of the signal shown in the graph is 1 volt RMS, or 4 volts P-P. RMS stands for "root-mean-square," a mathematical procedure for finding the average power in a signal. The ratio between the P-P and RMS voltage of a signal depends upon the waveform. It ranges 2:1 for a square wave to more than 10:1 for waveforms with skinny spikes. The
P-P: RMS ratio for a sine wave is 2.83:1, in case you're interested. The ratio suddenly becomes important when you try to squeeze a loud percussive sound with a high
P-P: RMS ratio through a signal processing chain, without undue distortion.
Audio signal levels are also measured in units of dBm. The B stands for Bel, a unit of signal power ratio named after Alexander Graham Bell, the same cat that Ma Bell got her name from. Two Bels equals a voltage ratio of 10:1. The d stands for deci-, which means 1/10. Thus twenty decibels is also a voltage ratio of 10:1. The m refers to an engineering convention for a reference level that harks back to telephone technology. Put them all together and you have dee-bee-em. Zero dBm happens to be 0.778 volts RMS. Positive dBm values are higher; negative dBm values are lower. Since dBm differences are voltage ratios, the use of dBm units makes it easy to talk about the net gain of a signal-processing chain. If, for instance, the starting signal is 0dBm and goes through two processors, the first with a gain of -1dB and the second with a gain of +10dB, then the output of the second processor is +9dBm. Typical music signal levels in volts RMS and dBm are shown below.
| Signal Source
|| RMS Volts
| Broadcast Studio Tape Recorder
|| + 8
|| + 2
| Fender Telecaster
|| - 18
| Lead Singer Microphone
|| - 38
Of course, the actual output level of a device depends upon how loud the sound is supposed to be.
Now let's have a look at the published level specifications of some typical signal modifiers to see what use we can make of the information.
Maestro MP-1 Phaser
Maximum Recommended Input Level: -10dBm
Insertion Loss: 0dBm
Mu-tron Phasor II
Signal Handling Capability
Minimum Feedback: 4V RMS, 11.2V P-P
Maximum Feedback: 2V RMS, 5.6V P-P
Gain: Unity (less than 1dB insertion loss)
Moog Three-Band Parametric Equalizer
Nominal Input Level: +4dBm
Maximum Input Level Before Clipping
Below 3kHz: +20dBm
Above 3kHz: Clipping level decreases at 6dB/oct due to input pre-emphasis
Maximum Output Level: +20dBm with THD* of less than 2%
Insertion Gain: Adjustable from -10 to +10dB
*Total Harmonic Distortion
The Maestro MP-1 Phaser is a battery-powered floor modifier, the Mu-tron Phasor II is AC line powered, and the Moog Three-Band Parametric Equalizer is a rack-mounted keyboard modifier. All three sets of specs tell what the allowable maximum signal level at the device's input is, above which the input circuitry may distort the waveform. The Maestro Phaser spec is called "Maximum Recommended Input Level" and is given in dBm. The Mu-tron Phasor spec is called "Signal Handling Capability," and is given in volts RMS and volts P-P for two different settings of the device's feedback control. The Parametric Equalizer spec is called "Maximum Input Level Before Clipping." The spec explains that the maximum allowable signal level goes down as the signal's frequency goes up. Many instrument designers deliberately incorporate this characteristic to improve noise performance.
When you compare these specs, you see that the maximum allowable signal levels of sound modifier accessories vary quite a bit from instrument to instrument, and that even for one particular instrument, the maximum level may depend on how the instrument is used. When you connect a given modifier to a signal source, you will want to adjust the source signal level low enough that it never exceeds the modifier's signal handling capability, but not so low that the modifier's internal noise becomes an appreciable part of the signal going through it. For instance, let's assume that the rated output level of your synthesizer is -4dBm (0.5V RMS). If you were to connect it to a Maestro Phaser, you would want to keep the output volume control turned down a bit to keep the signal from going above -10dbm and overloading the Phaser. Since the Mu-tron Phasor has a signal handling capability of +8dBm, your synthesizer would never overload it. Many synthesizers have an Output Level Adjustment pot. This control is usually at the rear of the instrument. You set it so that the maximum output level of which the synthesizer is capable never exceeds the signal-handling capability of the device, which the synthesizer output feeds into. You then use the loudness (or volume) control on the front panel during performance.
The gain of a modifier is how much the signal level is boosted or cut as it goes through the device. A modifier that does not change the level of a typical audio signal as it goes through it has unity, or 0dB, gain. The term insertion loss is also used. If the insertion loss is a positive number, then the signal level will be reduced. Phasers generally do not have gain controls because the phasing effect does not change the perceived loudness. However, a parametric equalizer may boost or cut a large portion of the signal spectrum. To compensate for this, the Moog Parametric Equalizer has a separate Gain control to enable the user to adjust the net instrument gain to unity.
Next month's column will discuss the specs related to impedance and dynamic range.