Moog Monday - On Synthesizers: Specmanship, Part II: Impedance & Dynamic Range

August 9, 2016

(This column originally appeared in the January 1978 issue of Contemporary Keyboard magazine.)


Impedance
When an audio signal is applied to an input, a current (actual movement of electrons in a wire) flows in via one conductor of the cable and out via the other conductor. The stronger the signal (voltage), the greater the current. The impedance of an input is a measure of the electrical voltage across the input that is required to result in the flow of a standard amount of current. The unit of impedance is the ohm (abbreviated Ω). A one-ohm impedance is one in which a current of one ampere flows when a voltage of one volt is applied across it. For audio inputs, one ohm is extremely low impedance. One volt is a typical signal level, but one ampere is a monster current for signal sources to provide. A 100-watt light bulb draws about one ampere from the power line. Audio signal currents are typically measured in microamperes (millionths of an ampere). Inputs of sound modifiers are typically 10,000 ohms (abbreviated 10k) to 1,000,000 ohms (abbreviated 1 meg). An input of 10k impedance draws 100 microamperes from a one-volt signal; an input of 1 meg impedance draws only 1 microampere from the same one-volt signal. What is important is that the impedance of an input is a property of the input itself. It does not depend upon the strength of the signal. It may depend somewhat upon the frequency of the signal, but you rarely see any mention of this in spec sheets.

Signal sources (such as microphones and pickups) and signal outputs also have impedances associated with them. The impedance of an output is a measure of how much the level of the output's signal drops when a standard current is drawn from it. Thus, if the output impedance of a signal source is much less than the impedance of the input, which it feeds, only a small amount of the signal is lost. If, on the other hand, the output impedance of the source equals the impedance of the input, which it feeds, then half the signal is lost. This fact gives us our first rule about impedances: When hooking the output of one device to the input of another, be sure that the output has a much lower impedance than the input. Otherwise you will weaken the signal. You may also degrade the frequency, because the output impedance may be frequency-sensitive. This is especially true if the signal source is a piezoelectric (ceramic) transducer or a magnetic pickup. As a general rule, an input impedance of less than, say, 500k will attenuate the low frequencies coming from a ceramic transducer, or the high frequencies coming from a magnetic pickup.

How about "impedance matching"? Many musicians have heard that a signal source with a certain impedance must be fed to an input of equal impedance. This rule harks back to the early days of the telephone, before there were such things as electronics or amplifiers. The rule still applies to sources that produce very weak signals that have to travel through long cables—microphones, for instance. Certain pieces of professional studio equipment are deliberately set up with equal input and output impedances (usually 600 ohms) so they are compatible with studio cable networks. However, most electronic equipment used by musicians has high-impedance inputs and fairly low-impedance outputs. Spec sheets will list both.

How does impedance affect your ability to connect more than one input to a signal source, or to connect more than one output together to combine the signals? In the case of connecting more than one input to a signal source, you must realize that the combined impedance of more than one input connected in parallel (as with a Y cord) is less than that of any one input by itself. Two 500k-impedance inputs will have a combined impedance of 250k; five such inputs in parallel will have a combined impedance of 100k, and so on. As a general rule, however, you can connect several inputs together and feed them from the same signal source if the value of the lowest impedance of the inputs is more than ten times larger than the output impedance of the signal source.

Connecting outputs together to mix their signals is a different story. The low impedance of one output soaks up ("shunts") signals from other outputs. The best of this bad situation is when the impedances of all outputs are equal. The level of the signal from one output is actually divided by the number of other outputs that it is connected to. If the impedance of one output is much lower than those of the other outputs, it will soak up even more of the other signals. So, rule No. 3: Avoid connecting outputs in parallel. Use a mixer to combine signals.

Generally, low-impedance outputs are better than medium- or high-impedance outputs. They are less susceptible to pickup of extraneous noises. More specifically, however, they allow you to use longer cables. Cables themselves draw more current from high-frequency signals than from low-frequency signals. If, for example, the output impedance of your source is 100k (not unusual in older vacuum tube stuff, or in low-power battery-operated modifiers), then a cable over five feet long will degrade the high-frequency response of your signal! If, on the other hand, the output impedance of your source is 1k or less (standard in professional audio equipment), your cable can run the length of a football field without high-frequency degradation. Our final rule regarding impedance is: Long cables should be connected only to outputs whose impedances are well under 10k.

Dynamic Range
The dynamic range of a signal modifier is the ratio between the strongest signal that the device will handle without audible distortion, and the noise that the device produces in the absence of a signal. Dynamic range is measured in decibels. Our ears have a dynamic range of about 120dB. The top of our hearing range is called the threshold of pain; the bottom is called the threshold of hearing. Of course, we can rarely make use of the entire range of human hearing at any one time. For instance, the high sound level of a performing group "masks" (prevents us from hearing) soft sounds. Noise levels 60dB below the audio signals in a typical performance are barely audible; noise levels 80dB below the desired sound material don't even get through the normal tape-recording process in making an album. Thus, musicians should be sure the dynamic range of their audio chain is at least 60dB. Twenty or thirty dB more will give the cleanliness and clarity of a good studio performance.

The dynamic range of a processing chain is always less than the dynamic ranges of the individual instruments. For instance, a chain of three modifiers, each having a dynamic range of 70dB, will have a composite dynamic range of 60dB if all levels are adjusted so the devices overload together. If careful adjustment of levels and gains is not made, the dynamic range of such a chain will be even less than 60dB. Systems with an overall dynamic range of less than 60dB produce a constantly audible hiss that clouds the sound and detracts from its appeal.

Dynamic range is rated in spec sheets in one of several different ways. The most common is "signal-to-noise ratio," a term which is synonymous with "dynamic range." For the reasons mentioned above, devices with a signal-to-noise ratio of less than 70dB should be avoided.

A second way of rating dynamic range is in terms of "equivalent input noise." Equivalent input noise is a fictitious quantity. It is the noise signal that you would need at the input of a signal processor to produce the observed noise at the output, if the internal circuitry of the processor itself were perfectly noise-free. Here is how equivalent input noise spec is shown on the Systech Phase Shifter spec sheet:


Maximum Recommended Input Level: 0dBm (Clip +10)

Equivalent Input Noise: -90dBm

This tells us that the maximum dynamic range is 100dB, and that, in a system where a 0dB signal level is not exceeded, the dynamic range is 90dB. This type of spec only indirectly tells you what the dynamic range is, but it supplies the explicit information necessary for adjusting levels within a signal processing chain.

A third, less common measure of dynamic range is "noise figure." The noise figure is the increase in equivalent input noise over the theoretical minimum. Thus, noise figure is a measure of how close to ideal a device is. A noise figure of 3dB means that the device produces only 40% more noise than the theoretical minimum.

 

For more articles by Bob Moog, please visit http://www.keyboardmag.com/analog/1318/moog-monday-hub-page/57761
 
 
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