Audio Terminology RECORDING
Bandwidth
The band of frequencies (difference between the lowest and the highest frequency in a signal) that pass through a device with a loss of less than 3dB, expressed in Hertz or in musical octaves. Described this way, the term is more or less synonymous to frequency range. This term is also used today to denote the maximum transfer capability of a digital connection or within a digital system. Used in this way, a computer network or Internet connection has a certain bandwidth, which denotes the maximum speed (rate) at which information can be transferred from one point to another or from input to output in a computer system/process.
Frequency Response
The measured accuracy within db limits of a piece of audio equipment. It states how much a piece of gear will respond in gain at precised frequencies. In fact, it doesn’t really make sense to use this specification when no indication of deviation in gain is precised with the frequency response since a piece of gear could respond to some frequencies but at a so low level in gain that it’s almost unnoticeable. The more frequencies are used during the measurement, the more the specification is precised. For instance, hi-fi loudspeaker manufacturers specify the tolerance limits (usually +/-3dB) of each model alongside the operating frequency range (typically 50Hz – 20kHz).
Phase Response
The phase response of a devise under test is quite similar to the frequency response except that instead of looking for change in gain of a signal at various frequencies, we are looking for change in phase. We measure it by comparing the input a devise under test to its output at various frequencies. The result of it is plotted on a graph similarly as the frequency response on a semi-log graph, with frequencies on the x-axis and phases on the y-axis.
Total Harmonic Distortion
This measurement specifies the total RMS value of the harmonic components of the output signal, excluding the fundamental, that is added to the audio signal by the equipment under given conditions. This measure is expressed as a percentage of the RMS value of the fundamental. The THD value is therefore quite a good measure on the unit’s quality. At low THD values – say at fractions of a percent, the THD added by the unit is not likely to be heard by any listener.
Total Harmonic Distortion + Noise
This measurement is quite similar to the total harmonic distortion specification except that this time, noise is being considered in the measurement. A single sine wave frequency of known harmonic purity is passed through the unit under test, and then patched back into the distortion measuring instrument. A measurement level is set; the instrument notches out the frequency used for the test (fundamental tone), and passes the result through a set of band-limiting filters adjusted for the bandwidth of interest (usually human threshold of hearing » 20Hz-20kHz). What remains is noise (generated by the devise under test, including AC line hum for example) and all harmonics generated by the unit. This composite signal is measured using a true RMS detector voltmeter, and the results displayed. Often a resultant curve is created by stepping through each frequency from 20 Hz to 20kHz, at some specified level (often +4 dBu), and bandwidth (usually 20 kHz; sometimes 80 kHz, which allows measurement of any 20 kHz early harmonics). THD+N is expressed as a percentage of the RMS value of the fundamental. THD+N is totally meaningless expressed by itself. To be meaningful, the frequency, level and measurement bandwidth must be specified.
Common Mode Rejection Ratio
The common-mode rejection ratio (CMRR) of a device (or other device) measures the tendency of the device to reject input signals common to both input leads. A high CMRR is important in applications where the signal of interest is represented by a small voltage fluctuation superimposed on a voltage offset, or when relevant information is contained in the voltage difference between two signals.
Ideally, a differential amplifier takes the voltages V + and V − on its two inputs and produces an output voltage Vo = Ad(V + − V − ), where Ad is the differential gain. However, the output of a real differential amplifier is better described as
where Acm is the common-mode gain, which is typically much smaller than the differential gain.
The CMRR is defined as the ratio of the powers of the differential gain over the common-mode gain, measured in positive decibels (thus using the 20 log rule):
As differential gain should exceed common-mode gain, this will be a positive number, and the higher the better.
The CMRR is a very important specification, as it indicates how much of the common-mode signal will appear in your measurement. The value of the CMRR often depends on signal frequency as well, and must be specified as a function thereof.
Gain
The measure of how much a circuit amplifies a signal. Gain may be stated as a ratio of input to output values, such as a voltage gain of 4, or a power gain of 1.5(expressed as a multiplier), or it can be expressed in decibels, such as a line amplifier with a gain of 10dB. An example of the former(as a multiplier) would be :
Input signal: 10kHz sine tone at 5mVrms
Output signal: 10kHz sine tone at 50mVrms
Gain (Out/In) = 50/5 = 10
An example of the latter (in dB) would be :
20 log (Out/In) = 20 log (50/5) = 20 log (10) = Gain of 20dB
Input Impedance
The input impedance is the combinaison of the resistance and of the reactance that a devise presents at its input, then acting as a load. This is optained by measuring the level of a sine tone (1kHz often used) generated by a function generator with a known output impedance, usually 600W . For example, if
Output Impedance
The output impedance of a devise is the combinaison of the resistance and of the reactance that it presents at its output to a load. To measure it we have first to send a signal of a specific frequency (let’s say 1kHz) to the devise under test and then get the level out of it. Let’s assume that the output is 2VRMS. If we do it again but this time adding a resistance of 500W across the output and then measure the level at the output. If the level is at 1VRMS, that means that the devise under test has an output impedance of 500W at 1kHz. The less the output drops we add the resistor, the lower the impedance. Output impedance are usually low, in the order of 50W .
Intermodulation Distortion
Intermodulation distortion (IMD) is the unwanted amplitude modulation of signals containing two or more different frequencies in a system with nonlinearities. The intermodulation between each frequency component will form additional signals at frequencies that are not, in general, at harmonic frequencies of either, but instead often at sum and difference frequencies of the original frequencies.
Slew Rate
This is a term used to define the maximum rate of change of an amplifier’s output voltage with respect to its input voltage. Slew rate is usually measured in V / msec. In essence, slew rate is a measure of an amplifier’s ability to follow its input signal. We measure it by applying a large amplitude step function (a signal starting at 0 volts and “instantaneously” jumping to some large level, creating a step-like look on an oscilloscope) to the amplifier under test and measuring the slope of the output waveform. For a very fast change at the input (often called “perfect” step input, i.e., one with a rise time at least 100 times faster than the amplifier under test), the output will not be vertical; it will exhibit a pronounced slope. The slope is caused by the amplifier having a finite amount of current available to charge and discharge its internal compensation capacitor.
Crosstalk
Crosstalk is a kind of interference caused by two signals becoming partially superimposed on each other due to electromagnetic (inductive) or electrostatic (capacitive) coupling between the conductors carrying the signals. A common example of crosstalk is when two cables(or wires) being close(parallel) together interfere with each other. In fact, what happens is that the magnetic field from changing current flow in one wire induces current in the other one. Crosstalk occurs a lot on magnetic tapes when elements on one track often tends to spread on an adjacent track. Crosstalk can be reduced by using shielded cables and increasing the distance between conductors. In stereo, the term relates to the breakthrough between channels measured as separation (in decibels) between the wanted sounds of the desired channel and the unwanted sounds from the second channel.
Clipping
Occurs when the input stage of an audio system is not capable to successfully handle the incoming signal due to the signal being too strong (too high in amplitude). Seen on an oscilloscope, the audio peaks appear clipped off. An analogue audio system has a certain capacity to handle signal overload by clipping in a way that is more acceptable to the human ear. An A/D converter in a digital audio system, however, has a maximum value it can handle. If the input signal exceeds this maximum value, the registered value will no longer be connected to reality , and the signal is therefore “clipped”. A clipped signal will almost always sound unnatural due to clipping. A clipped signal in a digital audio system is very likely to sound horrible, not just slightly distorted. To avoid distortion, reduce the system gain in or before the gain stage in which cheap cialis online the clipping occurs.
Gain Linearity
In theory, a devise under test should have the same gain on a signal irrespective of the level of the input. So, if we set a gain of 4 on a sine tone of 1kHz, then the gain should be of 4 at all the other amplitude levels of any sine tone of 1kHz. (It can change for other frequencies though). If it’s the case, then this devise under test is said to be have a linear gain response. This is usually not the case and in fact, the measurement is done at various amplitudes at its input. The results of these measurements can easily be graphed on an X-Y plot, where the input amplitude is represented on the x-axis and the deviation from the nominal gain is represented on the Y-axis.
Equivalent Input Noise
Let’s say you want to measure the input noise of a mic preamplifier. This noise level is usually too low to be measured accurately. The way to get it properly is by boosting it via the gain of the preamplifier and measure the noise from its output. Then the last operation would be to substract the gain from our noise measurement to calculate what is the gain at the input. The term “equivalent” relates to the fact that we are actually calculating it rather than measuring it. It typically ranges from –125 to –129.5dBm (» -120dBu).
Maximum Output Level
The maximum output level of a devise under test is the point when the signal gets either 0.1% or 1% distortion, (occuring before starting to clip) although this will change depending on the manufacturer. So it’s not really the maximum level before clipping. Usually, when a maximum output voltage is given, the percentage of distortion will be included to tell you what the company in question means by the word “maximum.” The maximum output of the device is independent of its gain. It’s a maximum level of the output signal.
Wow and Flutter
Measure of the deviation of frequency resulting from irregular motion(speed instability) in the recording or from deformation of the recording medium. It occurs a lot in turntables, tape recorders and cassette decks.
