Audio system measurements

From Academic Kids

The designer of a system for sound reproduction needs to be able to measure the system's performance in a number of areas. While this is not a measure of quality but of quantity, it is a fact that certain quantifiable measurements must be of a certain order to assure quality. That said, it is possible to design systems that sound terrible nevertheless — in other words, a quality system will attain certain measurements, but the existence of those measurements does not guarantee quality.

Contents

Measurable performance

Analog Electrical

Frequency response 
The signal should be passed at least over the audible range (usually quoted as 20 Hz to 20 kHz) with no significant peaks or troughs. The human ear can discern differences in level of about 3 dB, so peaks and troughs must be less than this. Modern equipment is capable of less than 1 dB variation over the quoted frequency range.
Total harmonic distortion (THD) 
For high fidelity, this is usually expected to be < 1% for electronic devices; mechanical elements such as loudspeakers usually have higher levels. Nowadays very low distortion is easy to achieve in electronics with use of negative feedback, but the use of heavy feedback in this manner has been the topic of much controversy among audiophiles - for more on this see electronic amplifier. Loudspeakers typically produce more distortion than electronics, and 1-5% distortion is not unheard of at moderately loud listening levels. Human ears are less sensitive to distortion in the bass frequencies, and levels are usually expected to be under 10% at loud playback. Even order distortion is usually less bothersome than odd order distortion.
Output power 
A genuine measurement quotes the root mean square power output per channel, which is a true value of the power output. Other measurements such as PMPO are meaningless and generally used in disingenuous marketing literature. The quoted power is usually the steady-state power into the rated load of the system.
Intermodulation distortion (IMD) 
A measure of the spurious signals resulting from unwanted multiplication of different input signals. This effect is contributed by non-linearities in the system. Again, heavy negative feedback can tame this effect, but many believe it is better to design to minimise it arising in the first place.
Noise 
The level of unwanted noise generated by the system itself, or by interference from external sources. Hum usually refers to noise only at power line frequencies (as opposed to broadband white noise), which is introduced through interference or inadequately regulated power supplies.
Crosstalk 
Caused by stray inductances or capacitances between components or lines, crosstalk results in things such as unintentional mixing of stereo signals or mixer channels. This is given in dB relative to a nominal level of signal in the path receiving interference.
Common-mode rejection ratio (CMRR) 
In balanced audio systems, equal and opposite signals (difference-mode) are used, which are subtracted, canceling out interference which affects both signals equally (common-mode). It is important to minimize the amount of common-mode signal which is passed through the system. This is usually measured in dB relative to a nominal difference-mode signal.
Dynamic range and Signal-to-noise ratio (SNR) 
A measurement of the range of signal levels the device is capable of. Dynamic range is the ratio (usually expressed in dB) between the noise floor of the device with no signal, and the maximum signal (usually a sine wave) that can be output without distortion. SNR, however, is the ratio between the noise floor and an arbitrary reference level. In "professional" equipment, this reference level is often +4 dBu, in "consumer-grade" equipment this level is −10 dBv. Different media exhibit different orders here - analogue cassette might give 60 dB, a CD almost 100. Nowadays most amplifiers have >110 dB dynamic range, which approaches that of the human ear, 160 dB.
Phase distortion, Group delay, and Phase delay 
A good system will maintain the phase coherency of a signal over the full range of frequencies. Phase distortion can be extremely difficult to reduce and eliminate. The human ear is actually largely insensitive to phase distortion, and so for many this figure lacks importance; however, there are always those who will argue the opposite.
Transient distortion 
A system may have low distortion for a steady-state signal, but distort sudden transients. This is often due to a lack of power delivery fast enough to supply the system during the transient. Related measurements are slew rate and rise time. Transient distortion can be hard to measure. Many otherwise good power amplifier designs have been let down by having an inadequately responsive power supply. Most typical loudspeakers generate significant amounts of transient distortion, though some exotic designs are less prone to this (e.g. electrostatic loudspeakers and plasma arc loudspeakers).
Damping factor 
A higher number is better. This is a measure of how well a power amplifier can control the reactive load of a loudspeaker. The amplifier must be able to damp out resonances caused by the mechanical inertia of the moving parts of the speaker. Essentially this involves ensuring that the output impedance of the amplifier is as close to zero as it can be made. Damping factor is actually just a different way of specifying the output impedance of an amplifier. It is significantly affected by the cables used to connect the speakers to the amplifier, as poor quality cables can have a large resistance compared to a typical amplifier output.

Mechanical

Wow and flutter 
This pertains to the drive mechanism of analogue media, such as vinyl records and magnetic tape. "Wow" is slow speed variations, caused by longer term drift of the drive motor speed, whereas "flutter" is faster speed variations, usually caused by mechanical defects such as out-of-roundness of the capstan of a tape transport mechanism. A lower number is better.
Rumble 
The measure of the low frequency noise contributed by the turntable of an analogue playback system. A lower number is better.

Digital

Note that digital systems do not suffer from many of these effects, even though the same processes occur in the circuitry, since the data being sent is symbolic. As long as the symbol survives the transfer between components, the data itself is perfectly maintained. The data is buffered by a memory, and is clocked out by a very precise crystal oscillator. The data usually does not degenerate as it passes through many stages, because each stage regenerates new symbols for transmission.

Digital systems have their own problems, however. Digitizing adds quantization noise (random data) which is measurable, depending on the resolution of the system. Clock timing errors (jitter) result in non-linear distortion of the signal. The quality measurement for a digital system basically revolves around the probability of an error in transmission. Otherwise the quality of the system is defined more by specifications than measurements, such as the sample rate and bit depth. In general, digital systems are much less prone to error than analog systems. The analog systems invariably at the inputs and outputs of the digital system can suffer analog effects, however.

Jitter 
A measurement of the variation in period between clock cycles, which should theoretically be exactly the same period. Less jitter is better.
Sample rate 
A specification of the rate at which measurements are taken of the analog signal. This is measured in samples per second, or hertz. A higher sampling rate allows a greater total bandwidth or flatband frequency response. It can also reduce the effects of jitter.
Bit depth 
A specification of the accuracy of each measurement. For example, a 3-bit system would be able to measure 23 = 8 different levels, so it would round the actual level at each point to the nearest representable. Typical values for audio are 16-bit, 24-bit, and 32-bit. The bit depth determines the theoretical maximum signal-to-noise ratio or dynamic range for the system. It is common for devices to create more noise than the minimum possible noise floor, however. Sometimes this is done intentionally; dither noise is added to decrease the negative effects of quantization noise by converting it into a higher level of uncorrelated noise.
To calculate the maximum theoretical dynamic range of a digital system, find the total number of levels in the system. Dynamic Range = 20*log(# of different levels). Note: the log function has a base of 10. Example: An 8-bit system has 256 different possibilities, from 0 - 255. The smallest signal is 1 and the largest is 255. Dynamic Range = 20*log(255) = 48 dB.
Sample accuracy/synchronization 
Not as much a specification as an ability. Since independent digital audio devices are each run by their own crystal oscillator, and no two crystals are exactly the same, the sample rate will be slightly different. This will cause the devices to drift apart over time. The effects of this can vary. If one digital device is used to monitor another digital device, this will cause dropouts in the audio, as one device will be producing more or less data than the other per unit time. If two independent devices record at the same time, one will lag the other more and more over time. This effect can be circumvented with a wordclock synchronization.
Linearity 
Differential non-linearity and integral non-linearity are two measurements of the accuracy of an analog-to-digital converter. Basically, they measure how close the threshold levels for each bit are to the theoretical equally-spaced levels.

See also

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