Success Stories

The level of unwanted noise generated by the system itself, or by interference from external sources added to the signal. Hum usually refers to noise only at power line frequencies (as opposed to broadband white noise), which is introduced through induction of power line signals into the inputs of gain stages. Or from inadequately regulated power supplies.

Crosstalk

The introduction of noise (from another signal channel) caused by stray inductance or capacitance between components or lines. Crosstalk reduces, sometimes noticeably, separation between channels (eg, in a stereo system). It is given in dB relative to a nominal level of signal in the path receiving interference. Crosstalk is normally only a problem in equipment in which several channels are handled in the same chassis.

Common-mode rejection ratio (CMRR)

All electronic equipment with inputs is susceptible to this problem. In balanced audio systems, there are equal and opposite signals (difference-mode) in inputs, and any interference imposed on both leads will be subtracted, canceling out that interference (ie, the common-mode). CMRR is a measure of a system's ability to ignore any such interference and especially hum which arises at its input. It is generally only significant with long lines on an input, or when some kinds of ground loop problems exist. Unbalanced inputs do not have common mode resistance; induced noise on their inputs appears directly as noise or hum.

Dynamic range and Signal-to-noise ratio (SNR)

The difference between the maximum level a component can accommodate and the noise level it produces. Input noise is not counted in this measurement. It is measured in dB.

Dynamic range refers to the ratio of maximum to minimum loudness in a given signal source (eg, music or programme material), and this measurement also quantifies the maximum dynamic range an audio system can carry. This 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 at a specified (low) distortion level.

Signal-to-noise ratio (SNR), however, is the ratio between the noise floor and an arbitrary reference level or alignment level. In "professional" recording equipment, this reference level is usually +4 dBu (IEC 60268-17), though sometimes 0 dBu (UK and Europe - EBU standard Alignment level). 'Test level', 'measurement level' and 'line-up level' mean different things, often leading to confusion. In "consumer" equipment, no standard exists, though ?10 dBV and ?6 dBu are common.

Different media characteristically exhibit different amounts of noise and headroom. Though the values vary widely between units, a typical analogue cassette might give 60 dB, a CD almost 100 dB. Most modern quality amplifiers have >110 dB dynamic range, which approaches that of the human ear, usually taken as around 160 dB. See Programme levels.

Phase distortion, Group delay, and Phase delay

A perfect audio component will maintain the phase coherency of a signal over the full range of frequencies. Phase distortion can be extremely difficult to reduce or eliminate. The human ear is largely insensitive to phase distortion, though it is exquisitely sensitive to relative phase relationships within heard sounds. Multi-driver loudspeaker systems have complex phase distortions, caused by crossovers, by driver placement relative to other drivers, and by internal driver characteristics.

Transient distortion

A system may have low distortion for a steady-state signal, but not on sudden transients. This problem can be traced to amplifier power supplies in some instances, to insufficient high frequency performance in amplifiers, to negative feedback in amplifiers, or in loudspeakers to the mass and resonances of drivers and enclosures. Related measurements are slew rate and rise time. Transient distortion can be hard to measure. Many otherwise good power amplifier designs have been found to have inadequate slew rates, by modern standards. Most loudspeakers generate significant amounts of transient distortion, though some designs are less prone to this (e.g. electrostatic loudspeakers, plasma arc tweeters, ribbon tweeters).

Damping factor

A higher number is generally thought to be better. This is a measure of how well a power amplifier can control the undesired motion of a loudspeaker driver due largely to mechanical reactance. The amplifier must be able to damp out resonances caused by the mechanical motions (eg, inertia) of the moving parts of the speaker. For the common voice coil drivers, this essentially involves ensuring that the output impedance of the amplifier is close to zero. Damping factor is actually a relative way of specifying the output impedance of an amplifier with a particular load. It is affected by the cables used to connect the speakers to the amplifier, and by the amount of negative feedback especially in solid state amplifiers.

Digital

Note that digital systems do not suffer from many of these effects at a signal level, though the same processes occur in the circuitry, since the data being handled is symbolic. As long as the symbol survives the transfer between components, and can be perfectly regenerated (eg, by pulse shaping techniques) the data itself is perfectly maintained. The data is typically buffered in 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.

But digital systems have their own problems. Digitizing adds noise which is measurable, and which depends on the resolution ('number of bits") of the system, regardless of other quality issues. Clock timing errors (jitter) result in non-linear distortion of the signal. The quality measurement for a digital system centers on the probability of an error in transmission or reception. Otherwise the quality of the system is defined more by design intent (ie, specifications) than measurements, such as the sample rate and bit depth. In general, digital systems are much less prone to error than analog systems. However, nearly all digital systems contain analog inputs and/or outputs, and certainly all of those which interact with the analog world do so. These analog components of the digital system can suffer analog effects and potentially compromise the integrity of a well designed digital system.

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 8-bit, 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