Editing Audio system measurements (section)

=== 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 (e.g., by [[pulse shaping]] techniques) the data itself is perfectly maintained. The data is typically buffered in a memory, and is [[Clock signal|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. Digitizing adds [[quantization noise|noise]], which is measurable and depends on the [[audio bit depth]] of the system, regardless of other quality issues. Timing errors in sampling clocks ([[jitter]]) result in non-linear distortion (FM modulation) of the signal. One quality measurement for a digital system (Bit Error Rate) relates to the probability of an error in transmission or reception. Other metrics on the quality of the system are defined by [[sample rate]] and [[Audio bit depth|bit depth]]. In general, digital systems are much less prone to error than analogue systems; However, nearly all digital systems have analogue inputs and/or outputs, and certainly all of those that interact with the analogue world do so. These analogue components of the digital system can suffer analogue effects and potentially compromise the integrity of a well designed digital system.

; [[Jitter]] : A measurement of the variation in period (periodic jitter) and absolute timing (random jitter) between measured clock timing versus an ideal clock. Less jitter is generally better for sampling systems.

; [[Sample rate]] : A specification of the rate at which measurements are taken of the analogue signal. This is measured in samples per second, or [[hertz]]. A higher sampling rate allows a greater total bandwidth or pass-band frequency response and allows less-steep anti-aliasing/anti-imaging filters to be used in the stop-band, which can in turn improve overall phase linearity in the pass-band.

; [[Audio bit depth|Bit depth]] : In [[Pulse-code modulation]] audio, the bit depth is the number of [[bit]]s of information in each [[Sampling (signal processing)|sample]]. [[Quantization (signal processing)|Quantization]], a process used in digital audio sampling, creates an error in the [[Signal reconstruction|reconstructed signal]]. The [[Signal-to-quantization-noise ratio]] is a multiple of the bit depth.

:[[Compact Disc Digital Audio|Audio CDs]] use a bit depth of 16-bits, while [[DVD-Video]] and [[Blu-ray]] discs can use 24-bit audio. The maximum [[dynamic range]] of a 16-bit system is about 96&nbsp;dB,<ref name="Middleton, Zak">{{cite book |url=https://books.google.com/books?id=dUw8oha56zwC&pg=PA54 |page=54 |last1=Middleton |first1=Chris |last2=Zuk |first2=Allen |title=The Complete Guide to Digital Audio: A Comprehensive Introduction to Digital Sound and Music-Making |publisher=Cengage Learning |year=2003 |isbn=978-1592001026 }}</ref> while for 24 bit it is about 144&nbsp;dB.

:[[Dither]] can be used in [[audio mastering]] to randomize the [[quantization error]], and some dither systems use [[Noise shaping]] to spectral shape of the quantization noise floor. The use of shaped dither can increase the effective dynamic range of 16-bit audio to around 120&nbsp;dB.<ref>http://xiph.org/~xiphmont/demo/neil-young.html {{Webarchive|url=https://web.archive.org/web/20150202095012/http://xiph.org/~xiphmont/demo/neil-young.html |date=2 February 2015 }} "With use of shaped dither ... the effective dynamic range of 16 bit audio reaches 120dB in practice"</ref>

:To calculate the maximum theoretical dynamic range of a digital system ([[Signal-to-quantization-noise ratio]] (SQNR)) use the following algorithm for bit depth Q:
:<math>\mathrm{SQNR} = 20 \log_{10}(2^Q) \approx 6.02 \cdot Q\ \mathrm{dB} \,\!</math>

:Example: A [[16-bit]] system has 2<sup>16</sup> different possibilities, from 0 – 65,535. The smallest signal without dithering is 1, so the number of different levels is one less, 2<sup>16</sup> − 1. 
:So for a 16-bit digital system, the Dynamic Range is 20·log(2<sup>16</sup> − 1) ≈ 96&nbsp;dB.

; Sample accuracy/synchronisation : 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, 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 or distortion 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 [[word clock]] synchronization.  It can also be corrected in the digital domain using a drift correction algorithm.  Such an algorithm compares the relative rates of two or more devices and drops or adds samples from the streams of any devices that drift too far from the master device.  Sample rate will also vary slightly over time, as crystals change in temperature, etc.  See also [[clock recovery]]

; 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.