| 词条 | Digital-to-analog converter |
| 释义 |
In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function. There are several DAC architectures; the suitability of a DAC for a particular application is determined by figures of merit including: resolution, maximum sampling frequency and others. Digital-to-analog conversion can degrade a signal, so a DAC should be specified that has insignificant errors in terms of the application. DACs are commonly used in music players to convert digital data streams into analog audio signals. They are also used in televisions and mobile phones to convert digital video data into analog video signals which connect to the screen drivers to display monochrome or color images. These two applications use DACs at opposite ends of the frequency/resolution trade-off. The audio DAC is a low-frequency, high-resolution type while the video DAC is a high-frequency low- to medium-resolution type. Due to the complexity and the need for precisely matched components, all but the most specialized DACs are implemented as integrated circuits (ICs). Discrete DACs would typically be extremely high speed low resolution power hungry types, as used in military radar systems. Very high speed test equipment, especially sampling oscilloscopes, may also use discrete DACs. OverviewA DAC converts an abstract finite-precision number (usually a fixed-point binary number) into a physical quantity (e.g., a voltage or a pressure). In particular, DACs are often used to convert finite-precision time series data to a continually varying physical signal. An ideal DAC converts the abstract numbers into a conceptual sequence of impulses that are then processed by a reconstruction filter using some form of interpolation to fill in data between the impulses. A conventional practical DAC converts the numbers into a piecewise constant function made up of a sequence of rectangular functions that is modeled with the zero-order hold. Other DAC methods (such as those based on delta-sigma modulation) produce a pulse-density modulated output that can be similarly filtered to produce a smoothly varying signal. As per the Nyquist–Shannon sampling theorem, a DAC can reconstruct the original signal from the sampled data provided that its bandwidth meets certain requirements (e.g., a baseband signal with bandwidth less than the Nyquist frequency). Digital sampling introduces quantization error that manifests as low-level noise in the reconstructed signal. ApplicationsDACs and ADCs are part of an enabling technology that has contributed greatly to the digital revolution. To illustrate, consider a typical long-distance telephone call. The caller's voice is converted into an analog electrical signal by a microphone, then the analog signal is converted to a digital stream by an ADC. The digital stream is then divided into network packets where it may be sent along with other digital data, not necessarily audio. The packets are then received at the destination, but each packet may take a completely different route and may not even arrive at the destination in the correct time order. The digital voice data is then extracted from the packets and assembled into a digital data stream. A DAC converts this back into an analog electrical signal, which drives an audio amplifier, which in turn drives a loudspeaker, which finally produces sound. AudioMost modern audio signals are stored in digital form (for example MP3s and CDs) and, in order to be heard through speakers, they must be converted into an analog signal. DACs are therefore found in CD players, digital music players, and PC sound cards. Specialist standalone DACs can also be found in high-end hi-fi systems. These normally take the digital output of a compatible CD player or dedicated transport (which is basically a CD player with no internal DAC) and convert the signal into an analog line-level output that can then be fed into an amplifier to drive speakers. Similar digital-to-analog converters can be found in digital speakers such as USB speakers, and in sound cards. In voice over IP applications, the source must first be digitized for transmission, so it undergoes conversion via an ADC, and is then reconstructed into analog using a DAC on the receiving party's end. VideoVideo sampling tends to work on a completely different scale altogether thanks to the highly nonlinear response both of cathode ray tubes (for which the vast majority of digital video foundation work was targeted) and the human eye, using a "gamma curve" to provide an appearance of evenly distributed brightness steps across the display's full dynamic range - hence the need to use RAMDACs in computer video applications with deep enough colour resolution to make engineering a hardcoded value into the DAC for each output level of each channel impractical (e.g. an Atari ST or Sega Genesis would require 24 such values; a 24-bit video card would need 768...). Given this inherent distortion, it is not unusual for a television or video projector to truthfully claim a linear contrast ratio (difference between darkest and brightest output levels) of 1000:1 or greater, equivalent to 10 bits of audio precision even though it may only accept signals with 8-bit precision and use an LCD panel that only represents 6 or 7 bits per channel. Video signals from a digital source, such as a computer, must be converted to analog form if they are to be displayed on an analog monitor. As of 2007, analog inputs were more commonly used than digital, but this changed as flat panel displays with DVI and/or HDMI connections became more widespread.{{Citation needed|date=October 2011}} A video DAC is, however, incorporated in any digital video player with analog outputs. The DAC is usually integrated with some memory (RAM), which contains conversion tables for gamma correction, contrast and brightness, to make a device called a RAMDAC. A device that is distantly related to the DAC is the digitally controlled potentiometer, used to control an analog signal digitally. MechanicalA one-bit mechanical actuator assumes two positions: one when on, another when off. The motion of several one-bit actuators can be combined and weighted with a whiffletree mechanism to produce finer steps. The IBM Selectric typewriter uses such a system.[1] TypesThe most common types of electronic DACs are:[2]
PerformanceDACs are very important to system performance. The most important characteristics of these devices are:{{cn|date=August 2016}}
Other measurements, such as phase distortion and jitter, can also be very important for some applications, some of which (e.g. wireless data transmission, composite video) may even rely on accurate production of phase-adjusted signals. Linear PCM audio sampling usually works on the basis of each bit of resolution being equivalent to 6 decibels of amplitude (a 2x increase in volume or precision). Non-linear PCM encodings (A-law / μ-law, ADPCM, NICAM) attempt to improve their effective dynamic ranges by a variety of methods - logarithmic step sizes between the output signal strengths represented by each data bit (trading greater quantisation distortion of loud signals for better performance of quiet signals) Figures of merit
See also
References1. ^{{cite web |author=Brian Brumfield |url=https://www.youtube.com/watch?v=uTjxwVfx8GA |title=Selectric Repair 10-3A Input: Keyboard |date=Sep 2, 2014 |via=YouTube |df=ymd-all}} 2. ^{{cite web |url=http://www.analog.com/media/en/training-seminars/design-handbooks/Data-Conversion-Handbook/Chapter3.pdf |title=Data Converter Architectures |work=Analog-Digital Conversion |publisher=Analog Devices |accessdate=2017-08-30 |df=ymd-all |ref={{sfnref|"Data Converter Architectures"}}|deadurl=no |archiveurl=https://web.archive.org/web/20170830032212/http://www.analog.com/media/en/training-seminars/design-handbooks/Data-Conversion-Handbook/Chapter3.pdf |archivedate=2017-08-30}} 3. ^{{Cite web |url=https://www.electronics-tutorial.net/analog-integrated-circuits/data-converters/binary-weighted-resistor-dac/index.html |title=Binary Weighted Resistor DAC |website=Electronics Tutorial |language=en-US |access-date=2018-09-25 |df=ymd-all}} 4. ^{{cite web |url=http://www.analog.com/static/imported-files/overviews/AnalogMultiplyingDACs.pdf |title=Multiplying DACs: Flexible Building Blocks |format=PDF |year=2010 |publisher=Analog Devices |accessdate=29 March 2012 |df=ymd-all}} 5. ^{{cite web |url=http://www.maxim-ic.com/appnotes.cfm/appnote_number/641/ |title=ADC and DAC Glossary |publisher=Maxim |df=ymd-all |dead-url=no |archive-url=https://web.archive.org/web/20070308223113/http://www.maxim-ic.com/appnotes.cfm/appnote_number/641 |archive-date=2007-03-08}} Further reading{{Refbegin}}
| title = The Data Conversion Handbook | last = Kester | first = Walt | isbn = 0-7506-7841-0 | url = http://www.analog.com/library/analogDialogue/archives/39-06/data_conversion_handbook.html }}
External links
2 : Digital signal processing|Electronic circuits |
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