digital audio introduction

8/14/2019 digital audio introduction

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An Introduction to Digital Audio

Digital audio has emerged because of itsusefulness in the recording, manipulation,mass-production and distribution of sound.

Distribution of audio as data files rather thanas physical objects has significantly reducedcosts of distribution.

In an analogue audio system, sounds begin asphysical waveforms in the air, are transformedinto an electrical representation of the

waveform, via a transducer (for example, amicrophone), and are stored or transmitted.

To be re-created into sound, the process isreversed, through amplification and thenconversion back into physical waveforms via aloudspeaker. Although its nature may change,

its fundamental wave-like characteristicsremain unchanged during its storage,transformation, duplication, amplification. Allanalogue audio signals are susceptible tonoise and distortion, due to the inherent noisepresent in electronic circuits.


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On the other hand, the digital audio chainbegins when an analogue audio signal isconverted into electrical signals on/off

pulses rather than electro-mechanicalsignals. This signal is then re-encoded (ratherlike a spy might use a code book), in order tocombat any errors that might occur in thestorage or transmission of the signal. It is this"channel coding" that is essential to the ability

of the digital system to recreate the analoguesignal upon replay. An example of a channelcode is Eight to Fourteen Bit Modulation asused in the audio Compact Disc.

Digital audio is the method of representingaudio in digital form.

An analog signal is converted to a digitalsignal at a given sampling rate and bitresolution; it may contain multiple channels (2channels forstereo or more forsurroundsound). Generally speaking: the higher thesampling rate and bit resolution the more

fidelity. Both systems introduce noise at thecapturing stage, in analogue recording this isdue to the noise floorof the circuit, and indigital recording due to quantization noise.

Much like an analog audio system, a digitalaudio system strives to reproduce the audio

perfectly but neither can ultimately prevail.
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Analog systems have inherent capacitanceand inductance which limit the bandwidth ofthe system and resistance limits the amplitude.

Digital systems' sampling rate limits thebandwidth and bit resolution limits the dynamicrange (resolution of amplitude creation). Bothsystems require increased cost and attentionto achieve higher fidelity.

A digital audio signal starts with an analog-to-digital converter(ADC) that converts ananalog signal to a digital signal. The ADC runsat a sampling rate and converts at a known bitresolution. For example, CD audio has asampling rate of 44.1 kHz (44,100 samples persecond) and 16-bit resolution for each channel

(stereo). If the analog signal is not alreadybandlimited then an anti-aliasing filterisnecessary before conversion, to preventaliasing in the digital signal. (Aliasing occurswhen frequencies above the Nyquistfrequency have not been band limited, andinstead appear as audible artifacts in the lowerfrequencies).

After being sampled with the ADC, the digitalsignal may then be altered in a process whichis called digital signal processing where it maybe filtered or have effects applied.
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The digital audio signal may then be stored ortransmitted. Digital audio storage can be on aCD, an iPod, a hard drive, USB flash drive,

CompactFlash, oranyother digital datastorage device. Audio data compressiontechniques such as MP3, Ogg Vorbis, orAAC are commonly employed to reduce thesize. Digital audio can be streamed to otherdevices.

The last step for digital audio is to beconverted back to an analog signal with adigital-to-analog converter(DAC). Like ADCs,DACs run at a specific sampling rate and bitresolution but through the processes ofoversampling, upsampling, and

downsampling, this sampling rate may not bethe same as the initial sampling rate.
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digital audio introduction