How Recorder Work

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Digital Data

In a CD (and any other digital recording technology), the goal is to create a

recording with very high fidelity (very high similarity between the original

signal and the reproduced signal) and perfect reproduction (the recording

sounds the same every single time you play it no matter how many times you

play it).

To accomplish these two goals, digital recording converts the analog wave into a

stream of numbers and records the numbers instead of the wave. Theconversion is done by a device called an analog-to-digital converter (ADC).

To play back the music, the stream of numbers is converted back to an analog

wave by a digital-to-analog converter (DAC). The analog wave produced by

the DAC is amplified and fed to the speakers to produce the sound.

The analog wave produced by the DAC will be the same every time, as long as the numbers are not corrupted. The analog wave produced by

the DAC will also be very similar to the original analog wave if the analog-to-digital converter sampled at a high rate and produced accurate

numbers.

You can understand why CDs have such high fidelity if you understand the analog-to-digital conversion process better. Let's say you have a

sound wave, and you wish to sample it with an ADC. Here is a typical wave (assume here that each tick on the horizontal axis represents one-

thousandth of a second):

When you sample the wave with an analog-to-digital converter, you have control over two variables:

The sampling rate - Controls how many samples are taken per second

The sampling precision - Controls how many different gradations (quantization levels) are possible when taking the sample

In the following figure, let's assume that the sampling rate is 1,000 per second and the precision is 10:

The green rectangles represent samples. Every one-thousandth of a second,

the ADC looks at the wave and picks the closest number between 0 and 9. The

number chosen is shown along the bottom of the figure. These numbers are a

digital representation of the original wave. When the DAC recreates the wave

from these numbers, you get the blue line shown in the following figure:

You can see that the blue line lost quite a bit of the detail originally found in

the red line, and that means the fidelity of the reproduced wave is not very

good. This is the sampling error. You reduce sampling error by increasing

both the sampling rate and the precision. In the following figure, both the rate

and the precision have been improved by a factor of 2 (20 gradations at a rate

of 2,000 samples per second):

In the following figure, the rate and the precision have been doubled again

(40 gradations at 4,000 samples per second):

You can see that as the rate and precision increase, the fidelity (the similarity

between the original wave and the DAC's output) improves. In the case of CD

sound, fidelity is an important goal, so the sampling rate is 44,100 samples per

second and the number of gradations is 65,536. At this level, the output of the

DAC so closely matches the original waveform that the sound is essentially

"perfect" to most human ears.

Page 2 of 4HowStuffWorks "Digital Data"

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10 comments

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Reply Like January 16 at 7:09pm

Christopher Topolovich Works at Digital Generation (DG)

At what sampling rate does a phonograph's 33 1/3 rpm begin to break down?

Reply Like October 9, 2012 at 2:53pm

Delfin Fiorito St. Andrew's Scots School

I'm amazed with your knowdlege! This article is fasinating, it`s completely changed my way of thinking.Congratulations duuude

Reply Like April 13, 2011 at 5:59am2

Ignacio Smart St. Andrew's Scots School

Congratulations to the author! Very helpful indeed!

Reply Like April 13, 2011 at 5:57am

Ignacio Smart St. Andrew's Scots School

Very good article, I found very interesting!


Lucas Roldan 18 years old

Great article, Thank you very much!


Lucas Roldan 18 years old

fasinating


Ty Fouse Team Leader at Legion Of Tunez

Thank you!

Reply Like April 6, 2011 at 10:19pm

Santos Mclean San Andrs, Buenos Aires, Argentina

Great point, although I preffer aristoteles theory.


Nacho Tertzakian St. Andrew's Scots School

quite mediocre in fact.



08/03/2013http://electronics howstuffworks com/analog-digital3 htm

How Recorder Work

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