Close up of a keyboard.

CD kbps

Image of an audio CD.How many kbps (kilobits per second) is a CD? The answer is 1411. For a non-technical explanation of where this figure comes from read on.

In order to understand the link between a CD and its 1411kbps bitrate it’s necessary to have a little background information.

When you listen to a CD you’re hearing a digital representation of the recorded sound. In order to digitize sound you have to sample it by taking snapshots of the audio signal. The more snapshots you take the better the representation. It’s a bit like digital cameras and pixels, the more pixels you have the clearer your image will be.

To create a good digital representation of the audio signal it’s necessary to sample it at a rate twice as high as the frequency you want to reproduce. Since the frequency that a healthy young adult can hear extends up to 20,000Hz, the sampling rate must be twice this, i.e. 40,000Hz. For a mixture of technical reasons and convenience a sampling frequency of 44,100Hz was chosen for CD audio (giving a comfortable margin above the 40,000Hz requirement). In other words, 44,100 ‘snapshots’ of the audio signal are taken every second!

So, what about kbps? Well, each snapshot, or sample, because it’s digital, is represented by a string of ones and zeros (i.e. binary), which are referred to as ‘bits’. To reproduce 44,100 in binary requires 16 bits, i.e. a mixture of 16 ones and zeros - 1010110001000100.

The number of samples taken per second x The number of bits required to record each sample = The total number of bits per second.

Closeup of information being read from a CD by a laser.

44,100 x 16 = 705,600 bps

But this is only for one channel of audio. A CD has two channels (i.e. stereo) so this number must be multiplied by two:

705,600 x 2 = 1,411,200 bps

This can be rewritten as 1,411.2kbps

Rounding the number down leaves 1411kbps, the number of kbps of an audio CD.

This means that if you are listing to a 128kbps MP3, over 90% of the audio information is missing! Without a doubt, CDs are vastly superior in sound quality.

 
See the related article: CD vs MP3 Sound Quality

 
Useful external links:

For more information on bit rates check out Richard Farrar's artice 'What are Bit Rates?'.

 


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