How MP3 can produce same quality as CD?
Ears of human can listen to signal in the range of 20Hz~20KHz. It is called "audible frequency". To
convey the audible sound in digital type CD, sampling frequency of 44.1KHz, about 2 times of audi-
ble frequency, should be used. It is the task of dividing sound signal to 44,100 pieces per second
and making the signal to digital format of 0 and 1. How delicately the sample can be expressed will
be determined by number of bit per sample. Audio CD is 16 bit. It means that 1 sample can be
expressed in 65,536(16 square of 2) stage.
When converting CD music to WAV file, the capacity is about 40MB(for 4 minutes). By converting it
to MP3, it reduces to 4MB since "loss compression", the feature of MP3, is used. Loss compression
type removes the sound beyond the range of audible frequency(20Hz~20KHz). It uses the features
that small sound cannot be heard after very strong sound. <Figure 1> is the sound wave before
loss compression. It can be regarded as the sound wave of music CD or cassette tape containing
sound people cannot listen. When it is made into MP3, it is as shown in <Fig.2>.
Digital information is cut between sections and location information is saved in each section.
"Sampling rate" is the standard of how many sub-section it will divide 1section. Divided frequency
part is called 8 bit and 16 bit. 8 bit sampling means that frequency is divided into 2 stage, that is,
sound pitch of 256 stage. 16 bit sampling divides into 65,536 sound pitch. In addition, 44.1KHz
means sampling of 44,100 times per second. To reduce the amount of information made at digital,
sampling bit number and frequency should be set low, but it will cause deterioration of sound quali-
ty. There is no difference in sound quality between MP3 and CD since encoding(converting CD
track to MP3) is done with 44.1KHz at 16 bit. Better CD sound quality cannot be obtained by lower-
ing the sampling rate, but the capacity can be reduced.
Capacity can be reduced much by adjusting the degree
of loss. However, it causes deterioration of sound qual-
ity. Music CD contains sound made with 16 bit
44.1KHz of stereo sampling. Stereo is the type of divid-
ing the sound into left and right. CD should change
sound to digital.