Lagring av Lyd


created by Frank and Mohamud(franko/mohamua)




Introduction.


This exercise we will look at how audio signals are created and how the limitations of human hearing help difine the usefull information content of an audio signals. we shall look at a basic phenomenon of sound. e.g. The result of droping a book on a floor is that sound is produced, and it is this motion of air that we perceive as a sound.

PART1.

This figure shows tree signals with different frequencies and amplitudes which we have used to generate a sound signals. For Sin1 is A=5000 and f=600, Sin2 is A=12000 and f=1000, and then Sin3 is A=15000 and f=1500.
The daigram is amplitude/time curve. The picture shows amplitude and frequence which we have used for the curve. And the formula we have used is A*sin(2*pi*ft)=x(t). (where x(t) represents sound values. f =frequency, A=amplitude). The Y-axis shows the amplitude and the X-axis shows the time of each sound signal movement.
Sin1
A*sin(2*pi*ft)=x(t)
5000*sin(2*3.142*600*t)=x(t)
Sin2
A*sin(2*pi*ft)=x(t)
12000*sin(2*3.142*1000*t)=x(t)
Sin3
A*sin(2*pi*ft)=x(t)
15000*sin(2*3.142*1500*t)=x(t)



This picture is magnitude/frequency curve, which we have used the same values as the picture of amplitude/time curve. The frequency spectrum explains which frequency component a sound contains. The spectrum is also symmetric. The frequency spectrum is based on the Fourier-transform of sound signals. Fourier-transform decompose sound signals into different basis in sin- and cosin. so Y-axis shows magnitude and the X-axis shows the frequency. The peroidic raise and fall of the sound signals is due to the time given.

PART2.


This is a sound file tale1.wav. The plot shows the sound with function of time. The sound changes with the time, the amplitude have low value due to the content of sound file. The signal changes faster becuase of X-axis in milliseconds. The sound files tale2.wav, tale3.wav and tale4.wav are sampling intervals which increase on every file.

LYD1 - Sample rate 44100 Hz.
LYD2 - Sample rate 22050 Hz.
LYD3 - Sample rate 11025 Hz.
LYD4 - Sample rate 8000 Hz.
Here we have four different Sampling rates, where we can hear a clear sound of the tale1.wav and tale2.wav. The second one gives as a very clear soud, even it can be difficult to differciate. The quality of third and fourth sound reduced because of lower sampling rate, even though we are using the same number of bits. we get different sounds due to differences in sampling rate. The sound becomes worse when you decrease the number of bits as well as the sampling rate in Hz. That is why tale4.wav is dificult to hear. As long as you move from tale4.wav to tale1.wav you hear better sound.




This picture has been sampled at rate of 44100 samples per second with 10bits.


This picture has been sampled at rate of 44100 samples per second with 8bits.

Here we have two different pictures which we sampled with 10 and 8 bits respectively. The first picture we have used 10bits which gives us 1024 level. When we used 10-bits, the quality of the sound was good, allthough we could not get a quality sound like when we increase the of bits. When we have used 8-bits, allthough we had a sound, but the quality of the sound was not as good as when we have used 10-bits. As we reduce the number of bits, we miss som of sounds, we are sampling.

kilder:

Foiler fra forelesningen.
Pensumboka.