W2D1

JPEG images Representing avg. pixel colour for large vs. small blocks Idea behind the Fourier transform: Pixel values:

W2D1 Fourier transform Takes a function f(x)Represents it as amplitude/frequency

W2D1 JPEG compression Small # bits for amplitude of high frequency functions Large # bits for amplitude of low frequency functions

W2D1 Digitizing Sound An object creates sound by vibrating in a medium such as air  Vibrations push the air  Pressure waves emanate from the object and vibrate our eardrums  The force, or intensity of the push determines the volume  The frequency (number of waves per second) is the pitch

W2D1

Sampling Take measurements at regular intervals Number of samples in a second is the sampling rate  The faster the rate, the more accurate the recording

W2D1 MP3 compression Use psychoacoustic model to eliminate sounds that cannot be heard. Experiment: (T26) Pure tones that are close together in pitch mask each other better than those widely separated. To demonstrate this fact, a single tone is played, followed by the same tone and a higher frequency tone. The higher frequency tone is reduced in intensity first by 12 dB, then by steps of 5 dB. The sequence above is repeated twice. The second time the frequency separation between the tones is increased

W2D1 MP3 compression Use psychoacoustic model to eliminate sounds that cannot be heard. Experiment: Pure tones also mask higher frequencies better than lower frequencies. Track 27 and 28 attempt to mask high and low frequencies respectively. The higher frequencies will mask more easily than the lower frequencies.