Mp3 player working by digital signal processing

Dipanjon Halder

1230630

MP3Player

WHAT IS

MP3 ??

MPEG-1 or MPEG-2 Audio Layer III

MP3, is an audio coding format for

digital audio which uses a form

of lossy data compression.

MP3 Cycle

Audio as Physical

Phenomenon Vibrations of object generate sound

Sound propagates as pressure wave

Ear can sense pressure wave

Audio as Analog

Signal Microphone translates waves into varying

voltage

Speaker converts electrical signal into

pressure wave

Digital Signal Recording

Need process to represent analog signal in binary

Steps for processing working mp3 Measure signal (“sampling”)

Translate into binary (“quantization”)

Store or transmit

Reconstruct signal (“excite filter”)

Sample and Sampling Rate

Sampling rate determines quality of representation

Low-rate sampling fails to capture high frequencies

Nyquist-Shannon sampling theorem

“If a function f(t) contains no frequencies higher than W hertz, it is completely

determined by giving its ordinates at a series of points spaced 1/2 W seconds

apart.”

Quantization Samples have continuous value

“Quantization” assigns discrete value to each

sample

Analog-to-digital (A/D) converter n-bit digital output

n bits have 2n possible values

Digital Representation

Digital representation Encode quantized values in binary

Concatenate binary codes of samples

Add meta-information (can be implied if standard is used)

Playback Digital-to-analog (D/A) converter Generates voltage of sample value

Voltage is held for duration of sample period

Low-pass filter to “smooth out” signal

Signal is amplified and sent to speaker

Aliasing

Difference between original and reconstructed

signal

Almost same to the original signal

Sampling and Quantization

Quality

Sampling rate and quantization levels impact

quality

Parameters

Telephony: pulse code modulation (PCM) ITU-T standard G.711 (sample 8000, quantazation-8 bit)

µ-law in U.S. (14-bit samples)

A-law in Europe (13-bit samples)

Encoded signal: 64 kb/s

CD-quality audio: (PCM) Sampling: 44,100 samples per second

Quantization: 16-bit samples

Encoded signal (stereo): (176.4kB/s)

How Can Reduced the Mp3 size???

Compression

Example for loss-less compression: Huffman coding

Variable-length code

Huffman coding for our example

2-bit symbol frequency:

10 (37%), 01 (30%), 00 (23%), 11 (10%)

New encoding

10→0, 01→10, 11→110, 00→111

Encoded sequence marginally better Only 1 bit (2%) shorter

Better on sequences with more redundancies

101001000110101010011101110000010110110001000000101010011001

00101111000001011010110111111101001101111011111111100010010

Conti…… Lossy compression

Uses perceptual coding

Reduces precision of audio components

less audible to humans

Sound is analyzed in a short windows

Analysis in time domain and frequency domain

Coding exploits masking effects

Simultaneous masking: loud sound masks soft sound

Temporal masking: Loud sound masks following soft sound

Reduces data rate considerably

MP3 uses 128kb/s for CD-quality audio

Typical 5-minute song: 4.8MB

16GB MP3 player: more than 3,000 songs