MP3 uses lossy data compression to reduce the size of digital audio files. It works by analyzing sound within short windows in both the time and frequency domains. It exploits the principles of simultaneous and temporal masking, where certain sounds can mask others that are softer. This allows it to reduce the precision of inaudible audio components. By doing so, MP3 is able to significantly reduce the data rate needed for audio files while still maintaining near-CD quality. For example, a typical 5-minute song compressed to MP3 at 128kbps would be only 4.8MB in size, allowing over 3,000 songs to fit on a 16GB MP3 player.

1. Dipanjon Halder
1230630
MP3
Player

2. 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.

3. MP3 Cycle

4. Audio as Physical
Phenomenon
 Vibrations of object generate sound
 Sound propagates as pressure wave
 Ear can sense pressure wave

5. Audio as Analog
Signal
 Microphone translates waves into varying
voltage
 Speaker converts electrical signal into
pressure wave

6. 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”)

7. 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.”

8. 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

9. Digital Representation
 Digital representation
 Encode quantized values in binary
 Concatenate binary codes of samples
 Add meta-information (can be implied if standard is used)

10. 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

11. Aliasing
 Difference between original and reconstructed
signal
Almost same to the original signal

12. Sampling and Quantization
Quality
 Sampling rate and quantization levels impact
quality

13. 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???

14. 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

15. 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