mp3 player working by digital signal processing
TRANSCRIPT
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Dipanjon Halder
1230630
MP3Player
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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.
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MP3 Cycle
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Audio as Physical
Phenomenon Vibrations of object generate sound
Sound propagates as pressure wave
Ear can sense pressure wave
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Audio as Analog
Signal Microphone translates waves into varying
voltage
Speaker converts electrical signal into
pressure wave
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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”)
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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.”
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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
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Digital Representation
Digital representation Encode quantized values in binary
Concatenate binary codes of samples
Add meta-information (can be implied if standard is used)
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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
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Aliasing
Difference between original and reconstructed
signal
Almost same to the original signal
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Sampling and Quantization
Quality
Sampling rate and quantization levels impact
quality
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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???
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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
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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
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