电子科技大学多媒体通信第二章讲解

Sampling frequency
Nyquist frequency: half of the Nyquist rate
Since it would be impossible to recover frequencies higher than Nyquist frequency in any event, most systems have an antialiasing filter that restricts the frequency content in the input to the sampler to a range at or below Nyquist frequency.
The SNR is usually measured in unit decibels (dB)
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Signal to Noise Ratio (SNR)
Usual levels of sound
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Signal to Noise Ratio (SNR)
Amplitude
8Δ 7Δ 6Δ 5Δ 4Δ 3Δ 2Δ
Low-pass signal, fs≥2fH
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Sampling at exactly the frequency produces a constant.
Sampling at 1.5 times per cycle produces an alias perceived frequency.
1. Data Capacity＝Sampling Rate (fs ) × Bits Number per Sample × Channels ×Time period/8 (Bytes)
2. Data Rate ＝Sampling Rate (fs ) ×Bits Number per Sample×Channels (bps)
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Signal to Noise Ratio (SNR)
The ratio of the power of the correct signal and the noise is called the signal to noise ratio (SNR) -- a measure of the quality of the signal.
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Digitization of Sound
What Is Sound? Digitization Nyquist Theorem Audio Quality versus Data Rate（数据率）
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Basics of Digital Audio
What is Sound?
Sound is a wave phenomenon like light, but is macroscopic （宏观的）and involves molecules（分子）of air being compressed and expanded under the action of some physical device.
Δ
2Ts 4Ts 6Ts 8T(s)
Quantization noise:
the difference between the actual value of the analog signal, for the particular sampling time, and the nearest quantization interval value.
Quantization
Sampling in the amplitude or voltage dimension is called quantization.
Phone Quality: 8-bit quantization CD Quality: 16-bit quantization
Frequency Amplitude
Based on frequency, there are periodic(周期性的) and nonperiodic sound. Examples:
Periodic: music, vowel sounds （元音）, whistling wind, bird songs…
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Data Rate
Phone Quality: 8-bit quantization
R=8000samples/second×8bits/sample =64kbps
CD Quality: 16-bit quantization
R=44100samples/second×16bits/sample =705.6kbps 2-channels (stereo): R=705.6kbps×2=1411.2kbps
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Undersampling and Oversampling
Oversampling
trying to capture fast edges, transients, and one-time events
Undersampling
sampling at a rate below the Nyquist frequency
frequency
1
period (Time)
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Basics of Digital Audio
Based on Frequency of sound
Subsonic(次声波，亚音速) signal（<20Hz） Audio signal（20Hz－20kHz） Speech signal（300－3000Hz） Ultrasonic（超声波） signal（>20kHz）
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Digitization：Overview
To digitize, the analog signal must be sampled（采样） in each dimension: in time (sampling，采样), and in amplitude (quantization，量化).
Sampling means measuring the quantity we are interested in, usually at evenly-spaced intervals.
The rate at which it is performed is called the sampling frequency.
Subsonic Audio Speech (Audio) Audio Ultrasonic
0Hz
20Hz 300Hz
3000Hz 20kHz
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Basics of Digital Audio
Digitization:
Conversion to a stream of numbers, and preferably these numbers should be integers for efficiency.
Contents
2.1 Digitization of Sound 2.2 Quantization and Transmission of Audio 2.3 MPEG(Moving Pictures Experts Group) Audio 2.4 Example for Voice Programming
At most, this error can be as much as half of the interval.
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Signal to Quantization Noise Ratio (SQNR)
SQNR: to characterize the quality of the quantization
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Contents and objectives
Contents to master
• Digitization of Sound； • Coding of Audio、Pulse Code Modulation 、Differential Coding of Audio 、
Lossless Predictive Coding 、DPCM 、DM 、ADPCM ； • Psychoacoustics: Non-uniform sensitivity、Spectral masking、Temporal
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Sampling frequency
Nyquist Theorem:
Band-limited signal, i.e., there is a lower limit f1 and an upper limit f2 of frequency components in the signal, then the sampling rate should be fs≥2(f2 − f1).
Analog Waveform
Sampling
Quantization
samples
0.75 0.5 0.25
0 -0.25 -0.5 -0.75
samples
Encoding
Digital Sequence
0100111…
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Digitization: Sampling
What is Sampling?
The relationship among the Sampling Frequency, True Frequency, and the Alias Frequency is as follows:
For audio:
Phone Quality: 8kHz sampling rate CD Quality: 44.1kHz sampling rate
Nonperiodic: unpitched percussion instruments（无调的打击 乐器）, coughs & sneezes…
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Basics of Digital Audio
Frequency
Unit: Hz( KHz, MHz, GHz, THz) or cps
Amplitude (loudness)
masking；MPEG-1 Layer III Coding；
Contents to learn about
• Vocoders(声码器); • MPEG-1 Layer I ,II Coding； • MPEG-1 Layer III File Structure； • MPEG-2 AAC、MPEG-4 Audio、MPEG-7 and MPEG-21； • Audio Programming
In short -- the vibration（振动）of matter
Vibration of Matter
create
Propagate in
Pressure a wave-like Human
Variation motion
ear
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Basics of Digital Audio
Two key parameters in a sound waveform:
For quantization accuracy of N bits per sample, the SQNR can be simply expressed:
Amplitude
8Δ
8
1000
7Δ
7
0111
6Δ
6
0110
5Δ
5
0101
4Δ
4
0100
3Δ
3
0011
2Δ
2
0010
Δ
1
0001
0
0
0000
2Ts 4Ts 6Ts 8Ts 10Ts 12Ts 14Ts 16Ts 18Ts 20Ts Time (s)
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Digitization：Overview
Analog to Digital Conversion
Multimedia Communications
Chapter 2 Basics of Digital Audio
National Key Laboratory of Science and Technology on Communications,
University of Electronic Science and Technology of China