2. Overview
Chapter 12
Digital Audio
Digitization of Audio Samples
Quantization
Reconstruction
Quantization error
3. Digitization of Audio Samples
Audio signals are continuous in time and amplitude
Audio signal must be digitized in both time and amplitude to
be represented in binary form.
Discrete in time by sampling – Nyquist
Discrete in amplitude by quantization
Once samples have been captured, they must be made
discrete in amplitude.
The two step digitization process
Step 1: Sampling Step 2: Quantization
Step 2: Quantization
4. Quantization
Quantization
Converts actual sample values (usually voltage
measurements) into an integer approximation
Process of rounding off a continuous value so that it can
be represented by a fixed number of binary digits
Tradeoff between number of bits required and error
Human perception limitations affect allowable error
Specific application affects allowable error
Two approaches to quantization
Rounding the sample to the closest integer.
(e.g. round 3.14 to 3)
Create a Quantizer table that generates a staircase pattern
of values based on a step size.
5. Consider an audio signal with a voltage range between -10 and
+10
Assume the audio waveform has already been time sampled, as
shown
How can the amplitude also be converted into discrete values?
6. For this example, let’s choose to represent each sample by 4 bits
There are an infinite number of voltages
between -10 and 10.
We will have to assign a range of voltages
to each 4-bit codeword.
There will be 16 steps. Why?
How large will each step be?
7. Reconstruction
Analog-to-Digital Converter (ADC) provides the sampled and
quantized binary code.
Digital-to-Analog Converter (DAC) converts the quantized
binary code back into an approximation of the analog
signal by clocking the code to the same sample rate as the
ADC conversion.
Quantization and Reconstruction example on next two
slides:
10. Quantization Error
After quantization, some information is lost
Errors (noise) introduced
The difference between the original sample value and the
rounded value is called the quantization error
A signal to noise ratio (SNR) is the ratio of the relative sizes of
the signal values and the errors.
The higher the SNR, the smaller the average error is with
respect to the signal value, and the better the fidelity.
Quantization is only an approximation.
11. Comments for next class
Chapter 13
The Telephone System: wired and wireless
Analog Telephone system
Digital telephone system
Cellular telephone system
12. Chapters Read So Far
Everyone should have read the following chapters thus
far:
Chapter 1: What is the Information in the Information
Revolution?
Chapter 3: Representing Information in Bits
Chapter 4: The Need and Basis for Data Protocols
Chapter 5: From the Real World to Images and Video
Chapter 7: Compressing Information
Chapter 8: Image Compression
Chapter 9: Digital Video
Chapter 10: Audio as Information
Chapter 11: Sampling of Audio Signals
Chapter 12: Digital Audio
Information Technology Inside and Outside
