02 - Analog vs. Digital Signals Flashcards
Analog Signals are continuous in _______ and in _______.
Time
Amplitude
What are examples of an Analog Signal?
Speech Sounds
Musical Tone
Displacement of the middle ear bones
What is a Digital Signal?
Discrete Time Signal
It only exists at discrete points. It is not continuous. There are empty moments in between.
How does digitizing work? (2)
You need a representative sample.
You just need to know how accurate the digital representation needs to be.
What does “digitization” literally mean?
To convert into numbers (digits) so that the information can be stored in numeric form
What are the two main operations in digitization?
Sampling
Quantization
Before sampling & quantization, the signal may be passed through a _______ and a ______.
Pre-Emphasis Filter
Pre-Sampling (Lowpass) Filter
What does a Pre-Emphasis Filter do?
Boosts the higher frequencies
What does a Pre-Sampling (Lowpass) Filter do?
Rejects the energy above the highest frequency of interest
In the long term average of speech, is there more energy in the high frequencies or the low frequencies?
Low Frequencies
Why do we need to boost the intensity of the high frequencies?
To make these levels to equal to that of the low frequencies
What is Sampling?
Taking “samples” at given intervals
This makes the analog signal discrete in time
What does a sampling rate of 10,000 Hz (10 kHz) mean?
That the analog signal has been sampled 10,000 times per second
What is Nyquist’s Sampling Theorem?
That the number of samples taken must be TWICE the highest frequency of interest
Why does the sampling rate need to be at least twice the rate of the highest frequency?
To avoid aliasing
What is Aliasing?
When the higher frequencies aren’t sampled frequently enough, they appear as lower frequencies
What are the two things that usually cause Aliasing?
Undersampling
No Lowpass Filter
What are three ways to avoid Aliasing?
Determine the highest frequency of interest
Filter the energy above the highest frequency of interest
Sample the signal at at least twice the rate of the highest frequency of interest.
What is a Quantum?
An increment of energy
What is Quantization?
It converts the amplitude (energy level) of the samples
It makes the amplitude of the signal discrete
What is the Quantization rate often called?
Resolution
What is Quantization measured in?
Bits
What are Bits?
Binary digits
How many Bits does a 1-bit system have?
2 levels
How many Bits does a 8-bit system have?
256 levels
(2⁸)
How many Bits does a 16-bit system have?
65,536 levels
(2¹⁶)
In Quantization, do you store the positive output, the negative output or both?
Positive
What are the steps to Analog-to-Digital Conversion? (6)
Acoustic Signal
Pre-Emphasis Filter
Pre-Sampling (Lowpass) Filter
Sampler
Quantizer
Encoder
What is Zipper Noise?
The noise that comes from square edging
What is Square Edging?
How the sinwave becomes “choppy” due to the lesser quantization
Do we listen to the digital signal?
No, it must be converted back into analog form
There is usually a final lowpass filter to smooth out the signal
What are the steps to A/D and D/A conversion?
(6)
Acoustic Pressure Wave (Sound)
Microphone
Analog-to-Digital Conversion
Processing & Storage
Digital-to-Analog Conversion
Audio Output
What kind of signal is this?
Analog
What kind of signal is this?
Digital
What is going on with the dotted line?
Aliasing
