Week 5 - PCM, analog-digital conversion Flashcards
What is digitization?
Converting analog signal to digital data.
PCM is a common method.
What are the 3 main elements of Pulse Code Modulation (PCM)?
Sampling
Quantizing
Encoding
What are the three sampling methods (PAM - Pulse Amplitude Modulation)?
Ideal Sampling (discrete point)
Natural Sampling (sampling with finite width pulse/high speed switch)
Flat-top Sampling (Similar to natural sampling but the sample value is discrete and held over the duration of pulse).
What is the restriction on sampling rate?
To reproduce the original analog signal, we must sample at x2 the highest frequency.
Why? To capture the peak and trough of each cycle.
f, 2f, 4f Which of these would be oversampling and undersampling? (f = highest frequency of analog wave)
f - under-sampling
4f- over-sampling
2f - required sampling rate
Why can’t we tell what the minimum sampling rate is for a bandpass signal given the bandwidth?
Because the lowest frequency is non-zero, we cannot assume the highest frequency = bandwidth.
Sampling rate = 2*highest frequency
What is Quantization?
Mapping signal amplitudes to discrete values. (Approximation)
How do we determine quantization levels?
(Max voltage - min voltage) / levels
What is the relationship between normalized amplitude and normalized quantized values?
The amplitudes are divided into quantization levels. Therefore, each quantization level represents a range of amplitudes.
How do we determine normalized quantized values?
What is the relationship between normalized quantized values, normalized PAM values, and normalized error?
Say we have 8 levels. Our quantization levels would range from 4 to -4 (to represent a sine wave)
The midpoint of each level would be a normalized quantized value.
Any normalized PAM values within that level will assume the normalized quantized value.
The difference in value between the two is the normalized error.
What is a quantization code?
Refers to each quantization level starting from 0.
How is quantization error related to the number of levels?
More levels means less error and more accurate signal recovery.
Quantization error changes the SNR of a signal (T or F)
True
What is the issue with uniform quantization?
Quantization levels are evenly spaced, this may not be practical for music for example where amplitudes are concentrated near zero.
What is PCM Encoding?
Converting each quantization level into an encoded word (1010)
Why might digitization increase the amount of bandwidth we need?
The more bits we use to represent each quantization level, the more bandwidth we need.
Why is baud rate <= bit rate?
S = N/r
What is a carrier signal?
A high frequency simple signal from the sending devices that acts as a base for information.
What is modulation/shift keying?
Modifying amplitude, frequency, or phase of the carrier signal to convey digital data.
What kind of signal does ASK create?
Composite non-periodic
What is the bandwidth for ASK?
Bandwidth is proportional to the signal rate (baud) S = N / r * case factor
How many bits per signal element are there in binary and multi-level ASK?
log(base2) # amplitudes
In FSK the frequency of the modulated signal is constant for the duration of a signal element (T or F)
True
What is the bandwidth for FSK?
The bandwidth(s) are centered around the various frequencies.
How many bits per signal element are there in binary/multi-level FSK?
Same as ASK
log(base)2 #frequencies
Why is PSK less susceptible to noise than ASK?
Noise affects amplitude.
Which analog to digital modulation technique requires more sophisticated hardware?
PSK
Phase-shift keying
What is the bandwidth required for PSK?
Same as ASK, bandwidth centered around carrier signal frequency. Requires less BW than FSK.
What is binary PSK?
Uses an in-phase, and out of phase wave for the signal element/ Offset by 180 degrees or 1/2 cycle
What is Quadrature Amplitude Modulation (QAM)?
Combines ASK and PSK.
Dominant method of digital to analog
modulation
One in phase carrier and one quadrature carrier offset by 90 degrees.
Why?
Same bandwidth as ASK and PSK.
Using phases of two carriers AND amplitude allows for more distinct values.
What does binary ASK look like on constellation diagram?
Two dots on the x axis representing a single in-phase carrier at two distinct positive values
What does binary PSK look like on a constellation diagram?
Two dots on the x-axis representing a single in phase carrier. Each dot will be a positive and negative amplitude respectively to represent the 180 degree phase shift.
What does a QAM look like on a constellation diagram?
You have an I component (in-phase) and a Q component (90 degrees offset). Each constellation point on the quadrant correlates to a specific amplitude, phase of the in-phase carrier, and phase of the quadrature carrier.
What is Amplitude Modulation (AM)?
Carrier amplitude is modulated in response to the input analog signal.
Bandwidth of modulated signal is twice that of input (think of how noise affects amplitude)
What is the bandwidth of an AM station and guards?
10kHz
To find the total # of stations we would take entire frequency range and divide by 10kHz. Then divide the result by 2.
What is the bandwidth of FM station and guards?
200kHz
The bandwidth of the input is equal to the modulated signal because we are altering frequency in response to input signal amplitude.
