Power spectral density in SciPy - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to compute the power spectral density (PSD) of the signal using scipy.

SciPy

from scipy.signal import [1]
import numpy as np

fs = 1000  # Sampling frequency
signal = np.sin(2 * np.pi * 50 * np.linspace(0, 1, fs))
f, Pxx = [1](signal, fs)
print(f[:5], Pxx[:5])

Drag options to blanks, or click blank then click option'

Aperiodogram

Bfind_peaks

Cspectrogram

Dwelch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'spectrogram' instead of 'welch' which returns a spectrogram, not PSD directly.

Using 'find_peaks' which is for peak detection, not PSD.

Using 'periodogram' which also computes PSD but is less robust than 'welch'.

The welch function from scipy.signal computes the power spectral density (PSD) of a signal.

2fill in blank

medium

Complete the code to generate a noisy sine wave signal before computing its PSD.

SciPy

import numpy as np
fs = 500
T = 1
n = int(T * fs)
time = np.linspace(0, T, n, endpoint=False)
signal = np.sin(2 * np.pi * 60 * time) + [1]

Drag options to blanks, or click blank then click option'

Anp.random.normal(0, 0.5, n)

Bnp.random.uniform(0, 1, n)

Cnp.random.binomial(10, 0.5, n)

Dnp.random.poisson(5, n)

Attempts:

3 left

💡 Hint

Common Mistakes

Using uniform noise which has a different distribution.

Using discrete distributions like Poisson or binomial which are not typical for noise.

Adding Gaussian noise with mean 0 and standard deviation 0.5 simulates a noisy sine wave.

3fill in blank

hard

Fix the error in the code to correctly compute the PSD with the correct sampling frequency parameter.

SciPy

from scipy.signal import welch
import numpy as np
fs = 1000
signal = np.sin(2 * np.pi * 100 * np.linspace(0, 1, fs))
f, Pxx = welch(signal, [1]=fs)
print(f[:5], Pxx[:5])

Drag options to blanks, or click blank then click option'

Afs

Bsampling_rate

Cfreq

Drate

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names like 'sampling_rate' or 'freq' causes errors.

Omitting the sampling frequency leads to wrong frequency axis.

The correct parameter name for sampling frequency in welch is fs.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps each word to its length only if the length is greater than 3.

SciPy

words = ['data', 'science', 'is', 'fun']
lengths = {word: [1] for word in words if [2]

Drag options to blanks, or click blank then click option'

Alen(word)

Blen(word) > 3

Cword.startswith('s')

Dword == 'fun'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the word itself as value instead of its length.

Using unrelated conditions like checking the first letter.

The dictionary comprehension maps each word to its length (len(word)) only if the length is greater than 3 (len(word) > 3).

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase words to their lengths only if length is greater than 2.

SciPy

words = ['Data', 'AI', 'ML', 'Science']
result = { [1]: [2] for word in words if [3] }

Drag options to blanks, or click blank then click option'

Aword.upper()

Blen(word)

Clen(word) > 2

Dword.lower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase instead of uppercase for keys.

Using incorrect conditions that do not filter by length.

The comprehension maps the uppercase version of each word (word.upper()) to its length (len(word)) only if the length is greater than 2 (len(word) > 2).

Practice

1. What does the power spectral density (PSD) represent in signal analysis?

easy

A. The phase shift of a signal at various points

B. The total energy of a signal over time

C. How the power of a signal is distributed over different frequencies

D. The amplitude of a signal at a single point in time

Solution

1. Step 1: Understand the definition of PSD

   Power spectral density shows how the power of a signal is spread across frequencies, not just total energy or amplitude at one time.

2. Step 2: Compare options with PSD meaning

   Only How the power of a signal is distributed over different frequencies correctly describes PSD as power distribution over frequencies.

3. Final Answer:

   How the power of a signal is distributed over different frequencies -> Option C

4. Quick Check:

   PSD = power distribution over frequency [OK]

Hint: PSD shows power spread by frequency, not time or phase [OK]

Common Mistakes:

• Confusing PSD with total signal energy
• Thinking PSD shows amplitude at one time
• Mixing PSD with phase information

2. Which of the following is the correct way to import the Welch method for PSD calculation from scipy?

easy

A. from scipy.signal import welch

B. import scipy.welch

C. from scipy import welch

D. import scipy.signal.welch as welch

Solution

1. Step 1: Recall scipy.signal module structure

   The Welch method is inside scipy.signal, so we import it from there.

2. Step 2: Check import syntax correctness

   Only 'from scipy.signal import welch' is valid Python syntax for this function.

3. Final Answer:

   from scipy.signal import welch -> Option A

4. Quick Check:

   Correct import = from scipy.signal import welch [OK]

Hint: Welch is in scipy.signal, import directly from there [OK]

Common Mistakes:

• Trying to import welch directly from scipy
• Using incorrect import syntax
• Assuming welch is a standalone module

3. What will be the output type of the following code snippet?

from scipy.signal import welch
import numpy as np
fs = 100
x = np.sin(2 * np.pi * 10 * np.arange(1000) / fs)
freqs, psd = welch(x, fs)
print(type(freqs), type(psd))

medium

A. (, )

B. (, )

C. (, )

D. (, )

Solution

1. Step 1: Understand welch output types

   The welch function returns two numpy arrays: frequencies and power spectral density values.

2. Step 2: Check printed types

   Printing type(freqs) and type(psd) will show numpy.ndarray for both.

3. Final Answer:

   (<class 'numpy.ndarray'>, <class 'numpy.ndarray'>) -> Option D

4. Quick Check:

   welch returns arrays = numpy.ndarray [OK]

Hint: Welch returns frequency and PSD as numpy arrays [OK]

Common Mistakes:

• Expecting lists instead of numpy arrays
• Thinking output is a tuple of floats
• Confusing output with single float values

4. Identify the error in this code snippet that calculates PSD:

from scipy.signal import welch
import numpy as np
fs = 50
x = np.random.randn(500)
freqs, psd = welch(x)
print(freqs)

medium

A. The print statement should be print(psd) instead

B. Missing sampling frequency (fs) argument in welch call

C. Random data cannot be used with welch

D. Import statement for welch is incorrect

Solution

1. Step 1: Check welch function call parameters

   The welch function needs the sampling frequency (fs) to correctly compute frequencies.

2. Step 2: Identify missing argument

   Here, fs=50 is defined but not passed to welch, causing incorrect frequency calculation.

3. Final Answer:

   Missing sampling frequency (fs) argument in welch call -> Option B

4. Quick Check:

   Pass fs to welch to get correct frequencies [OK]

Hint: Always pass fs to welch for correct frequency output [OK]

Common Mistakes:

• Forgetting to pass fs argument
• Assuming random data is invalid input
• Confusing print output with error

5. You have a noisy signal sampled at 200 Hz. You want to find the dominant frequency component using scipy's Welch method. Which parameter adjustment helps improve frequency resolution in the PSD estimate?

hard

A. Increase the segment length (nperseg) to use longer data chunks

B. Decrease the sampling frequency (fs) to reduce data size

C. Use a smaller window type to reduce computation

D. Set detrend to False to keep original signal shape

Solution

1. Step 1: Understand frequency resolution in Welch method

   Frequency resolution improves with longer segments (nperseg) because it provides finer frequency bins.

2. Step 2: Evaluate options for improving resolution

   Increasing nperseg increases segment length, improving resolution. Decreasing fs reduces max frequency but not resolution. Window type affects leakage, not resolution. Detrend affects baseline, not resolution.

3. Final Answer:

   Increase the segment length (nperseg) to use longer data chunks -> Option A

4. Quick Check:

   Longer nperseg = better frequency resolution [OK]

Hint: Longer segments (nperseg) give finer frequency detail [OK]

Common Mistakes:

• Reducing sampling frequency to improve resolution
• Confusing window type with resolution effect
• Ignoring segment length impact on frequency bins