# (ENGR-E 511) Homework 4 kNN Source Separation solution

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P1: kNN Source Separation [5 points]
1. Prepare your S and N matrices by using the procedure in Homework 3 P3.1. In addition to
them, let’s create another matrix G, the magnitude spectrogram of trs.wav+trn.wav. Also,
let’s create yet another matrix called Ideal Binary Masks (IBM), B defined in Homework 3
P3 (or repeatedly as follows):
Bf,t =

1 if Sf,t ≥ Nf,t
0 otherwise.
(1)
My hope is that S ≈ B ⊙ G. This sounds a bit too rough, but you know it works to some
degree as shown in Homework 3 P3.
2. Load x nmf.wav and convert it into a complex-valued spectrogram X and its magnitudes Y ,
respectively. This time, we compare each column of Y with all columns in G to find out k
nearest neighbors. Then, for t-th given test frame in Y we have a list of indices to the k
nearest frames in G, e.g. {i1, i2, · · · , ik}, where i1, i2, · · · , ik ∈ {1, · · · , 990}, where 990 is the
number of frames in G (yours can be slightly different depending on the STFT setup). For
the comparison, I used Euclidean distance for no good reason, but you can choose a different
(more proper) one if you want.
3. What we want to do with this kNN search is to predict the unknown IBM of the test signal,
D ∈ B
513×131 (again 131 is the number of frames in Y ). By using the nearest neighbor indices,
I can collect the IBM vectors from B, i.e. {B:,i1
, B:,i2
, · · · , B:,ik
}. Since Y :,t is very similar
1
to {G:,i1
, G:,i2
, · · · , G:,ik
}, I hope that my D:,t should be similar to {B:,i1
, B:,i2
, · · · , B:,ik
}.
How do we consolidate them though? Let’s try median of them, because that’ll give you one
if one is the majority, and vice versa:
D:,t = median({B:,i1
, B:,i2
, · · · , B:,ik
}). (2)
4. Repeat this for all your test frames (131 frames in my STFT) and predict the full D matrix.
Recover the complex-valued spectrogram of your speech by masking the input: Sˆ
test = D⊙X.
Convert it back to the time domain. Listen to it. Is the noise suppressed? Submit this wavefile
P2: Motor Imagery [5 points]
1. eeg.mat has the training and testing samples as well as their labels. Use them to replicate
my BCI classification experiments in Module 5 (not the entire lecture, but from S3 to S8 and
S37). But, instead of PCA dimension reduction, we’re going to use locality sensitive hashing
to extract binary features. Also, instead of na¨ıve Bayes, we’ll do kNN classification.
2. For the kNN classification, for every test example you have to find the K nearest neighbors
from 112 training samples. You’re lucky. This kNN search is not a big deal, because your
training set is tiny. However, as your professor I have to help you guys be prepared for your
future big data projects at your fancy new job after your graduation (don’t forget me). So, I’m
giving you this homework assignment that will help you come up with a speed-up technique
for the kNN search. It’s not that complicated.
3. Come up with a random projection matrix A ∈ R
L×M, where M denotes the number of
dimensions of your data samples (5 rows of your magnitude STFT, vectorized), 2551
, and
L is the new dimension after this random projection. So, you’re replacing PCA with this
random projection. Note that L doesn’t always have to be smaller than M. Although A
is a matrix with random values, you should make sure that the row vectors (the projection
vectors) should be unit vectors, i.e. ||Ai,:
||2 = 1. Make sure of it.
4. Do the random projection and take the sign of the result (element-wise), so that your random
projection produces a bunch of +1’s and -1’s:
Y = sign(AX1:M,:) (3)
5. Use this A matrix and the sign function wrapper to convert your 255 dimensional TRAINING
samples into L-dimensional bipolar binary hash codes. Ditto for the TEST samples (use the
same A matrix). If you’re not comfortable with bipolar binaries, you can turn them into 0-1
binaries by replacing -1’s with 0’s. It’s up to you.
6. Do your kNN classification using these binary version of your data samples. Note that you can
compare the test bit string with the bit strings of the training data by using the Hamming
distance, instead of the Eucleadian distance between the original real-valued vectors. You
know your computer prefers binary values, right? This way you can speed up the comparison,
1Note that this number comes from the calculation [# channels] × [# frames] × [# subbands], while # frames
can slightly vary depending on your STFT implementation. Mine resulted in 3 × 17 × 5 = 255
2
and eventually the kNN search (although you might not be able to see the actual speed-up due
to the too small dataset size and your script language that doesn’t exploit binary variables).
Report your classification results with different choice of K and L. I think a table of accuracies
will be great. You don’t have to submit all the intermediate plots. What I need to see is the