How to average a signal to remove noise with Python
<p>I am working on a small project in the lab with an Arduino Mega 2560 board. I want to average the signal (voltage) of the positive-slope portion (rise) of a triangle wave to try to remove as much noise as possible. My frequency is 20Hz and I am working with a data rate of 115200 bits/second (fastest recommended by Arduino for data transfer to a computer).</p>

<p>The raw signal looks like this:</p>

<p><a href="https://i.stack.imgur.com/Bgotg.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Bgotg.png" alt="Raw Signal"></a></p>

<p><a href="https://i.stack.imgur.com/yT1Cy.png" rel="noreferrer"><img src="https://i.stack.imgur.com/yT1Cy.png" alt="Raw Signal Zoom"></a></p>

<p><a href="https://i.stack.imgur.com/vrHIW.png" rel="noreferrer"><img src="https://i.stack.imgur.com/vrHIW.png" alt="Raw Signal Zoom Plus"></a></p>

<p>My data is stored in a text file, with each line corresponding to a data point. Since I do have thousands of data points, I expect that some averaging would smooth the way my signal looks and make a close-to-perfect straight line in this case. However, other experimental conditions might lead to a signal where I could have features along the positive-slope portion of the triangle wave, such as a negative peak, and I absolutely do need to be able to see this feature on my averaged signal. </p>

<p>I am a Python beginner so I might not have the ideal approach to do so and my code might look bad for most of you guys but I would still like to get your hints / ideas on how to improve my signal processing code to achieve a better noise removal by averaging the signal.</p>

<pre><code>#!/usr/bin/python
import matplotlib.pyplot as plt
import math

# *** OPEN AND PLOT THE RAW DATA ***
data_filename = "My_File_Name"
filepath = "My_File_Path" + data_filename + ".txt"

# Open the Raw Data
with open(filepath, "r") as f:
    rawdata = f.readlines()

# Remove the \n     
rawdata = map(lambda s: s.strip(), rawdata)

# Plot the Raw Data
plt.plot(rawdata, 'r-')
plt.ylabel('Lightpower (V)')
plt.show()

# *** FIND THE LOCAL MAXIMUM AND MINIMUM
# Number of data points for each range 
datarange = 15 # This number can be changed for better processing
max_i_range = int(math.floor(len(rawdata)/datarange))-3

#Declare an empty lists for the max and min
min_list = []
max_list = []

min_list_index = []
max_list_index = []

i=0

for i in range(0, max_i_range):
    delimiter0 = i * datarange
    delimiter1 = (i+1) * datarange
    delimiter2 = (i+2) * datarange
    delimiter3 = (i+3) * datarange

    sumrange1 = sum(float(rawdata[i]) for i in range(delimiter0, delimiter1 + 1))
    averagerange1 = sumrange1 / len(rawdata[delimiter0:delimiter1])
    sumrange2 = sum(float(rawdata[i]) for i in range(delimiter1, delimiter2 + 1))
    averagerange2 = sumrange2 / len(rawdata[delimiter1:delimiter2])
    sumrange3 = sum(float(rawdata[i]) for i in range(delimiter2, delimiter3 + 1))
    averagerange3 = sumrange3 / len(rawdata[delimiter2:delimiter3])

    # Find if there is a minimum in range 2
    if ((averagerange1 &gt; averagerange2) and (averagerange2 &lt; averagerange3)):
        min_list.append(min(rawdata[delimiter1:delimiter2])) # Find the value of all the minimum 

        #Find the index of the minimum
        min_index = delimiter1 + [k for k, j in enumerate(rawdata[delimiter1:delimiter2]) if j == min(rawdata[delimiter1:delimiter2])][0] # [0] To use the first index out of the possible values
        min_list_index.append(min_index)


    # Find if there is a maximum in range 2
    if ((averagerange1 &lt; averagerange2) and (averagerange2 &gt; averagerange3)):
        max_list.append(max(rawdata[delimiter1:delimiter2])) # Find the value of all the maximum

        #Find the index of the maximum
        max_index = delimiter1 + [k for k, j in enumerate(rawdata[delimiter1:delimiter2]) if j == max(rawdata[delimiter1:delimiter2])][0] # [0] To use the first index out of the possible values
        max_list_index.append(max_index)



# *** PROCESS EACH RISE PATTERN ***

# One rise pattern goes from a min to a max
numb_of_rise_pattern = 50 # This number can be increased or lowered. This will average 50 rise patterns

max_min_diff_total = 0

for i in range(0, numb_of_rise_pattern):
    max_min_diff_total = max_min_diff_total + (max_list_index[i]-min_list_index[i])

# Find the average number of points for each rise pattern
max_min_diff_avg = abs(max_min_diff_total / numb_of_rise_pattern)

# Find the average values for each of the rise pattern
avg_position_value_list = []


for i in range(0, max_min_diff_avg):

    sum_position_value = 0
    for j in range(0, numb_of_rise_pattern):

        sum_position_value = sum_position_value + float( rawdata[ min_list_index[j] + i ] )
    avg_position_value = sum_position_value / numb_of_rise_pattern

    avg_position_value_list.append(avg_position_value)


#Plot the Processed Signal
plt.plot(avg_position_value_list, 'r-')
plt.title(data_filename)
plt.ylabel('Lightpower (V)')
plt.show()
</code></pre>

<p>At the end, the processed signal looks like this:</p>

<p><a href="https://i.stack.imgur.com/nFZmS.png" rel="noreferrer"><img src="https://i.stack.imgur.com/nFZmS.png" alt="Averaged Signal"></a></p>

<p>I would expect a straighter line, but I could be wrong. I believe that there are probably a lot of flaws in my code and there would certainly be better ways to achieve what I want. I have included a link to a text file with some raw data if any of you guys want to have fun with it.</p>

<p><a href="http://www108.zippyshare.com/v/2iba0XMD/file.html" rel="noreferrer">http://www108.zippyshare.com/v/2iba0XMD/file.html</a></p>