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Rewrote RC filter in Python as a refresher
#/usr/bin/env python
from sys import stdout
from math import pi, log10, pow
Tau = 2 * pi
TableWidth = 20
## Calculate Decibels
def db(ref, val):
return 20 * log10(val / ref)
## Represent a resistor
class Resistor:
def __init__(self, ohms):
self.r = ohms
## Get the value of r
def value(self):
return self.r
## Get the resistance
def resistance(self):
return self.r + 0j
## Return the impedance (will just be resistance)
def impedance(self, frequency = 0):
return complex(self.r, 0)
## Represent a capacitor
class Capacitor:
def __init__(self, farads):
self.c = farads
## Get the value of c
def value(self):
return self.c
## Get the resistance (none)
def resistance(self):
return 0
## Get the impedance
def impedance(self, frequency = 0):
reactance = -1 / (Tau * frequency * self.c)
return complex(0, reactance)
## Represent and RC filter
class RCFilter:
def __init__(self, resistor, capacitor):
self.set_input(1.0)
self.resistor = Resistor(resistor)
self.capacitor = Capacitor(capacitor)
## Set the AC voltage and frequency
def set_input(self, voltage = 1.0, frequency = 0.0):
self.voltage = voltage
self.frequency = frequency
## What is the cutoff for this filter?
def cutoff(self):
return (1 / (Tau * self.resistor.value() * self.capacitor.value()))
## What is the total impedance, Z, of this circuit?
def total_impedance(self):
return self.resistor.impedance(self.frequency) + self.capacitor.impedance(self.frequency)
## Total current in this circuit, similar to I = V/R, I = V/Z
def total_current(self):
return self.voltage / self.total_impedance()
## Voltage across the resistor
def voltage_across_resistor(self):
return self.total_current() * self.resistor.impedance(self.frequency)
## Voltage across the capacitor
def voltage_across_capacitor(self):
return self.total_current() * self.capacitor.impedance(self.frequency)
## Do a frequency sweep, I'm using MIDI notes because they are logarithmic
def frequency_sweep(self):
output = []
for midi in xrange(0, 127, 2):
## Find the frequency of this midi note number
## Set the AC voltage to this frequency, 1 volt
f = 440 * pow(2, (midi - 69) / 12.0)
self.set_input(1.0, f)
output.append(self.voltage_across_capacitor().real)
return output
## print an ascii graph of the frequency response
def print_frequency_response_graph(self):
line = '-' * 64
## First map the 0.0 - 1.0 voltages to 0 - 10 integers
## Do I need me = self?
frequency_response = map(lambda voltage: int(voltage * 10), self.frequency_sweep())
## Print out a 64x10 ASCII graph
print "Frequency Response:"
print line
for voltage in reversed(xrange(11)):
stdout.write('|')
for response in frequency_response:
stdout.write('*' if voltage == response else ' ')
print("|")
print line
r1 = 220.0
c1 = 0.000001
filter = RCFilter(r1, c1)
cutoff_frequency = filter.cutoff()
filter.set_input(1.0, cutoff_frequency)
attenuation = db(1.0, filter.voltage_across_capacitor().real)
filter.print_frequency_response_graph();
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