# Control-Rate Envelope Generator

I wanted to generate a control-rate signal with my Python-Slipmat prototype. To my surprise, doing so was fairly straight forward; Python iterator classes have a control-rate mechanism built right in.

I designed a simple envelope that generates a rise-fall shape. By default, the rise and fall times are identical, though users can specify a peak value relative to the duration. Here’s the code:

```class RiseFall:
'''A rise-fall envelope generator.'''

def __init__(self, dur, peak=0.5):
self.frames = int(dur * sr / float(ksmps))
self.rise = int(peak * self.frames)
self.fall = int(self.frames - self.rise)
self.inc = 0
self.v = 0

def __iter__(self):
self.index = 0

if self.inc <= self.rise and self.rise != 0:
self.v = self.inc / float(self.rise)
else:
self.v = (self.fall - (self.inc - self.rise)) / float(self.fall)

self.inc += 1
return self

def next(self):
if self.index >= ksmps:
raise StopIteration

self.index += 1
return self.v
```

To make sense of this, I’m going to compare this Python iterator class to Csound. More or less, Csound works at three rates: init (i), control (k) and audio (a). All three of these are represented in class RiseFall. The i-rate of RiseFall is __init__(), the k-rate is __iter__() and the a-rate is next().

What makes RiseFall a k-rate unit generator is that the code that calculates the current value of the envelope resides in __iter__(), which gets executed at the beginning of each new frame of audio. If you look at class Sine, you’ll see that the code responsible for generating the sine wave is in the class method next().

And here is RiseFall added to our graph:

```if __name__ == "__main__":
t = 0.002
a1 = Sine(0.5, 440)
a2 = Sine(0.5, 440 * 2 ** (7 / 12.0))
amix = Sum(a1, a2)
aenv = RiseFall(t, 0.5)
aout = Multiply(amix, aenv)

for frame in Run(t):
print '%d:' % frame
for i in aout:
print 't%.8f' % i
```

I also refactored class Mixer; It is now known as Sum, which can now sum two or more signals. Additionally, I added the class Multiply, which takes after Sum.

# Rendering 1 Second of Audio Data

Yesterday’s python script rendered 1 frame of a rudimentary additive synthesizer. Today’s script renders 1 second. You can download the complete example at textsnip or at snipt.

To render multiple frames of audio, I added two classes: Mixer and Run.

```class Mixer:
'''A simple mixer.'''

def __init__(self, source1, source2):
self.s1 = source1
self.s2 = source2

def __iter__(self):
self.index = 0
self.iter_1 = (i for i in self.s1)
self.iter_2 = (i for i in self.s2)
return self

def next(self):
if self.index >= ksmps:
raise StopIteration

self.index += 1
return self.iter_1.next() + self.iter_2.next()

class Run:
'''Render frames over time.'''

def __init__(self, dur=1.0):
self.dur = dur

def __iter__(self):
self.index = 0
return self

def next(self):
if self.index >= (self.dur * sr) / ksmps:
raise StopIteration

self.index += 1
return self.index
```

Mixer is an iterator class that takes two iterator objects as its input. The values yielded by the inputs are summed together. Yesterday’s method was only good for one frame; A Mixer object does not have this restriction.

The Run iterator class is designed to loop through the iterator/audio graph over a user-defined duration of time, creating multiple frames of audio data.

The follow snippet of code resembles yesterday’s example as it creates a graph of two sine waves (a1 & a2) being fed into a mixer (amix.) The Run object is given a duration of 1 second, which produces 4410 frames (sr / ksmps) and 44100 samples (10 samples per frame.)

```if __name__ == "__main__":
a1 = Sine(0.5, 440)
a2 = Sine(0.5, 440 * 2 ** (7 / 12.0))
amix = Mixer(a1, a2)

for frame in Run(1.0):
print frame, ':'
for sample in amix:
print 't', sample
```

Still no output to an audio file. What it does output is a printed list of frames and samples. Here’s frame 4276:

```4276 :
0.128148365438
0.138541849949
0.14709747835
0.153592896452
0.157826911985
0.15962183375
0.158825589584
0.155313602303
0.148990404968
0.139790979215
```

BTW, I’m trying a new service, textsnip.com, for storing and sharing my scripts online. If you have a better recommendation, let me know. Update: textsnip seems to add a couple of gremlins to the autodocs, so I’m trying out snipt.org as well.