CSE1710 Lab

CSE 1710.03A Programming for Digital Media

Lab 08

Due date: Nov 9, 2009 at 20:00

Making an "old" sound

For this lab, you have to modify a sound file with two effects to make it sound "old", like in old recordings. The first effect is to add some noise to the sound, which is one characteristic of old recording and playback technologies. The second effect is to modulate the volume of the sound to vary in a rythmic pattern. This is to simulate how old record players played a vinyl record.

Your starting point is a function that changes the volume of a sound by a constant factor.

def changeVolume(sound):
  len = getLength(sound)
  for sampleIndex in range(0,len):
    value = getSampleValueAt(sound,sampleIndex)
    setSampleValueAt(sound,sampleIndex,value * 1.1)

You can test this function as usual:

>>> filename = pickAFile()
>>> sound = makeSound(filename)
>>> decreaseVolume(sound)
>>> play(sound)

Now modify this function to add a uniformly distributed random number between -1000 and +1000 to each sample value. For this, you have to use the random number generator provided by Python. The documentation for this module can be found in the Python documentation. Start by searching for "random" on that page and follow the link. On the page documenting the facilities of the random number generators, search for "uniform". First, add the statement
```
import random
```
at the very top of your program, before the defintion of the function(s). Then use random.uniform(....) with appropriate arguments to generate a random noise value for each point in time. Simply add the result of the random generator to each sample to generate the desired effect.
The second step is to vary the volume over time. For this lab, we are going to use a sinusoid, i.e. a sine function, to vary the volume, with a cycle length of one cycle per second. To generate that, you will need to use the number of samples per second, as returned by getSamplingRate(). Details for the sine function in Python are again available in the Python documentation, under "Mathematical functions".
- The basic idea is to use the sample index as the argument to the sine function. However, as the sine function has a much shorter cycle length than desired (once every 2*π, instead of the number of samples per second), you will need to lengthen the cycle to one per second by first dividing the sample index by the number of samples per second and then multiplying that by 2*π. math.pi is a predefined constant that contains a very accurate definition of π.
  One of the little "gotcha's" you may encounter is that you need to make sure that the result of the division is a floating point value. This has been discussed in a previous lab.
- Assign the result of the sine function, as modified above, to a variable called "factor" inside the loop where you process the samples. Then multiply the sample value at each point in time with that factor, after you have added the noise to the sample value.
- If you play the sound now, you will see that the sound can fade completely, as the sine function can yield zero.
- To fix this, first multiply the result of the sine function with 0.5 and then add a constant of 0.75 to the factor (before it gets multiplied by the sample value).

Here are a couple of sample sounds files. You have to right-click and select "Save Link As..." to download them.

An example how the result should sound can be found here (for the second sound from the list above).

Lab Questions and What to Turn in

As mentioned previously, you have to add comments with your own identification (name, student ID, date). Remember to save the file after you modify it!

How to submit the lab

Exactly like in lab02, you hand in this lab the submit command on the Linux syst ems that are behind the PRISM lab infrastructure. For details on how to submit, please refer to lab02. However, this time please submit to lab08, i.e. issue the command

    submit 1710 lab08 lab08.py

Note: You must do all the above steps correctly for receiving full credit for this lab.