Turn in a binaural sound file which demonstrates the auditory streaming illusion. Here's a visual analogy to the illusion you'll be dealing with:

Here are two references for FM synthesis:

• John Chowning's paper on FM Synthesis
• Introduction to FM by Bill Schottstaedt

Fire up the example hw4-starter.ck and study how it selects musical attributes (pitches, pitch register, timings, loudness, location, instrument). It plays an accelerating cycle of four pitches with everything else held constant.

The spectogram shows each note's attack and its first three harmonics

The example loops through 3 instruments while playing a cycle of 4 pitches. But out-of-the-box, the example plays 3 instruments which are absolutely identical and sound as if played with a single instrument (monophonic). There is no sense of breaking into multiple streams (polyphonic) because the streaming illusion requires different sounds which can be “grouped by similarity.”

As shipped, there is only one group of sounds and therefore only one stream. Modify your copy of the file to slightly disturb the similarity of the 3 instruments. At a faster speed in the accelerating tempo, you will begin to hear a descending pitch line as your changed sound, if distinct enough, aliases against the ascending pitch cycle. Try modifications to individual parameters and listen to whether your modifications are sufficiently salient to cause grouping:

• Loudness change one of the loud array elements... for example, 80 dB up to 100 dB, how much of a change is required before it separates into more than one voice?
• Register Transposition shift the octave of one of the tran array elements, do you get a polyphony effect?
• Location try assigning to different channels in the 4-channel space to see if source location is enough for the effect.
• Timbre experiment with FM parameters index and ratio.
• Pitch/Timbre Details can amounts of vibrato, jitter or pitch skew create the effect (or their time-varying envelopes such as amplitude, index, ratio, vibrato ADSR envelopes and skew envelope)?

Turn in an FM synthesis study which demonstrates auditory streaming. Use four channels and reduce them to binaural stereo as in homework 2 & homework 3. We'll be listening to hear the effect of monophonic alternation which blossoms into illusory polyphonic lines. Choose your own dimension(s) for the effect e.g., timbral, spatial and/or envelope qualities. In addition, make it a musical study in which the illusion is present but it's not the only interest. Concoct at least one big musical change (your choice of what that is, but it should be obvious to the listener) and shoot for a total duration of under 1 minute.

The following psuedo-code shows one way to create two contrasting sections in sequence. Here, the sections are 10 seconds long and only their pitches differ. Use your imagination to create sectional contrast in more flavors than just a single dimension. (e.g., combine pitch, rhythmic, timbral, spatial and/or envelope qualities)

// -----------------------------------------------------------
// section 1

// section 1 pitches
[ 56, 57, 58, 59 ] @=> keyn;

// when to change
10::second + now => time endOfSection1;

while (now < endOfSection1) {
  // TODO: code which makes a note from the next pitch in the 
  //       sequence in keyn[]
}

// -----------------------------------------------------------
// section 2

// section 2 pitches 
[ 66, 67, 68, 69 ] @=> keyn; 

10::second + now => time endOfSection2;

while (now < endOfSection2) {   
  // TODO: code which makes a note from the next pitch in the 
  //       sequence in keyn[]
}

The following files should be viewable via Homework Factory. Upload them to your CCRMA public web space. Note that the HTML file should be named as "hw4.html" in order to be retrieved by our script. You can name the rest of files whatever you want, but make sure that your links are active and valid.

• Stereo audio file (.wav) - binaural-encoded stereo
• ChucK source codes (.ck) - the codes you wrote
• HTML page (hw4.html) - includes links to all the files above.