Telecom programs Oscillator and Mixer

You are to write the following two programs. Use my WavReader.java library, and look at the other programs for ideas on how to proceed. Here are the programs:

WavReader.java: my general library for reading wav files into an array of samples and writing wav files from arrays of samples. Some key methods:

• WavReader.writeWav(short [] samples, int samplerate, File outfile);    // static method; no object constructed
• WavReader wr = new Wavreader(FileInputStream fis);               // create reader object
• wr.getNumSamples():    number of samples, eg 24,000 for 3 seconds at a rate of 8000.
  
• wr.getChannels():          number of channels, usually 1
• wr.getSampleRate():      sampling rate, for us it will usually be 8000.
  
• wr.readValue(): read one sample
• wr.getShortSamples(): returns a short[] containing all the samples

Oscillator program

• frequency
• samplingrate (eg 8000)
• duration (eg 1 second; can be specified as a number of samples)
• volume (eg 16000; multiply all the sines by this to get sample levels in a reasonable range)
• output file name

Create an array of short (short []) to hold the data, fill it in with a loop (using Math.sin() to create the sine wave), and then use WavReader.writeWav() to create the file.

The formula for a sine wave of frequency f is sin(2𝜋fx). Use 𝜋 = Math.PI. Note, though, that for filling a discrete array short[] samples you will need to express the frequency f in terms of the sampling rate (because the values samples[i] are expressed in terms of the sampling rate). To make this conversion, f should be the original frequency (in Hertz) divided by the sampling rate (in samples per second). Be sure you use floating-point division! (You will also use the array index i instead of x, and multiply the sine result by the volume level.)

Those registered for graduate credit should also generate sawtooth and square waves (using separate programs is fine). Which one sounds the most musically interesting? This is easiest if you write your saw(t)/sqr(t) functions with period 1 (not 2𝜋!), in which case you will set samples[i] = saw(freq*i) (that is, without the 2𝜋). To find saw(t)/sqr(t), first, scale t so that it is in the range [0,1]: t = t - Math.floor(t). Then shift so that t is in the range [-1/2, 1/2]: if (t>1/2) t = t-1. Then saw(t) = 2*t; sqr(t) = 1.0 if t>=0, -1.0 otherwise.

To help you get started, I've given you osc_skel.java.

Mixer program

To mix WavReader objects wr1 and wr2 (constructed from the corresponding files), form the sum
      sample[i] = wr1.readValue() + wr2.readValue();
Note that you'll also have to divide by 2, the number of files, in order to avoid overrunning the 32767 16-bit-maximum-sample-value limit.

You will need to create an array of WavReader objects, one for each file, so you can read from each file in turn, and compute the above sum in a loop.

The files listed on the command line will appear in the String[] args parameter to main. The number of files is args.length, which should be 3;  the input file namess are args[0] and args[1] and the output file name is args[2].

To help you get started, most of the file I/O is done for you in mixer_skel.java.

Demo