Lesson 31 · Capstone: synthesize, shape, write, read

Capstone: synthesize, shape, write, read

The finale drives the whole toolkit end to end - synthesize two tones, shape one with gain and a fade, mix them, write a stereo 16-bit WAV, and read it back to confirm the exact samples and header. Every chapter proves itself at once.

The goal

Run the full pipeline and assert the round-tripped stereo samples and format exactly.

Start here - the target

TO DO

Scenario: The full pipeline round-trips a shaped stereo signal

Givena sine (1000 Hz, amp 10000, sr 4000, 4 samples) = [0,10000,0,-10000] and a sawtooth (1000 Hz, amp 8000, sr 4000, 4 samples) = [-8000,-4000,0,4000]

Whenthe sine is gained by 0.5 then faded out (factor (N-n)/N) to make the left channel, the right channel is that left mixed with the sawtooth, and the stereo pair is written to a 16-bit 4000 Hz WAV and read back

Thenthe read-back left channel is [0, 3750, 0, -1250] and the right channel is [-8000, -250, 0, 2750]

Andthe read-back Format is numChannels 2, sampleRate 4000, bitsPerSample 16, blockAlign 4, byteRate 16000, with RIFF size 52 and data size 16

Background

This is the promise the project was built to keep: a real WAV/PCM toolkit that makes sound, shapes it, writes it, and reads it back byte for byte. The pipeline touches every chapter. Synthesis builds a sine and a sawtooth. Sample math shapes them: the sine is halved by gain and tapered by fadeOut to [0, 3750, 0, -1250], which becomes the left channel; mixing that with the sawtooth (clamped, though nothing here overflows) gives the right channel [-8000, -250, 0, 2750].

Then the codec closes the loop. WriteSamples interleaves the two channels, encodes them 16-bit, and wraps them in a valid file whose data size is 16 and whose RIFF size is 52; ReadSamples walks it back to the exact two channels and the exact Format, with blockAlign 4 and byteRate 16000 derived correctly. Every value is reproducible, no audio hardware or sample file involved. From four ASCII bytes read off a header in lesson 1, you have built a genuine audio library - a RIFF/WAVE reader and writer, a five-format PCM decoder, an interleaver, and a bank of DSP and synthesis tools - the honest core that libraries like libsndfile wrap around. That is a real toolkit, and it is yours.

Make it work

left := fadeOut(gain(sine(1000, 10000, 4000, 4), 0.5))  // [0,3750,0,-1250]
saw := sawtooth(1000, 8000, 4000, 4)                     // [-8000,-4000,0,4000]
right := mix(left, saw)                                  // [-8000,-250,0,2750]
f := Format{NumChannels: 2, SampleRate: 4000, BitsPerSample: 16}
file := WriteSamples(f, [][]int{left, right})
gotF, chans, _ := ReadSamples(file)
// chans[0]==left, chans[1]==right, gotF==f (byteRate 16000, blockAlign 4)

CheckpointDONE

The whole toolkit runs end to end and round-trips exactly. The project is complete; commit and stop here.