Add a simpletest: a synthio chord saved as a WAV

docs/get-started/first-program.md

@@ -72,6 +72,11 @@ That renders a block from a source; it doesn't select a speaker. A host can
 pull the graph for offline rendering, give it to a mixer or a pump, or
 attach it to a platform-specific audio output.
 
+[`examples/audiodsp_simpletest.py`](../../examples/audiodsp_simpletest.py)
+takes it to something you can hear: a chord through a low-pass, pulled
+block by block into a WAV file. It runs unchanged on MicroPython,
+CircuitPython and CPython.
+
 ## Next
 
 - [Play computed audio](../guides/play-computed-audio.md) with `audiopump.Ring`.

examples/audiodsp_simpletest.py

@@ -0,0 +1,63 @@
+# SPDX-FileCopyrightText: 2026 Brad Barnett
+#
+# SPDX-License-Identifier: MIT
+"""Play a chord on synthio, filter it, and save it as a WAV file.
+
+A synthio Synthesizer plays three notes, an audiofilters low-pass softens
+them, and the program pulls the result one block at a time and writes it
+to a WAV file you can open in any audio player. No speaker is needed.
+
+Needs: audiodsp, built into MicroPython or installed on CPython with
+pip install pydevices-audiodsp. CircuitPython has these modules built in.
+
+Run it:
+    micropython audiodsp_simpletest.py
+
+Tested on: the unix port of MicroPython, the unix port of CircuitPython,
+and CPython 3.12 on Linux.
+"""
+
+import struct
+
+import audiocore
+import audiofilters
+import synthio
+
+# Change these to match your setup.
+FILENAME = "chord.wav"
+NOTES = (60, 64, 67)  # MIDI note numbers: C, E and G
+SAMPLE_RATE = 22050
+SECONDS = 2
+CUTOFF = 1200  # Hz; lower it for a darker sound
+
+# A synthesizer with a gentle attack and release.
+envelope = synthio.Envelope(attack_time=0.05, release_time=0.5)
+synth = synthio.Synthesizer(sample_rate=SAMPLE_RATE, envelope=envelope)
+
+# A low-pass filter that takes the synthesizer as its input.
+lowpass = synthio.Biquad(synthio.FilterMode.LOW_PASS, CUTOFF)
+effect = audiofilters.Filter(filter=lowpass, sample_rate=SAMPLE_RATE)
+effect.play(synth)
+
+# Write the WAV header now; the data size is filled in at the end.
+out = open(FILENAME, "wb")
+out.write(b"RIFF\0\0\0\0WAVEfmt ")
+out.write(struct.pack("<IHHIIHH", 16, 1, 1, SAMPLE_RATE, SAMPLE_RATE * 2, 2, 16))
+out.write(b"data\0\0\0\0")
+
+# Hold the chord, then let it go for the last half second.
+synth.press(NOTES)
+size = 0
+while size < SECONDS * SAMPLE_RATE * 2:
+    if size >= (SECONDS - 0.5) * SAMPLE_RATE * 2:
+        synth.release_all()
+    status, block = audiocore.get_buffer(effect)
+    size += out.write(block)
+
+# Go back and fill in the two sizes.
+out.seek(4)
+out.write(struct.pack("<I", 36 + size))
+out.seek(40)
+out.write(struct.pack("<I", size))
+out.close()
+print("Wrote %d samples to %s" % (size // 2, FILENAME))