Reading the pulse
A pulse wave jumps between two levels. The rate (the frequency box) says how many pulses arrive each second; the duty cycle says what fraction of every period sits on the high level. The pulse width follows from both: the period (1 ÷ rate) times the duty cycle. The card prints both under the frequency, so 1 kHz at 12.5% reads “Period 1.00 ms · pulse width 125 µs”.
The duty cycle also decides which harmonics are missing. A pulse that is high for 1/n of its period has no energy at every nth harmonic: 25% skips the 4th, 8th and 12th, 12.5% skips the 8th and 16th, and 50% skips every even one, which is why the square sounds hollow. Narrow pulses keep many strong upper harmonics but carry less energy overall, so they sound thin and quieter at the same volume setting.
What to use it for
- Chiptune voices. The NES sound chip’s two pulse channels offer exactly four duty settings: 12.5%, 25%, 50% and 25% negated (75%), as documented on the NESdev wiki’s APU Pulse page. The buttons above set each one at 1 kHz; type a note such as E5 into the card to hear it at a musical pitch.
- Hearing PWM. Start a tone, then drag the slider slowly between about 10% and 50%. That moving timbre is pulse width modulation, the effect behind many analog synth string and pad sounds.
- Checking audio gear on a scope. Feed a 1 kHz pulse into an amplifier or interface and look at the output: rounded edges point to limited high-frequency bandwidth, a tilted top to the low-frequency roll-off of its coupling capacitors.
- Slow ticks. 1 Hz at 1% gives a short tick once a second, 10 Hz gives ten a second.
Where it stops being a lab instrument
This is a pulse wave played through a sound output, and that sets its limits. Most audio interfaces use AC-coupled outputs, where a capacitor filters out the extreme low frequencies (Sound On Sound and RME both describe this; only some interfaces have DC-coupled outputs). So the wave has no DC offset: it swings around zero instead of from 0 V up to a logic level, and at a few hertz its flat parts droop on a scope. The edges are band-limited to keep the sound free of aliasing, so the rise time depends on the sample rate and the top of each edge shows a small overshoot. The voltage follows your volume setting and headphone amplifier, so measure it. There is no sync output, single-shot or burst mode. For driving logic, timing circuits or anything that needs a defined voltage, use a bench pulse or function generator.
Saving it
Download as WAV renders the pulse with the duty cycle you set, scaled so its highest sample lands on the peak level you pick. The file name carries the rate and duty (for example tone-1000Hz-pulse25-5s.wav), and Share result links to the same rate and duty cycle.