Online Oscilloscope
Turn your browser into a two-channel audio oscilloscope. View a microphone, line input or audio file in real time with time/div and volts/div controls, a stable trigger, XY mode for stereo Lissajous figures and automatic frequency and amplitude readings. A built-in signal generator lets you try it right away.
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Showing the built-in test signal. Start the microphone or open a file to view real audio. Nothing is recorded or uploaded.
Vertical units are fractions of the input's digital full scale (±1.0), not calibrated volts. Only connect normal audio signals to your computer's inputs.
How to use the Online Oscilloscope
- Look at the test signal that runs on load, or press Start microphone and allow access, or choose Open audio file.
- Set time/div so one to three cycles fill the screen, and volts/div so the wave is about half the screen high.
- Adjust the trigger: move the level slider into the wave and pick the rising or falling slope until the trace stands still.
- Read the measurements under the screen: frequency, period, peak-to-peak and RMS for each channel.
- Switch to XY mode to compare left and right, press Freeze to hold a trace, and Save PNG to keep a picture.
Reading the screen
The display is a 10 × 8 division graticule, just like a bench oscilloscope. Everything you measure by eye comes from counting divisions:
- Time across the screen = 10 × time/div. At 1 ms/div the screen spans 10 ms.
- Period = divisions between two matching points × time/div. Frequency = 1 ÷ period. A cycle that spans 2.3 divisions at 1 ms/div has a period of 2.3 ms, so f ≈ 435 Hz.
- Peak-to-peak = vertical divisions from the lowest to the highest point × volts/div.
- RMS of a sine = peak ÷ √2 ≈ 0.707 × peak. For a square wave RMS equals the peak; for a triangle or sawtooth it is peak ÷ √3 ≈ 0.577 × peak.
The automatic readings below the screen are calculated from the full capture buffer (about 0.7 seconds of audio), so they are usually more precise than counting divisions.
Choosing time/div for a signal
| Signal frequency | Period | Good time/div (about 2–4 cycles on screen) |
|---|---|---|
| 50–60 Hz hum | 16.7–20 ms | 5 ms |
| 100 Hz | 10 ms | 2–5 ms |
| 440 Hz (A4) | 2.27 ms | 0.5–1 ms |
| 1 kHz test tone | 1 ms | 0.2–0.5 ms |
| 5 kHz | 0.2 ms | 0.1 ms |
| Speech or music envelope | varies | 10–50 ms |
At very short time/div settings only a few dozen samples cross the screen (48 samples per millisecond at 48 kHz), so waveforms above roughly 5 kHz look angular: that is the sample rate showing, not distortion in your signal.
Lissajous figures in XY mode
With two sine waves, the figure tells you their frequency ratio and phase. Imagine a box around the figure and count where the curve touches it: f(CH1) ÷ f(CH2) = touches on a side edge ÷ touches on the top edge. A sideways figure eight touches the top twice and each side once, so CH2 runs at twice the frequency of CH1.
| CH1 : CH2 frequency | Phase difference | Shape |
|---|---|---|
| 1 : 1 | 0° | Diagonal line rising to the right |
| 1 : 1 | 90° | Circle (ellipse if volts/div differ) |
| 1 : 1 | 180° | Diagonal line falling to the right |
| 1 : 2 | 0° | Figure eight lying on its side (∞) |
| 1 : 2 | 90° | Single arch shaped like a parabola |
| 2 : 3 | any fixed value | Closed pretzel-like curve with 2 and 3 lobes |
For a 1 : 1 ellipse, the phase can be found from sin φ = B ÷ A, where A is the full height of the ellipse and B is the height where it crosses the vertical centre line. Try it with the test generator: set CH2 phase to 30° and you will measure B ÷ A ≈ 0.5.
Tips for clean, stable traces
- Turn off voice processing. The tool requests raw audio, but some systems still apply noise suppression or automatic gain; disable "audio enhancements" in your OS sound settings for accurate shapes.
- Use Normal trigger mode for occasional events such as a hand clap: the last captured event stays on screen until the next one crosses the level.
- Avoid clipping. If the tops of the wave are flat at ±1.0, reduce the input gain in your system settings.
- Safety first. A computer's mic and line inputs accept only small audio-level signals. Never connect them to mains wiring, chargers, car electrics or any circuit you are unsure about.
Want to see the frequency content instead of the waveform? Use the Audio Spectrum Analyzer. To produce a clean reference signal from another device, the Tone Generator is handy.
Frequently asked questions
Can this measure real voltages?
No. A sound card measures signal level relative to its digital full scale, so the vertical axis here is in full-scale units (1.0 = the largest value the input can record). The real voltage depends on your input gain and hardware. Never connect anything except normal audio signals to a computer's mic or line input, and never mains or high-voltage circuits.
What frequency range can a browser oscilloscope show?
It is limited by the sound card: usually about 20 Hz to 20 kHz at a 44.1 or 48 kHz sample rate. Many inputs block DC and very low frequencies, and microphones roll off at both ends. For radio-frequency or logic signals you need a hardware oscilloscope.
What do time/div and volts/div mean?
The screen is divided into 10 horizontal and 8 vertical divisions. Time/div is how much time one horizontal division represents, so the whole screen shows 10 × time/div. Volts/div (here, full-scale units per division) sets how much signal one vertical division represents.
Why does the trace keep moving?
The trigger is not finding a clean edge. Move the trigger level so it sits inside the waveform (between its minimum and maximum), try the other slope, or select the channel that carries the signal. In Normal mode the screen only updates when a trigger occurs; Auto mode free-runs when no trigger is found.
What is XY mode used for?
In XY mode the left channel moves the dot horizontally and the right channel vertically. Two sine waves make Lissajous figures whose shape reveals their frequency ratio and phase: a diagonal line means in phase, a circle means 90° apart. With music it shows stereo width: a vertical-ish line is mono, a wide cloud is wide stereo.
How is the frequency measured?
The tool finds the moments where the signal crosses its average level going upward (with a little hysteresis to ignore noise), interpolates between samples and divides the number of complete cycles by the time they take. It is very accurate for steady periodic signals and less reliable for noise, speech or chords.
Why is channel 2 flat when I use the microphone?
Most built-in microphones are mono, so only channel 1 receives a signal. Use a stereo line input or audio interface to see two different channels, or open a stereo audio file.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.