Audio Modulation Generator

Amplitude Modulation Explained

AM varies carrier amplitude while leaving carrier frequency fixed. The conceptual equation is output(t) = A × [1 + m × sin(2πfmt)] × sin(2πfct), where fc is carrier frequency, fm is modulation frequency, and m is depth from zero to one. This modulation generator uses A equal to half the selected output gain, reserving enough headroom for the envelope maximum at full depth.

At zero depth, the output is an unmodulated sine. At 50%, the envelope moves between half and one-and-a-half times A. At 100%, it reaches zero and twice A without reversing polarity. Depth above 100% would create overmodulation, where the envelope can become negative; this first version deliberately caps the control at 100%.

Frequency Modulation Explained

FM varies instantaneous frequency while keeping the nominal amplitude constant. Here, instantaneous frequency equals fc + Δf × sin(2πfmt). The deviation Δf is the distance in hertz from the carrier to either extreme. A 440 Hz carrier with ±50 Hz deviation moves between approximately 390 and 490 Hz. A 5 Hz modulator repeats that excursion five times each second.

The oscillator accumulates phase continuously as frequency changes. Simply substituting a changing frequency into an unrelated sine calculation would give the wrong phase trajectory. The modulation generator uses Web Audio frequency-parameter modulation for that integration. Setting deviation to zero produces a stable sine carrier regardless of the modulator rate.

Carrier Frequency vs Modulation Frequency

Carrier frequency sets the central reference; modulation frequency sets the speed of change. Raising a 1000 Hz carrier to 2000 Hz moves the reference higher. Raising a 5 Hz modulation rate to 10 Hz makes the modulation repeat twice as fast. Neither action is the same as increasing modulation depth or deviation. The signal diagram shows these separate roles using the current settings.

Modulation Depth in AM

Depth is a relative amplitude ratio, expressed as a percentage. It does not specify how loud your speakers will be. With the default 10% output level and 50% depth, the AM envelope ranges from 0.025 to 0.075 digital amplitude. At 100% depth it spans zero to 0.10. Keeping this fixed headroom makes depth comparisons predictable without clipping the generated samples.

For a sine carrier whose frequency is above the modulation rate, ordinary AM creates sidebands at carrier minus rate and carrier plus rate. A 1000 Hz carrier with a 100 Hz modulator therefore has components at 900, 1000, and 1100 Hz. Their relative magnitudes depend on depth. With lower modulation rates, the sidebands may be too close for a small FFT to resolve.

Frequency Deviation in FM

Deviation is an absolute frequency distance, not a percentage. The ratio Δf/fm is the FM modulation index. Increasing that ratio generally makes more sidebands significant, though individual sidebands can rise and fall. A wider instantaneous range is not the same as a spectrum with only two outer peaks. FM can contain many sidebands spaced by the modulation frequency.

This modulation generator rejects a range that reaches zero or Nyquist, half the sample rate. It also checks an approximate upper significant band using carrier + deviation + rate when deviation is nonzero. That practical guard reduces risky combinations but is not a mathematical guarantee of zero aliasing: FM has infinitely many theoretical sidebands, with most becoming very small.

Slow Modulation vs Audio-Rate Modulation

Rates from 0.1 to 1000 Hz cover slow low-frequency-oscillator behavior and audible modulation. A 0.1 Hz cycle takes ten seconds, so the short waveform window cannot show its entire envelope at once. Around a few hertz, AM resembles tremolo and FM resembles vibrato. At higher rates the ear increasingly hears combined spectral components as timbre rather than individual pulses.

Listening to AM and FM Effects

Begin with a 1000 Hz AM carrier and reduce depth to zero before increasing it slowly. Then switch to FM and compare ±10 Hz with ±50 Hz around 440 Hz. Keep the listening level unchanged during each comparison. The canvas displays actual output samples, with each screen column retaining local sample extremes. Its fixed amplitude scale makes AM envelope changes visible without auto-normalizing them away.

For a spectral comparison, export the modulation generator output and load it into the analyzer. A 100 Hz modulator gives more widely separated sidebands than a 5 Hz modulator. The first and last 15 milliseconds are faded to reduce clicks; analyze the middle of the file when you want a sustained example.

Use the Audio Spectrum Analyzer to view the exported file and choose suitable FFT resolution.

Common Audio and Educational Uses

The modulation generator can illustrate synthesizer building blocks, compare tremolo and vibrato concepts, and supply bounded examples for signal-processing lessons. In an audio or electronics demonstration, document the carrier, modulation rate, depth or deviation, output gain, and sample rate. These settings make the digital example reproducible; they do not calibrate the connected amplifier or loudspeaker.

Audio Modulation vs Radio Transmission

The letters AM and FM describe modulation methods, which are used in many fields. This page is an audio-frequency demonstration and generator. It creates samples for your browser audio output or a WAV file. It does not transmit radio-frequency signals, broadcast a radio station, control wireless spectrum, or turn your device into regulated communications equipment.

Digital Signal and Speaker Limitations

A valid digital frequency may still be poorly reproduced by your playback system. Device resampling, speakers, headphones, and operating-system effects can alter what you hear. High modulation rates or wide FM deviation can create components near the digital bandwidth limit. The modulation generator uses sine carriers to keep examples simple; it offers no square-wave or sawtooth mode with additional carrier harmonics.