The Roland TR-808 bass drum circuit gets analyzed constantly. What gets less attention is the hi-hat, which uses a completely different approach and contains a small surprise: the closed hi-hat, the open hi-hat, and the cymbal all share the same source.
The Problem With Metallic Sounds
Real cymbals produce inharmonic spectra. A harmonic sound, like a plucked string or a blown horn, has overtones at integer multiples of the fundamental: 100 Hz, 200 Hz, 300 Hz, 400 Hz. The spacing is even and mathematically clean.
A cymbal plate does not behave this way. The vibration modes of a thin bronze disc with a raised central dome are governed by the physics of circular plates, described mathematically by Bessel functions. The frequencies at which those modes occur do not fall on a harmonic series. They are incommensurate: no two of them have a ratio that is a simple fraction.
The density of these inharmonic partials, spread across a wide frequency range, is what makes a cymbal sound metallic rather than tonal. To approximate this with oscillators, you need multiple oscillators tuned to frequencies that avoid simple integer ratios. The more oscillators, and the more carefully the ratios are chosen, the more convincingly metallic the result.
Roland chose six.
One Chip, Six Oscillators
The TR-808 hi-hat and cymbal circuit uses a single HD14584 hex Schmitt trigger inverter IC. This chip contains six inverter gates. Each gate, paired with one resistor and one capacitor, forms an astable multivibrator.
An astable multivibrator built from a Schmitt trigger inverter works like this. The inverter output feeds back to its input through an RC network. When the output is high, the capacitor charges through the resistor until the voltage at the input rises above the gate's upper switching threshold. The output then flips to low. The capacitor discharges until the voltage drops below the lower threshold, and the output flips back to high. This process repeats continuously, producing a square wave.
The frequency of oscillation is set by the RC time constant. With the HD14584 operating at 5 volts, each oscillator produces a square wave with a duty cycle of approximately 47.98% and an amplitude of 5 volts.
Six RC networks. Six time constants. Six oscillators. All on one chip.
The Specific Frequencies
The six oscillators are tuned to these frequencies:
- 205.3 Hz
- 304.4 Hz
- 369.6 Hz
- 522.7 Hz
- Approximately 540 Hz (factory-adjusted via trimpot TM1, range 254 to 627 Hz)
- Approximately 800 Hz (factory-adjusted via trimpot TM2, range 359 to 1150 Hz)
The first four are fixed by permanently soldered resistor and capacitor values. The last two have small trimmer potentiometers on the circuit board, used at the factory to calibrate the frequencies to their target values.
What matters is the ratio structure. A harmonic series starting at 205.3 Hz would place the next partials at 410.6 Hz, 615.9 Hz, 821.2 Hz, and so on. The 808's oscillators instead land at 304.4 Hz, 369.6 Hz, 522.7 Hz, 540 Hz, and 800 Hz. The ratios relative to the lowest frequency are approximately 1.00, 1.48, 1.80, 2.55, 2.63, and 3.90. None of these are clean integers.
This inharmonicity is not accidental. The 808's cowbell voice uses two oscillators tuned to 800 Hz and 540 Hz, the same two adjustable frequencies used in the cymbal circuit. The cowbell is also designed to sound metallic, and the designers applied the same principle to produce that character.
Two Bandpass Filters
The outputs of all six oscillators are mixed through a passive resistor network. The combined signal then passes through two bandpass filters.
Bandpass filter 1 has a center frequency of approximately 3440 Hz. Bandpass filter 2 has a center frequency of approximately 7100 Hz.
Square waves are harmonically rich. A square wave at 205 Hz contains energy at 205 Hz, 615 Hz, 1025 Hz, 1435 Hz, and up through all odd multiples. Six square waves at six different base frequencies produce a dense harmonic spectrum with energy distributed across hundreds of partials.
The two bandpass filters suppress the low-frequency fundamental content, where the oscillator base frequencies live, and emphasize the upper range around 3 to 8 kHz. After filtering, there is very little energy below 1 kHz and significant energy in the range that gives the hi-hat its characteristic tick and sizzle. The exact spectral texture in that range, shaped by the intermodulation of six inharmonic frequencies, is what produces the metallic quality.
Three Voices, One Source
Here is what surprises most people. The closed hi-hat, the open hi-hat, and the cymbal all come from the same six oscillators.
After bandpass filtering, the signal branches into separate envelope generators and VCAs, one for each voice. The differences are purely in the VCA decay times.
The closed hi-hat has a fixed decay time of approximately 50 milliseconds. There is no front-panel control. The VCA opens briefly and closes, producing a short metallic tick.
The open hi-hat decay is adjustable from approximately 90 to 600 milliseconds. The longer the VCA stays open, the more of the oscillator mixture you hear before it closes.
The cymbal has an adjustable decay from approximately 350 to 1200 milliseconds. The decay control specifically affects the VCA on the 3440 Hz bandpass filter path, independently controlling how long the lower-frequency component of the sound sustains.
Three distinct sounds. Zero additional oscillators.
What the 909 Did Instead
The Roland TR-909, released in 1983, faced the same metallic synthesis problem and took a different path. Its hi-hats and cymbals are 6-bit PCM samples of real cymbals, run through analog envelope and VCA circuits.
The 909's metallic sounds are more realistic in the obvious sense: they begin as recordings of actual cymbals. But they are also fixed at the exact characteristics of those samples. You cannot retune the oscillators, alter the bandpass filter frequencies, or adjust the spectral balance of the metallic content independently.
The 808's synthesized approach offers more flexibility precisely because it is built from adjustable components. A large part of the 808 modification and circuit-bending culture targeted the cymbal circuit: replacing resistors to change the oscillator frequencies, modifying the bandpass filter components to shift the spectral emphasis, or altering the decay curves. The 909 has far fewer accessible parameters in its metallic voices because those sounds live in sample ROM rather than in analog components.
Digital Emulation and the Werner Paper
In 2014, Kurt Werner, Jonathan Abel, and Julius O. Smith from Stanford CCRMA published "The TR-808 Cymbal: A Physically-Informed, Circuit-Bendable, Digital Model" at the International Computer Music Conference. The paper analyzed the cymbal voice circuit at the component level, confirmed the oscillator frequencies and filter parameters described here, and implemented a working digital model in Cycling 74's Gen.
Their analysis also documented nonlinear behavior at the switching transitions of the Schmitt trigger oscillators. The HD14584's upper and lower thresholds are not perfectly symmetric, and the RC charge and discharge curves are exponential rather than linear. These properties contribute to the sonic character in ways that a naive model of six ideal square waves does not capture.
This is why 808 emulations built from circuit models can sound different from emulations that use a sample of a real 808 hi-hat, even when both are accurate in their own terms. A sample captures what a specific unit produced on a specific day. A circuit model describes how the circuit behaves over its full parameter range.
Understanding the difference between these two approaches matters for anyone using or developing 808 emulations. The synthesized nature of the original circuit is not a limitation compared to using a sample. It is the feature that made the sound modifiable in the first place.