808 Cowbell Synthesis

Percussion808
Written By Peter Barata

Oscillators

The 808 cowbell uses two Schmitt trigger based oscillators as its sound source. A Schmitt trigger is a comparator that flips its state when the incoming voltage passes its upper/lower thresholds. It outputs either a high or low signal. These oscillators work by creating a continuous RC (resistor-capacitor) charge-discharge cycle. A low voltage capacitor causes the Schmitt trigger inverter to output a high state, which charges the capacitor through a resistor. Once the capacitor’s voltage crosses the inverter’s upper threshold, the output flips low, draining the capacitor until it hits the lower threshold, where the cycle repeats. When quickly switched, this creates a square wave oscillator. The frequency is determined by the capacitor and resistor values. You can read more about how these oscillators work on page 9 of the Erica Synths DIY HiHat manual.

The 808 uses an array of six Schmitt trigger based oscillators to create the cymbal and hi-hat’s metallic noise. The cowbell taps two of these.  They are tuned to an inharmonic relationship: 800 Hz (G5 + 35 cents) and 540 Hz (C#5 -45 cents). This is a detuned perfect fifth. Because their frequencies aren’t exact multiples, they beat against each other, creating the complex dissonance we associate with metallic instruments.

Envelope:

The 808 cowbell has a two-stage decay envelope. This vaguely mimics a real bell. There is a fast (though not quite immediate) initial transient snap, followed by a sustained clangorous ringing tail.

The envelope follows a standard R/C discharge slope. However, the tail has some sustain added. The envelope’s behaviour is shaped by two capacitors of different values discharging simultaneously. Each has its own discharge time. The first lasts approximately 50ms (C9, R28/R29), dropping the volume by half. This is the cowbell’s initial clunk. This is followed by a more sustained decay, lasting about 500ms (C34, R82). The envelope has an exponential curve, however, the accent level changes the slope of the decay, though the overall shape remains the same.

VCA

Each oscillator feeds into its own VCA. Both VCAs are modulated by the same envelope.

The cowbell uses two of the same ‘swing type’ VCAs Roland used throughout the 808 metal noise based sounds. The service manual states that the ‘swing type’ VCA is designed to produce extra high harmonics that will accentuate the metallic-ness of the sound. It’s a simple VCA circuit with high gain. This causes higher volume signals to generate a lot of fizzy drive/distortion. Watch the Moritz Klein video about the Erica Synths EDU DIY Hi-Hat for a good explanation of how it works.

The swing type VCA is a key part of the cowbell’s sound. It provides the 808 character. It gives the bite.

Bandpass Filter:

At this point it sounds like a cowbell, but if you A/B with an 808 it’s still off. The tone isn’t quite right. The bandpass filter finishes the sound. It’s used to cut out the lower harmonics and highest fizz.

The output from both VCAs are mixed together into an asymmetrical bandpass filter that cuts the low frequencies more harshly than the higher ones. It looks like a 6 dB highpass combined with a 3 dB lowpass. The filter is fairly gentle and its more about shaping than cutting the sound.

The bandpass filter has a centre frequency of approximately 880 Hz (A5). The higher cutoff is at approximately 977 Hz (B5 minus 19 cents). The lower cutoff is at approximately 794 Hz (G5 plus 22 cents). Notice how the centre frequency is just above the frequency of the higher oscillator (880 vs 800 Hz), and the lower cutoff is right on the same frequency as the higher oscillator. This will cause the 800 Hz oscillator to sound more prominent in the mix than the 540 Hz oscillator.

The filter has a moderately-high/narrow resonance. Keep the resonance low enough that it doesn’t screech or squeal, but high to feel resonant and add some character. Because the centre frequency is close to the frequency of the 800 Hz oscillator, the resonant peak will again accentuate that oscillator more.

Buffer:

The buffer amplifies the signal and sets its maximum level.

Roland CR-5000/8000 Cowbell:

The cowbell on the CR-5000 and 8000 use a different sound source, but are otherwise the same as on the 808. The CR-5000/8000 creates two square wave oscillators out of NAND gates. The oscillators are tuned to 800 Hz (G5 +35 cents), and 555 Hz (C#5 +2 cents).

Further Reading

Peter Barata
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808 Cowbell Bandpass Filter Frequency