CR-5000 Cymbal, Hi-Hat, and Cowbell Synthesis
The Roland CR-5000 and 8000 were preset drum machines released in 1981, one year after the 808. Compared to the 808 they have simpler circuit designs, and almost no control of the sounds. The 5000 was the budget option, while the 8000 included additional features such as tempo sync, more presets, and the ability to program 8 custom patterns. Both machines share identical voice circuits.
The primary design brief of the CR-5000 appears to have been simplification rather than innovation. According to the service manual, most of the CR-5000/8000’s voices were directly based on the 808’s, with the exception being the cymbal, hi-hat, and rimshot. One significant change was new tunings for the hex Schmitt trigger based oscilators. The 808 generated square waves at the following frequencies: 800 Hz, 540 Hz, 522.7 Hz, 369.6 Hz, 304.4 Hz, and 205.3 Hz. Whereas the CR-5000 uses: 794 Hz, 649 Hz, 524 Hz, 444 Hz, 368 Hz, and 283 Hz.
The manual refers to signals being “gated at choppers” and shows “gates” in the block diagrams. My understanding is that a chopper is a switch, but the circuit diagram shows the same transistor-based VCAs found in the 808. In my block diagrams I’ve change ‘gate’ to ‘VCA.’
CR-5000 Cymbal Circuit:
The CR-5000 cymbal is significantly simplified from the 808. Where the 808 employs 3 envelopes, 3 VCAs, and 5 filters (2x BPF, 3x HPF), the CR-5000 uses just 1 envelope, 2 VCAs, and 4 filters (2x BPF, 2x HPF).
The circuit begins by mixing all outputs from the hex Schmitt trigger in equal proportions to create a metallic hum.
The signal is then shaped with a bandpass filter. I calculated the approximate frequency for the bandpass filter using the following formula from page 5 of the 808 service manual (Figure 11): f = 1000000/(2π√(82,000 * 680 * 0.0033 * 0.0033)) = 6,458.695 Hz. The result is only approximate, but is good enough for replicating a close-enough copy of the sound on a synth.
The signal then splits into two parallel paths, each feeding a separate VCA. Both VCAs are modulated by a common decay-only envelope. The first path routes through a high-pass filter, creating some brightness and shimmer. The second path is the main body of the sound. It first passes through a second bandpass filter and then a high-pass filter. Running the same formula as above, the centre frequency of this bandpass filter is: 1000000/(2π√(82,000 * 560 * 0.0068 * 0.0068)) = 3,453.904 Hz.
Both paths are then summed together. While simpler than the 808’s triple-path design, still provides a convincing enough cymbal voice.
CR-5000 Hi-Hat Circuit:
The CR-5000’s hi-hat is also aggressively streamlined. The 808 has 2 independent open and closed hi-hats, whereas on the CR-5000 both hi-hats share the same signal path.
The signal begins identically to the cymbal, mixing all Schmitt trigger outputs in equal amounts, and then going through the same bandpass filter. The filtered sound then goes into a single VCA. The open and closed hi-hat each trigger their own envelope, but both envelopes modulate the same shared VCA. The closed hi-hat has a decay time of 74 ms. The open hi-hat has a decay time of 300 ms. After the VCA, a high-pass filter removes additional low-frequencies.
CR-5000 Cowbell Circuit:
Of the metal percussion voices, the CR-5000’s cowbell is closest to the 808’s. They have different sound sources, but otherwise, are the same.
The 808 extracts 2 square waves from its hex Schmitt trigger. The CR-5000 builds 2 square wave oscillators using NAND logic gates configured similar to a bridged-T network. However, these oscillators run continuously without requiring a trigger pulse to excite them. The CR-5000’s oscillators are tuned to 800 Hz and 555 Hz. 808 frequencies are: 800 and 540 Hz.
The rest of the signal path is the same as on the 808. The 2 oscillators pass through independent VCAs modulated by a common envelope, then summed and passed through a bandpass filter. I assume the bandpass filter has the same 3440 Hz centre frequency as the 808’s cowbell because the component values in both circuits are the same.
CR-5000 Rimshot Circuit:
The CR-5000 rimshot is also dramatically simplified.
The 808 rimshot’s sound source is 2 bridged-T oscillators. This is a pretty complex setup for such a short “tick” sound. The CR-5000 avoids this by just taking 2 square waves (CP5/368 Hz and CP6/283 Hz ) out of the hex Schmitt trigger array. They are summed together, then go through a VCA which is modulated by a short 30ms envelope. A high-pass filter is then used to make the rimshot brighter and more cutting.
Further Reading
Pg. 4 has a brief description of the voice circuits.
Pg. 10 has the circuit diagram (Pg. 19 has a revised board).
Pg 13 has a block diagram.
Pg. 12 has a chart displaying the various pitch and decay times.
Pg. 6 has a brief description of the various voice circuits.
Pg. 7 has a block diagram.
Pg. 9 has the circuit diagram of the rimshot (bottom right).
Pg. 13 has the circuit diagram for the cymbal, hi-hats, and cowbell.
Pg. 14 has a chart displaying the various frequencies and decay time.