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Sunday, January 21st, 2018 12:29 am

Synth Glossary

Frequency Modulation (FM):
Frequency modulation allows the output of one oscillator to drive the frequency of another oscillator. It can generate very complex sounds from very simple waveforms and is the basis of the Yamaha DX range of synthisizers and OPL range of synthesizer chips (used in Adlib and most other soundcards)....


Begun in 1979, after Alan R. Pearlman returned as the president of ARP, the Chroma was supposed to bail the company out of its financial crisis. It had a good chance to do just that. But in 1981, before they could get the product to the manufacturing stage, ARP folded. Philip Dodds remained at ARP to help in the liquidation of its assets. "While... cleaning up the financial and loose-end disorder that remained, he managed to sell the Chroma design to CBS - and get himself hired as the director of its production." He reassembled Chroma's design team (21 people in all) and set up shop for CBS in Woburn, MA. There the first 50 (serial numbers zero through 50) and, according to Dodds, the best, were hand- built."

"The Chroma had 16 discrete synthesizer channels, complete analog synthesizers. There were two VCOs, a cool multimode VCF, a VCA, and software-created envelope generators. There were 64 sample-and-holds that controlled all of those channels, and they were driven directly off the central computer [built on Intel's new 80186 computer chip (the PCs at the time only had the 8086)]. Not only were [the VCOs] under computer control, but [they] had implemented a frequency sampler, a zero-crossing detector, that permitted the computer to measure the wave lengths of all the VCOs and the VCF, so that it could figure out what frequencies they were at. That was for auto tuning. When the Chroma first woke up, it would go to sleep for a minute while it read the frequency of each VCO.... It could actually read frequencies of each of the oscillators in real time and it could automatically put the filters in resonant mode and read those frequencies as well. So all the tuning was computer-controlled automatically in the background."

"The Chroma represented what could be thought of as the second generation of analog/digital hybrid synthesizer instruments. It came along at the peak of the Sequential Prophet-5's popularity, offering then-radical functions like multitimbral operation, voice layering, and keyboard splitting, not to mention velocity response - common synth features that we take for granted now. Voice structuring wasn't limited to the typical Minimoog configuration (two oscillators, a filter, two ASDR's, and a VCA); the Chroma's internal computer could route signals through two low-pass filters, in parallel or series, or position the VCA before or after the filters. Its original keyboard was a wooden weighted-action design (by Dodds) manufactured by Kimball [and then later, by Prat-Read]

"In addition, there was a computer interface that allowed Chroma performances to be digitally recorded and played back, and voicing software was even developed for the Apple II and the IBM-PC. Digital sequencing and editor/librarian programs are commonplace today, but the Chroma came out at least a year before MIDI was accepted as a de facto protocol.

"The Chroma also helped usher in a concept that annoyed some synthesizer programmers: menu-driven voice programming with a single data slider and multiple dual-function membrane switches (undeniable influences on the Yamaha in their design of the DX7). Previously, single-function knobs and sliders (one control per parameter) abounded, and single-line LCD displays were foreign and unfriendly to affirmed knob-twisters."

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