A modular synthesizer is one where the structure of the synthesizer isn't fixed. There are a variety of modules available which can be plugged into each other. Tradionally these modules were physical boxes, but they can now be emulated with software and patched with on-screen cables....
Chroma
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."