Switched-On Bach: How a transgender synth pioneer changed music: "Wendy Carlos’s 1968 album of Bach music for Moog synthesiser brought electronic music into the mainstream and influenced artists from Giorgio Moroder to Daft Punk"
Since its release 50 years ago, Switched-On Bach has been relegated to the realm of kitsch curiosity; blips and blops produced from tinkerings on primitive electronic components. Many academics and commentators believe that part of the reason the album isn’t more widely celebrated is because of the gender of its creator. Wendy Carlos released her pioneering electronic album in 1968; at the time of release, she was six months into transgender hormone therapy. When the album hit the shelves, the person responsible for Switched-On Bach was still known as Walter.
The Moog synthesizer wasn't the first electronic musical instrument, and before all-electronic ones, there were electromechanical ones. The best-known electromechanical one is the electric guitar. Its strings power miniature electric generators: small permanent magnets with coils wrapped around them. That's why electric guitars need magnetic strings like steel ones.

Electric organs were common back then, but they had very simple tone generators, as they are called. The Moog synthesizer had a much more elaborate and general one, one that could be programmed by connecting modules with patch cables. It was all-analog; integrated circuits were in their infancy back then, and the first CPU chip ever, the Intel 4004, did not come out until 1971.

The modules were oscillators, filters, amplifiers, and envelope generators, as they are called. That latter one is for controlling the time behavior of sounds. To make a guitar-like sound, you'd make a fast attack (initial rise) and a slow decay (fall afterwards), with zero sustain (a plucked string doesn't sustain) and a slow release (after one stops pressing the key), so the sound will continue. To get a more guitarlike sound, one can use two of these setups, one for making the initial plucking sound and the other for the continuing sound. To sound like a string section, one has a slow attack, no decay, full sustain, and a slow release. One can also add noise to the sound to imitating the rubbing of bows against strings. Etc.
Carlos couldn’t afford to buy one of Moog’s hand-crafted, polished walnut-finished, incredibly expensive instruments. Walter Sear, who was responsible for selling the synths, recalled, “You could buy a nice house and a nice car for what these cost back in those days.” Instead, Moog and Carlos developed a barter system that saw Carlos recording pieces of music showcasing the capabilities of the instrument. For this work and for advice that aided the development of the synth, Moog knocked a few bob off the price.

The modular synth was about the same width as an upright piano, and 1½ times the height. The front panels were covered in dials, switches and patch cables. The instrument resembled a particularly chaotic and antique telephone exchange.
Johann Sebastian Bach's music was a good choice, because of the style of many of his pieces. He often wrote in a quasi-choral style, where each part has only one note each at a time. That fit in well with a big limitation of the Moog synthesizer: it was monophonic, meaning that it could make only one note at a time.

Wendy Carlos, who was Walter Carlos back then, painstakingly played one part at a time, layering them with a multitrack tape recorder, something that was already a common studio practice. If you've ever wondered how someone like Prince had played many of the instruments on his albums, that's how one does it.

So if you've ever wondered why "Toccata and Fugue in D Minor" isn't on it, that's why -- it would be very difficult to record in that fashion.

Switched-On Bach became one of the best-selling classical-music albums of all time, and it was a big influence. Producer Giorgio Moroder bought one and used it in Donna Summer's album I Fell Love. Mick Jagger brought one, but he and his fellow Rolling stones found it rather difficult to use. He then sold it to German electronic-music pioneers Tangerine Dream, who then went on to use it in several albums. TD's albums often have a wispy background-music sort of style, a style now known as ambient music.

Yes keyboardist Rick Wakeman got one secondhand from someone who grumbled that it made only one note at a time.

Walter Carlos became Wendy Carlos in 1972, and she has made some more albums, like "Switched-On Bach 2000", with much fancier synthesizers.

Not many others have done classical music on synthesizers, but TV-music composer Don Dorsey has done some nice albums of such music, Bachbusters (1986) and Beethoven or Bust (1988). The former one has "Toccata and Fugue in D Minor" on it, and it helped that he had more advanced synths: polyphonic ones. They have several tone generators, as they are called, and a system for dispatching note commands from the keyboard to them. This made it possible to play chords on them. So DD could play TaFiDm much more easily than WC could back then. In sheet music for TaFiDm, you can easily see its several chords. DD's version does depart a little bit from pipe-organ-like sound behavior, however, and I don't think that I want to spoil its ending.

Scores for Bach's Brandenburg Concertos -- WC did #3 in SOB, complete with some imaginative improvisation for the second part of it. But overall, you can see why it was easier for her (him back then) -- it has several monophonic parts.

Electronic musical instruments are now just about everywhere outside of traditionalist genres like classical music. Not just synthesizers, but also samplers and sequencers, and hybrids like drum machines. Those are boxes that play drum sounds in programmable sequences. With the development of digital synthesizers in the 1980's came the Musical Instrument Digital Interface, or MIDI. That way, one could connect a keyboard to some standalone tone generators, and one can use that to make a thicker sound, for instance.

With advances in CPU chips came the ability to do more and more in software. Sequencing software was the first, since it is software for managing commands to make notes. Sequencing software typically uses a piano-roll-like display, showing which pitches of sound are active an inactive at each time. One can also have musical-note display, making it possible to use a sequencer as a sheet-music editor. One can either enter note data directly, or from some MIDI instrument like a MIDI keyboard. One can then edit the note data.

Tone-generator quality has also advanced, from samples and simple oscillators to more complicated algorithms like FM synthesis (one oscillator running another oscillator at a variable rate), and to physical-modeling simulations of the electric circuits of analog synths and the working parts of acoustic or mechanical instruments, like strings and air columns and drumheads. One simulates them by finding their equations of motion and then solving those equations numerically.