The Korg Lambda ES-50: what is it?
The Korg ES-50 (also known as the Lambda), sold between 1979 and 1982 (according to Pete Forrest’s A-Z), is a type of synthesizer known commonly as a ‘stringer’. It provides a small selection of sounds with minimal parameter tweaking, but available simultaneously and with full polyphony. More details can be found here, and a good demonstration of its sonic capabilities can be seen here (thanks Pepe!).
The technology behind this kind of instrument is descended from electronic organs. A Top Octave Generator (TOG from here on) outputs a wave at each of the 12 frequencies of a scale. These 12 waves are then processed by dividers, meaning they are reproduced at progressively lower octaves – one octave being a halving of frequency. This gives a square wave for every note on the keyboard.
The Lambda does not sound only square waves, however. It combines layered octaves at different levels to generate pseudo-sawtooth waves. If one combines a square at say C3 with C4 at 50%, C5 at 25% and C6 at 12.5%, the result is a passable approximation to a C3 sawtooth. I have quickly made a shoddy illustration to show how this works, and hopefully you should get the idea despite its lack of artistic merit.
After summing, the signal is passed through various parallel filters to achieve the tonality of each instrument – strings, choir, organ, etc. Each sound uses different combinations of octaves and waves and filters them using simple active circuits. The sounds are switched in and out and the result is mixed, fed through a chorus circuit based on Bucket Brigade Device technology (a popular type of circuit at the time, a discussion of which is not in keeping here – perhaps another post), and from there to the audio output. There is more to it than this, but on a fairly basic level, that’s the structure of the Lambda in a nutshell.
Other points of note include the envelopes that shape the volume of each sound as it is played. There are both preset and adjustable rates for attack and decay, and each note is articulated individually. This is achieved using multiple (obsolete, naturally!) integrated circuits, each of which handles a small number of notes in the Lambda and in combination with a few external components provides basic but useful articulation for each key.
There is another fixed envelope for the Brass sound. This is routed to the brass’ filter, which gives it that slightly resonant (indeed, ‘brassy’) tone that brightens and darkens again. Here, the articulation is global, and each new note retriggers the envelope. Any keys already held down are affected by this and will be shaped along with the new notes. This is a limitation, but is not so problematic in practice. To provide each key with its own filter envelope would have greatly increased the complexity of the instrument, and after all the Korg PS-3100, 3200 and 3300 were fully polyphonic true synths in that regard. By comparison, the Lambda was a budget instrument and has a very good feature set for its type.
Next up: inside the Lambda, more technical details, and some repairs.