There were a good few stringers released through the 70’s and into the early 80’s. With most of them following the same principle. One or two Top Octage Generator LSI chips that literally generate the top 12 notes as square or pulse waves. These are then fed to frequency divider chips so you end up with full polyphony across the keyboard, usually with the option to go up or down an octave too (to simulate viola, violin and cello registers)
The dividers feed pulse shaping and filtering circuits which is where some of the sonic differences between models start to show up. It’s worth noting that those filter circuits have different cut off frequencies depending on the octave. From memory most have 3 or 4 sets of filter circuits. Similarly the pulse shapers are different in the higher registers than the lower ones.
Some instruments actually feed the divided waves directly to the key switches, sometimes upto three at a time on each key while others use the keyboard to control individual VCAs or a global VCA. As I’ve already pointed out VCA per key is infinitely more expressive that being stuck with a single VCA.
Some offer attack and decay (release really) controls and again, these lend a more expressive quality, especially with a single VCA per key. Others may have a release control but no attack controls.
Beyond the architecture of the different instruments available the other major factor in their sound is the Ensemble effect the majority used. To generalise the effect Is usually made up of three BBD chips in parallel modulated by two discrete LFO’s, one quite slow (less than 1Hz) and the other a bit faster at several Hz. The choice LFO speeds and the filtering applied before the signal hits the BBD chips combined with the differences in the filtered and shaped waveforms are what make each offering sound different.
The sonic qualities are subjective and sometimes rather than full on wobbly ‘fairground ride’ chorus you want a shallower, more subtle effect, especially in the higher registers. If you’re string machine has only got on/off or a basic level control you’re out of luck. Sure, it’ll do that classic fast modulation wobbly chorus sound or if you switch it off a very static pulse wave. Some machines offer more control over the Ensemble effect and it can make a huge difference, especially when coupled with attack and decay controls.
Although the Solina has its DNA in the Eminent Unique organ (Eminent were a Dutch organ company) the majority of offerings came from the Adriatic coast of Italy, the spiritual home of Italian accordion manufacture. Some of the better known instruments were the Crumar Performer (Crumar had a thing about including string sections in many of their multi instrument keyboards and synths with differing levels of control), Elka Rhapsody 490 and 610, the Logan String Orchestra and String Orchestra II, the Siel Orchestra, Jen Stringer, various models built in Italy by Elex and badged as Hohner and my personal favourite and IMO the ultimate string machine, the Godwin String Concert. The SC749 has three footages, attack and decay controls, a VCA per key, treble and bass controls but it’s crowning glory is the two faders for the Ensemble section. They’re marked ‘Chorus’ and ‘Tremolo’ and essentially control the two LFO’s that modulate the BBD’s. That means you can go from full on wobble to beautiful soft almost phasey chorus. With some judicious settings you can gate lovely warm pad sounds, especially on just the 8’ voice. The subtle effect settings are something most other stringers just can’t do.
The Italians and Dutch weren’t the only nations at the String synth game. (The basic principles were first seen in the Freeman String Symphonizer from the UK). Roland has the RS range and included a string section on the VP vocoders, Korg had the Lambda and Delta synths and Yamaha had their basic string section in their SK range. They saved the best for the dedicated SS 30 string synth. You can hear it in action at the very end of ‘Vienna’ by Ultravox. It’s got two Top octave generators as well as an ensemble section. It’s a rare and exotic festival of lush strings and fake rosewood
I’ll shut up now.