anotherscott2022 wrote:
But by reputation, at least, Kurzweil always seemed to me to be the leader of that pack, e.g. with its rearrangeable algorithms and 32 layers which I believe better emulates modular analog synths as opposed to the more common fixed path emulations. But as I said, my knowledge here is limited.
Kurzweil VAST is interesting and not as capable as it might seem. The PC3 series added VA functionality to the existing VAST engine in 2008, and it was a good synth engine for that period. It is basically still the same engine in the K2700 except for DSP processing power.
It is useful to distinguish between audio and control/modulation signals. In non-modular synthesizers, they are usually separate except for a few cases (e.g. oscillators that can be used as LFOs). That's also the case for Kurzweil VAST. The DSP algorithms are about audio signals and there is a separate mechanism for control/modulation signals.
Both audio and control/modulation signals are important. VAST has good support for the latter, similar to Korg AL-1 and Roland Zen-Core.
The situation is different for the VAST DSP algorithms that control audio signals. A VAST DSP algorithm consists of 0-4 DSP functions, and each DSP function consumes 1-4 DSP blocks (unit for DSP resources) with a limit of 4 DSP blocks per layer/partial/element. The signal flow between DSP functions is sample-based instead of virtual analog. This virtual A/D conversion works well with some DSP functions like filters and low-frequency amplitude changes. Due to aliasing it works poorly with functionality that would add harmonic content. That means basically everything that adds harmonic content based on audio signals needs to happen within a single DSP function. VAST has a number of DSP functions that combine certain features. For example a DSP function that combines a square wave oscillator with PW, a DSP function that combines a saw wave oscillator with internal oscillator sync, another DSP function that combines oscillator sync with a triangle wave and another one for a square wave. However, you can't do any combinations that are not directly supported by a DSP function (without aliasing). For example, frequency/phase modulation of a PWM wave with a sine wave.
Similar to VAST, Roland Zen-Core is also a hybrid engine, which brings some extra functionality like feature-rich envelopes and LFOs. The difference is: Roland properly integrated their previous generation sample-based and VA engines, whereas Kurzweil shoehorned VA into the existing engine.
Yamaha AN-X has only fixed per-voice modulation routings (except for poly AT). Basic modulations are possible, but not more advanced configurations. For example key tracking to PW, or controlling PWM depth with key-on velocity.