The CS-01 is a funky little synth!
It may seem a toy but is a real analog synth in a small case.
The structure is quite simple, justa simple LFO, a “VCO”, VCF and ADSR.
I wrote “VCO” because even if the yamaha labeled it VCO but it is in fact a DCO.
The sound of the oscillator is however real analog, and “phat sounding” in a typical japanese way, and it reminds the classic analog sounds of the early 80’s.
The oscillator offers a good selection of waves (TRI, SAW, SQUARE, PULSE, PWM) and feets(4′ 8′ 16′ 32′ and noise).
The most powerful is the sound of PWM, which is really fat and “wide” sounding.
PWM has its own speed control too (indipendent from lfo).
The LFO is only a tri wave useful for vibratos and wah effects, its not extremely fast, but works well for all the classic sounds.
VCO also have a “glissando” instead of portamento/glide, which means that it can jump from a note to another passing through the notes in the middle.
The VCF is a 12dB lowpass with resonance and EG depth.
The resonance is controlled by a switch that turns it on or off, but it’s possible to modify the control to have a real resonance control adding a knob.
On the MK2 model the filter use a different structure (it has a 24 dB slope instead of 12) and has the resonance controlled by a slider.
The filter is, as other  old yamaha monosynth, very smooth and “bubbling”, even with resonance tutned on the filter never eat the low frequency range, offering a wide variety of colour even as a bass machine.
The CS01 is mainly a bass and lead machine, the bass is always rounded, and punchy, for the classic 80’s arpeggio or for techno bass sounds.
On the lead side it can sound quite minimal and smooth or rich using the pwm, it can also reproduce easily the classic retrocomputer  sounds !
Despite the toys look with the built in speaker this a lightweight classic 80s monosynth that can do most of the analog mono-sound without any problem of space in the studio and being very reliable.