How can i change the effects to add a tremelo on some strings?

Rik

With a synthesizer like the Motif there are going to be several ways to accomplish this goal
1) you can apply a Low Frequency Oscillator to vary the Amplitude Modulation Depth
2) you can use an Effect Processor to vary the amplitude
3) you can build a Voice (or load a Voice from a sample library) using actual samples of a string sections performing with tremolo

Depending on how deep you are into synthesis you may opt for one over the other. In general, it is not even a question of which one gives the best results - its a matter of which one gives you the results you are happy with.

That may sound strange but is true. Sampling a string section performing a tremolo gesture is one of the things that was a common request back in 2001 when the Motif came out. In the ensuing models I beliieve strings was one of the main areas of improvements. I do not recall if the original Motif had a Tremolo string section sample (but you may want to look in the Waveform list and build a Voice from it).

Programming the LFO to modulate the amplitude (or loudness) can be a great way to learn about how the synthesizer engine works. There is the COMMON LFO that can be applied to vary AMD (Amplitude Modulation Depth) basically that is synth talk for what musicians call “tremolo”

Using an Effect processor may be the least natural way but as we said it is all a matter of the result for your particular need at the time. There is an INSERTION EFFECT in the “MOD” (modulation) category “Tremolo” - you have control over LFO speed (how fast the volume changes loud/soft)) and you have the ability to apply it to AM (Amplitude Modulation).

You will find that listeners will not really care or know how the tremolo got applied to the string sound, they will only accept what they hear on its face as it is presented. After all it is a synthesizer and we are emulating reality.