I am looking to buy an MIDI keyboard. I want to use it with VST as a Synth. Please guide me. I am completely new in this space.
TL;DR Extra knobs and buttons are not absolutely necessary, but then again a MIDI keyboard is not absolutely necessary either, because you can just use a mouse. But since you're buying a MIDI keyboard, you most likely want to make it easier to play virtual synthesizers, and for that purpose the extra knobs and buttons are very useful.
Even something very simple like using a modulation wheel or control pedal for filter cutoff can raise your musical expression to a completely new level compared to just pressing keyboard keys.
Physical controllers like pitch-bend and modulation wheels or a combined bend/modulation stick, buttons, sliders, pedals and knobs that can send MIDI Control Change messages are very useful for playing virtual instruments. Usually you'll be able to remap or assign any controller knob/slider/wheel/stick/pedal/button to any parameter in a virtual instrument, because they usually send CC messages, just with different controller numbers. (Pitch Bend being an exception that might need special handling, because it has a 14-bit range as opposed to the 7-bit range of CC. Program Change / Bank Change, for switching sounds (programs/patches) might have separate buttons and/or knobs, on some keyboards or controllers. Controller-mapping capabilities are somewhat dependent on the DAW software, but usually modern DAW applications and virtual instruments are well equipped to map almost any physical controller to almost any feature in a virtual instrument.
Sliders can be useful for
- Drawbar organ instruments like NI B4. Each slider controls one drawbar, so a keyboard with at least nine sliders side-by-side would give you a nice drawbar controller.
- ADSR envelope and other parameters on synthesizers, though knobs will do just as well. It's a matter of preference.
- Adjusting channel balance when mixing. You'll typically want to assign each channel (or group/bus) its own slider, so 8 sliders lets you mix 8 channels.
- Performance expression parameters, for things like expression dynamics, i.e. how hard a virtual player is blowing her flute. (a modulation wheel/stick can be equally good for this, particularly if it's spring-loaded like a Roland/Korg style modulation stick, providing some natural physical tension)
- Effect parameters like rotary speaker ("Leslie") speed, or effect wet/dry level.
Knobs can be useful for
- Synthesizer tone control parameters, most commonly filter cutoff frequency and resonance
- Effect parameters like effect strength, dry/wet
- "Type" parameters like switching between filter types (low-pass/band-pass/etc.)
- Program Change i.e. switching sounds, depending on whether the particular keyboard/controller can send Program Change messages with one of its knobs. Many keyboards have a dedicated program change wheel (and sometimes bank change buttons).
Push buttons can be useful for
- Toggle-type things like rotary speaker slow/fast, or effect on/off
- Controlling the DAW's playback and recording ("transport control"): start/stop/pause, play/rec, seek forwards/backwards
- Track record arming
- Trigger-type things like launching sequences or samples, though it might be preferrable to use dynamic "drum/trigger pads" for that
Drum/trigger pads are for triggering notes, drums, clips or samples, but they can usually also be used like push-buttons. Many hip-hop and EDM artists use trigger pads for beat-making. The trigger pads usually send Note-On/Off messages just like piano keyboard keys, sometimes even with After-Touch, so you can keep the pad pressed and push it harder or softer and it will keep sending information about how hard it is being pushed.
Pitch Bend wheel or stick is useful for ... pitch bends.
Modulation Wheel or mod wheel can be spring-loaded or not, and it is useful for
- modulation, usually meaning "vibrato" but also things like brightness or resonance
- expression, for dynamics in virtual wind instruments, strings, accordion, etc. Many virtual instruments like Garritan Personal Orchestra (GPO) use the mod wheel for dynamics (strength) by default.
In addition to those, a keyboard/controller can have connectors for plugging in pedals and other inputs like breath control, and additional controllers can greatly enhance your musical expressivity with synth plugins.
Switch/toggle-type pedals can be used as
- Sustain or sostenuto pedal
- "Punch in/out" i.e. toggle recording on/off while the transport is rolling
- Transport control, e.g. start/stop playback
- Rotary speaker (Leslie) speed high/low (nice when you're playing Hammond type sounds and holding down keys with both hands)
- or anything where you might also use push-buttons
Controller/expression pedals can be used for
- Synthesizer parameters like filter cutoff or attack speed.
- or anything where you might also use rotary knobs or sliders
Breath control input can be used for musically expressive control for synth sounds and particularly virtual wind instruments. If you have any experience at all with playing wind instruments, even a school recorder, a breath controller can raise your musical expression to a new level.
You're most probably going to learn what you really need and prefer only after using a controller for awhile.
I recommend reading up on MIDI to know the difference between various MIDI message types: Pitch Bend, Control Change, Program Change, Note On/Off, etc.
Rather more succinctly:
You need sufficient controllers for the parameters of the VSTi that you need to control in real-time, actually while playing. Give us more details of how you intend to work - is this for live performance or are you always recording into a sequencer? Then we can be more helpful.