There are many great virtual instrument libraries out there.  They can take old 80s synths for your violins and make them sound a million times better.  They and all other MIDI playback devices can bring life to a piece.  But, beware the MIDI.  It can provide you with a more realistic sound, but not a very realistic sound.  Don’t get sucked into believing that what is playing through your computer speakers is what the end result will be.  Here are some ways in which most MIDI I’ve encountered skews the sound of a piece (as compared to real performance with human players):

  1. Presence: the MIDI signal is usually too dry.  This creates a more nasal, focused, piercing sound that doesn’t decay naturally.  This is an increase in the tone’s pithiness, and leads to a feeling that certain instruments might sound brighter or louder than they actually are.
  2. Reverb: the reverberation automatically added to MIDI is oftentimes like a cathedral times ten.  This creates a much more voluminous sound, with unrealistic decay times (note that #1’s lack of decay is overcompensated by this #2’s exceedingly long decay).  This can also counterbalance #1 in its depriving the instruments/voices of tone color and presence they might naturally have.
  3. Beginnings/attacks: these elements are often overly precise, and depending on the synth/virtual instrument you’re using, can be too sudden or have too long of an attack, leading to a too precise (unhumanly so) or too elongated (eg. string attacks) beginning.  This can throw of the timing of a piece.
  4. Endings: the cutoff of a whole note in 4/4 is not the same in MIDI and human performance.  Human players will often cut it short, fade to niente, or cut off on beat 1 of the next bar (overlapping the barline).  MIDI doesn’t do this; when the 4 beats are up, only reverb can save the sound from sudden demise.
  5. Synchronicity and preciseness: this is true in increasing amounts as the size of your ensemble grows.  It is extremely hard for large ensembles to synchronize complex “hits” (although they do this quite well despite the acoustic odds against them), and MIDI will generally give hits and unison preciseness that will not happen in a large ensemble (where synchronized timings will be more fluid).
  6. Range: in the case of non-sampled sounds, the range of an instrument may be smaller than the sounds that you get–don’t push those trombones too low or those french horns too high!!
  7. Balance: many instruments stick out of the texture or are hidden unnaturally in MIDI for a few reasons.  The first is that they may be a naturally loud or naturally soft instrument, but MIDI doesn’t take this into account.  The second is that most instruments sound louder or softer depending on the register (try throat tone clarinets sounding loud, or soft oboe on a low Bb–it fails to match up).  The third is through doublings and orchestration–2 flute staves a due may not stick out as much as the MIDI makes them, but motion in power-chord-like fifths and fourths in the horns may be hidden.
  8. Percussion: I’ve found that metallic percussion is generally too loud in MIDI, and skin percussion, esp. bass drums, is too soft.  Marimbas have also been problematic in either sustaining too long on being too loud.
  9. Voice: voices generally sound more like hollow reverb tubes than human voices.  (I’m sure you’ve encountered examples of this in your musical life.)
  10. Difficulty: MIDI can play back extremely complex tuplets, huge leaps, extremely fast passages, or other non-idiomatic passages with no problem.  The problem, however, lies in performers not being computers and therefore being unable to execute certain passages (or being very angry with you when given said passages to play/sing).  If you have questions about a passage’s playability, ask a performer, not the MIDI.

MIDI and playback realization are great things.  But, don’t fall prey to relying on the sound as the “final product”.  I’ve made that mistake, and I hope by writing this other people can enjoy their playback, but understand its faults, limitations, and hear things with a grain of salt.

Thanks for reading!