Many times, I am asked the following questions by beginners:
Hopefully, I can explain the difference here.
Analog audio is an electrical signal that is (supposed to be) an exact copy of the sound waveform that was recorded.
Digital audio is a periodic sampling of an analog audio signal. Once per cycle of the sampling rate, the audio signal is sampled and converted into a number, which is then stored as part of the signal.
Here is an example of digital audio instructions (44.1 thousand samples every second, as on a CD):
"The instantaneous voltage is now .0398735224 volts."
"The instantaneous voltage is now .0398735964 volts."
"The instantaneous voltage is now .0398736032 volts."
"The instantaneous voltage is now .0398736349 volts."
"The instantaneous voltage is now .0398736064 volts."
"The instantaneous voltage is now .0398735932 volts."
When the digital audio instructions are run through a player at the correct rate, they recreate an approximation of the waveform of the original performance.
16 bit audio is used, because that is sufficient to recreate most recordings. 24 bit audio is used if the audio will be mixed or otherwise altered, to provide guard digits that prevent audible errors.
MIDI (Musical Instrument Digital Interface) is an entirely different animal. It doesn't record the actual sounds, but what actions the musician took to make those sounds. It is sort of like computerized sheet music, or the little pins in a music box.
MIDI does NOT carry the audio signal. It carries instructions that tell the MIDI instruments what to play.
Here is an example of MIDI instructions (sent whenever a change is needed):
"Press the middle C Key on the piano."
"Tell the violin to start playing the second G above middle C."
"Strike the kick drum."
"Release the middle C key on the piano."
"Tell the violin to stop playing the note."
"Close the hi-hat."
"Change the violin on channel 12 into a saxophone."
"Turn down the piano 6 dB."
"Play the E above middle C on the piano."
"Some clown pressed the STOP key. Everybody stop playing."
"Start playing at 432 quarter notes from the beginning of this song."
But the MIDI codes have no way of knowing what voices you have in your playing modules (unless MIDI codes put them there). That piano could be a concert grand, or a child's toy piano, depending on how it is set up.
The neat part about MIDI is that you can record the instructions for several instruments in a sequencer, and put a synchronization signal on one track of your multitrack. Then, when you play that track into the sequencer, which then sends MIDI codes to the playing modules, the modules obey the MIDI codes and play the MIDI recorded parts in sync with your recorded parts on the tape, but with NO generation loss. The modules actually create the tones during mixdown, piped right into the mixer and onto the 2-track master!
If you play a MIDI sync track itself through the speakers, it sounds like when a computer modem or a fax machine calls your telephone number by mistake.
"Squeepshshshirshshshooshshshshsh." or "Zugrugchugsmuthugurruzzaplug" depending on what CODEC you bought. You definitely don't want that in the final mix (unless it's a song about computers).
What MIDI can do:
What MIDI can not do: