When you compare the size of a MIDI file to that of an MP3, you will notice that the former is much smaller, usually just around a few kilobytes. This is because MIDI files are really just a set of instructions called event messages that tell a sound generating device like a synthesizer what to play. These instructions cover musical properties such as pitch and tempo and other parameters such as what channel to use, what voice is assigned to that channel, and the volume, velocity, and other audio qualities that can be attributed to a note. MP3 files on the other hand usually take up a few megabytes. This is due to the fact that an MP3 is a compressed format of digital audio, meaning that the file actually contains a digital representation of the actual sound waves.
- Install audio or music software that can handle both MIDI and digital audio. Converting MIDI to MP3 is not a simple and direct process, as the former did not originate from sound waves. It's not similar to converting a .wav file to MP3 where you're simply changing the format of what is already digital audio. Audio or music software that has both MIDI and digital audio capabilities will be necessary. Take note that not all audio software is capable of handling MIDI files. Some exclusively edit digital audio and won't even recognize MIDI files. Some examples of audio software that have both features are Sonar and Cubase.
- Have a full-duplex soundcard installed in your PC. The audio software will be used to either simultaneously playback and record the MIDI file, or export the MIDI file into a digital audio format. For most audio software to do this, it must be supported with a soundcard that is full-duplex. The term simply means that the soundcard has enough memory and capacity to playback and record audio at the same time.
- Load the MIDI file into the audio software. If the MIDI file was saved in MIDI format 0, what you'll see in the audio software's interface is a single track of jumbled notes all sounding like piano. The reason for this is that MIDI format 0 is a stripped down format that took out the instructions regarding channel segregation and instrument voice assignation. In such a case, you're going to have to either rearrange the music and separate it into proper tracks, or look for a MIDI format 1 version of the file. MIDI format 1 is a more complete set of instructions and follows the General MIDI standard. If the file was in this format, then you don't have a problem, as you'll see separate tracks for each channel and the right instrument assigned for each.
- Playback and record or export the MIDI file. With the MIDI file properly loaded in the audio software, you now have the choice of either playing the file and simultaneously recording it, or simply exporting it to turn it into digital audio. What option is available will largely depend on what features are available on the audio software you're using. Remember that what you're seeing on the audio software's interface are not sound waves but event messages. These messages need to be read by the PC's built in sound synthesizer in order for any audio to be produced. This 'synthesized' production of the MIDI event messages is what can be converted to the MP3 format. Sonar and Cubase, the two examples of audio software mentioned earlier are capable of directly exporting the MIDI file to the .wav digital audio format. The process of exporting is really just an internal and speeded-up method of simultaneous playback and record. The software executes the MIDI instructions and stores them onto the hard disk as it is being rendered. The procedure bypasses putting the audio out through the speakers. You'll most likely only see a task bar filling up to 100% and then a prompt that says the procedure is completed.
- Load the .wav version of the MIDI file into the audio software. When you started the exporting process, you were probably asked to input a file name and a location for the exported file. Look for the exported file in the location you specified and load that into the software. You can play and listen to it first to make sure every sound and instrument in the MIDI file was recorded properly. Now that you have the .wav digital audio version, you can use the same software to finally convert it to MP3. The same exporting procedure is used, the difference is that this time you're putting out a .wav file and turning it into MP3.
The specific steps to be taken may vary, as each audio software application has its own features and capabilities. The general procedure, however, remains the same. First render the MIDI file into digital audio, and only then can it be converted to the MP3 format.