SynthFont is a commercial MIDI editor and "MIDI to Waveform" converter, developed by European software developer Kenneth Rundt.
VST (Virtual Studio Technology) instruments can also be used instead of a sound source file.
A MIDI file is a collection of notes and instructions for how to play them.
SynthFont combines this data with the audio data in a SoundFont to produce ("render") an audio version of the musical piece.
SynthFont is freely available as trialware, and requires a licence in order to use it after thirty days.