The RTX ringtone file format is compatible with Nokring tones (.txt) and which are available on the web. The specification for RTX files is here.
Example:
Alert:B715:32C,32P,32D,32P,32E,32P,32F,32P,32G,32P
This file describes a ringtone whose name is 'Alert'. The control section sets the beats per second at 715, and the note definitions describe a rising sequence series of 5 notes and 5 pauses of length 1/32 of a full note. Each note is constructed of three parts: the note fraction (optional), the note name itself or rest, the octave to use (optional). If optional entries are omitted then the default is used from the section at the start of the ringtone.