There are two steps involved in generating a TRM ID for an audio file.  One of the steps involves licensed, proprietary, closed-source software; hence it is impossible to generate a TRM ID using only open-source software.

1. Generate an Audio Signature

This step is performed by the MusicBrainz client software (mb_client and tunepimp modules).

The audio file is opened, read, and analysed.  Various things are measured, including the duration of the audio file, the frequency distribution, "beat" detection, and so forth.  Together these measurements are the Audio Signature.

2. Turn the Audio Signature into a TRM ID

The MusicBrainz client software now sends the Audio Signature to the Relatable TRM Signature Server.  The Signature Server is closed-source proprietary software, licensed to MusicBrainz by Relatable.  Our licensed copy of the Signature Server runs on the same hardware as the MusicBrainz Server, but please understand that it is quite separate software.

After receiving the Audio Signature data, the Signature Server does whatever it does (remember, it's a "black box" to us), and it returns a TRM ID to the MusicBrainz client software.  The client software can then do whatever it wants with the TRM ID, such as make a TrackInfoFromTRMId RDF request to the MusicBrainz Server in order to find out what track it is.

If the TRM ID returned is one of these special TRMs, do not look these TRM up on the server:

• f9809ab1-2b0f-4d78-8862-fb425ade8ab9: File is too short to calculate a TRM for.
• c457a4a8-b342-4ec9-8f13-b6bd26c0e400: The TRM server is too busy at the moment. Please retry again later.

The last special TRM is reserved for silence: 7d154f52-b536-4fae-b58b-0666826c2bac. It is OK to look up this track in MB, but be warned: there will be many hits for this one.

Further Reading

The document Dissecting a Tagger Session analyses the various HTTP (web, RDF, TRM) requests made by the MusicBrainz Tagger as a file is tagged.