Trackers have a different kind of tempo and time signature mechanism than normal musical notation -- much of this comes from the tradition set down by old Amiga trackers. If you're familiar with traditional musical notation, music typically is structured around two markings we'll call "speed markings":
- The Tempo - often given a mood term like allegro or allegro agitato. In more recent music a specific measurement is given, "Beats per Minute" or BPM.
- The Time Signature - which specifies which kind of note is called "a beat" and how many of them fit into a measure. A common time-signature is 4/4, which means there are 4 quarter notes per measure, 3/4 means three quarter notes, 6/8 means six eighth notes and so on.
A BPM of 120 and a Time Signature of 4/4 thus means that there will be 120 quarter notes played per minute and all other note lengths will be derived from this (a half note will be twice as long, an eighth note half as long, etc.)
Trackers use a different system that uses a combination of "ticks", BPM, and Lines-per-Beat (from the Milky Tracker Docs1).
- Tick - The base time unit in traditional trackers like MilkyTracker, originating from Amiga. Notes are triggered on the first tick of a row (unless delayed) and effects are applied on the following ticks. (1)
- BPM - Traditionally Beats Per Minute, but in tracker terminology it defines the speed of ticks. (1)
- Lines-per-Beat - the number of lines available for note entry per beat.
Different trackers may use Ticks and BPM, or BPM and Lines/Beat, or may use both in different contexts or may use all three. Renoise uses either one, while OpenMPT supports all three. Needless to say, it's complex. A quick couple examples:
- BPM of 125 + Lines / Beat of 4 means you can write music at 125 BPM using "sixteenth" notes.
- BPM of 133 + Lines / Beat of 8 means you can write music at 133 BPM using "32nd" notes.
- BPM of 125 + Tick Speed of 6 means you can write music at 125 BPM using "sixteenth notes"
- BPM of 125 + Tick Speed of 3 means you can write music at 125 BPM using "32nd" notes.
This isn't quite exact, but it should be close enough to understand how this work. I put the note length in quotes because trackers don't exactly have a concept of sixteenth or 32nd notes. This is really how many lines you have to enter note data between beats, and there's all kinds of things that you might have to enter that aren't just notes -- for example, note-off commands. I've found that a good rule of thumb is to target notes that are twice as fast as you'll practically need -- if your composition needs sixteenth notes, set up your tracker to support 32nd notes and you'll generally have enough space to work.
One other factor to keep in mind, pattern length -- the number of notes you can store in a pattern. This is typically 64 for most trackers, or 16 beats with a BPM of 125 and Lines/Beat of 4. This number gets halved when the Lines/Beat is 8, you only get 8 beats worth of music. Fortunately this is adjustable, many composers will set this to 32 or 128 depending on the song speed and their phrase length. Many trackers will let you set this to different lengths for each pattern, or cut to the next pattern early (usually when the composer is using a fill or a bridge of some kind).
I had to find some kind of common ground to merge the two pieces, and to merge the pieces within. I felt like the introduction was important enough that I needed 125BPM -- if it was too fast it would feel rushed. But I wanted to accommodate the higher note volume in Purple Motion's portion, so I chose a Lines/Beat of 8, giving me up-to 32nd notes to play with. However, this functions more like a tick rate of 3 or 4, meaning that a note-by-note transcription of Skavens music would simply be too fast. The solution was to extend the patterns to 128 lines and simply double the length of each of Skaven's notes, unless some specific attack, effect or note cut required me to use a finer grained note. As it turns out, most of Skaven's more creative use of BPM and ticks occur in the end-credits, so by cutting those portions out, I didn't have to worry about them.
I used a similar technique for Purple Motion, where he used a tick rate of 3, I could use a pattern length of 64 and directly transcribe his notes, where he used a tick rate of 6 I simply doubled the pattern length along with his typical note length.
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