Tempo Modes

Tempo Modes

TODO:

• Talk about the BPM calculator
• Create a FAQs section and add a step by step guide to fix the song's tempo to a specific BPM.

Concepts

To understand tempo modes, it is important to understand the concept of 'ticks'.
The tick is the smallest time unit known to the pattern editor. Each row is subdivided into a number of ticks. The duration of a tick and the number of ticks per row are therefore factors that affect playback speed.
The number of ticks per row also affects the behaviour of many pattern effects.

In total, four parameters can affect playback speed.

Value		Description
IT	XM
Txx	Fxx with xx>20	Often referred to as "tempo". It affects the playback speed, but only corresponds to the exact BPM of the song with the Modern tempo mode.
Axx	Fxx with xx<20	Traditionally referred to as "speed". Under the covers, its purpose is to set the number of ticks per row.
Rows per beat		Tells OpenMPT how many rows represent a "beat" in your track. Only used by plugins and the Modern mode.
Rows per measure		This is used in combination with Rows per beat to calculate the "beats per measure", which is sent to some plugins, but it does not affect OpenMPT's playback algorithm.

Note that by default the pattern row highlighting is linked to the Rows Per Beat and Rows Per Measure values. This can be disabled in the colours settings.

Below is a description of how each parameter is interpreted in the different tempo modes:

1. Classic

This is the default mode in OpenMPT.

Value		Impact
IT	XM
Txx	Fxx with xx>20	Fixes the duration of a tick.
Axx	Fxx with xx<20	Sets the number of ticks per row. This affects playback speed, because the tick duration is fixed.
Rows per beat		This tempo mode is not affected by the Rows Per Beat.
Rows per measure		This is used to calculate the "beats per measure" which is sent to some plugins, but it does not affect OpenMPT's playback algorithm in any way

For performance reasons, most older trackers were implemented with very little floating point arithmetic. Some rounding errors when calculating the number of samples that should constitute a buffer of a given duration mean that tempos could be inexact. In OpenMPT, the Classic tempo mode preserves this "old-school" behaviour.

2. Modern (accurate)

Value		Impact
IT	XM
Txx	Fxx with xx>20	Fixes the exact beats per minute of your song.
Axx	Fxx with xx<20	Sets the number of ticks per row. This does not affect playback speed, because the BPM is fixed. It therefore affects the duration of a tick.
Rows per beat		Sets the number of rows constituting a beat in your song. OpenMPT cannot guess how many rows you consider to represent a beat in your song. You have to tell it explicitly by setting this value.
Rows per measure		This is used to calculate the "beats per measure" which is sent to some plugins, but it does not affect OpenMPT's playback algorithm.

This tempo mode compensates for the rounding errors described above, so the real BPM should nearly always correspond to the exact value you specify. This is the most intuitive mode for new users and closest to other modern music software packages. Some argue that this should be the default mode in OpenMPT.
A current limitation is that it's impossible to set fractional BPMs with this mode (e.g. 136.46 BPM).

3. Alternative

Value		Impact
IT	XM
Txx	Fxx with xx>20	Fixes the ticks per second.
Axx	Fxx with xx<20	Sets the number of ticks per row. so Txx/Axx = rows per second
Rows per beat		This tempo mode is not affected by the Rows Per Beat.
Rows per measure		This is used to calculate the "beats per measure" which is sent to some plugins, but it does not affect OpenMPT's playback algorithm.

So if tempo/speed = 128/16, then 128 rows will last 16s - in other words, tempo will be exactly 120 BPM if you track at 4 rows per beat.
This is a slightly more complex tempo mode devised by Ericus for the mathematically minded. It allows for very fine tuning.