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MIDI Clock and Tempo Map
MIDI Clock

MIDI Clock is a measure/beat driven pulse clock or "beat clock". It includes Play, Stop, Forward, and Backward commands as well as tempo data. MIDI clock also has the unique ability to slew to tempo changes, i.e. speed-up or slow-down when driven by a tempo map for example. A master sequencer such as Tracktion generates MIDI clock and controls the tempo map. This clock is then sent to hardware-based rhythm machines, outboard sequencers and other software-based synth/sequencer combo programs such as Reason. These devices and programs usually benefit from external tempo/measure control because they are often slaved to a master sequencer and this is why they are capable of receiving MIDI clock.

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 The Tempo Map

Why use a tempo map? Just listen to a rhythm machine - playing at a perfect tempo. Is that really the 'feel' you want? Have you noticed a phantom tempo?

So what is a real drummer (or band) really doing with respect to tempo when playing a song? Does he/she suddenly speed-up at the beginning of the chorus? Not at all. The drummer actually anticipates the arrival of the chorus and ramps-up the tempo in the last measure (or half measure) leading up to the chorus. At the end of the chorus, the ramp-down is (in many cases) even quicker. The tempo of the second verse (and subsequent verses) may be 1 or 2 beats per minute faster than the first. These tempo changes are very subtle. Towards the end of the last chorus - the tempo might also need to be a few beats per minute (bpm) faster than the preceding choruses. If the chorus is extended - the tempo changes would happen (approximately) at the top of each 8 or 16 measure section. Also, at the very end of the song you might want to dramatically ramp-down the tempo just as a drummer might.

Note: Tracktion includes a tempo ramp feature just for the reasons stated above.

What affect might a tempo map have on program/sysex changes, or controller data originating from your master sequencer? As the tempo slews and the MIDI project slows down or speeds up - program/sysex/controller changes are also being altered in time just as MIDI notes are (and anything else in the project) so happily, all outboard effects patches (for example) also change with respect to tempo and time in your project. It should be noted that the 'audio event start time' is also affected (if your sequencer is also playing audio) but the speed in which the audio event plays is not - this has to do with logic/DSP regarding 'time compression/expansion while retaining pitch' and affects synchronization (see below). This is an insurmountable problem for most software programs to overcome and there are no simple solutions. It is more a question of forecasting or anticipating obstacles that accompany tempo maps, and is a discipline in and of itself - hard earned by engineers, producers and composers alike. Some MIDI/audio programs offer time/pitch tools for adjusting audio tracks but keep in mind, this involves heavy DSP and should be avoided unless absolutely necessary.

See Locking Tempo Map below.

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 Setting and Locking-Down the Tempo Map
The axiom "less is more" is particularly applicable to tempo changes in your map. Once the global (average) tempo has been identified, you can start to get an idea as to what tempo changes might need to be made. Things get tricky if you are planning to add 'real players' after MIDI preproduction. Once you begin tracking (recording) musicians, it is not really possible to change the map because the previously recorded players would then fall out of synch as the newly recorded tracks follow the new tempo changes. Hence, the need to lock-down the map.

"Don't be tempted to change the map mid-stream i.e., after you have started to record musicians"

So it becomes clear - the difficulty is, getting the map 'right' when you haven't even started tracking the session players. How can this really be done? Well, using a drum machine when building the tempo map is very important. But even more important - setup a session just prior to tracking where the drummer and bass player (or the entire band) can jam to the map. They can then, 'weigh-in' and help you fine tune the map before the dreaded lock-down.

The bottom line - when used correctly, the tempo map should be transparent i.e., not noticeable unless you are going for a 'dramatic change'. Tempo maps help add a more organic feel to music that originated from a software-based sequencer.

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 MIDI Timecode (MTC)
Unlike the measure/beat driven pulse of MIDI clock, MIDI timecode (MTC) is time-based only relating to hours, minutes, seconds and frames, not measures, tempo or transport commands. SMPTE is basically a higher resolution time-based clock however, MTC has been widely adopted by the audio industry and is slowly becoming the timecode norm.
 

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