“How long does a second last?”
“Unless you want to get into Einstein and relativity”
“Great. So how long does a beat last?”
“That depends on the tempo”
I’ve had this conversation many times and if you followed that you already understand tick vs sample based tracks - Simple!
Ticks vs Samples
To put it another way, the job of a DAW is to make a collection of “events” (audio files, midi notes or automation) play in the right order and at the right time. When an event should happen depends on what units you are using to measure how far you are from the beginning of the session. If you are measuring absolute time (minutes and seconds) then this will not change if you change the tempo. If the explosion sound effect for a video is supposed to play 30 seconds after the beginning of the session it will still play 30 seconds in whether the tempo of the music underneath is 138 or 142bpm. A second is a second and always will be (no discussions of Einstein please…). If the music is 16 bars long and the tempo is 138bpm it won’t last for quite as many seconds if the tempo were to change to a faster tempo of say 142bpm. This is because measuring time in bars and beats is relative to the tempo (a bar of 90bpm tales longer to happen than a bar of 160bpm). The clue is in the name of the unit - Beats Per Minute.
Pro Tools is first and foremost an audio platform. It has been doing audio far longer than it has MIDI (unlike Logic or Cubase which were both MIDI sequencers first and gained audio features over time, Pro Tools did it the other way around). Pro Tools is a sample-based editor and as such it measures time in samples. Your session’s sample rate is the smallest division of time Pro Tools understands so if you are working at 44.1KHz, Pro Tools understands one second along the timeline as being at the 44100th sample of the session. Timing for relative time (bars and beats) is less precise and the recent story on the site about errors when looping because of “sample rounding” illustrates this.
If you measure absolute time in samples, what about relative time? Relative time is measured in ticks. A tick is the smallest fraction of a beat that Pro Tools (and other DAWs) can work in and there are 960 ticks to a quarter note (so basically nearly 1000th of a beat). 960 is used because it divides more neatly that 1000 would. You will already have seen reference to ticks in the bars and beats counter (ever wonder what those last three numbers were?) and more significantly in the grid and nudge settings. If you select 16th note as your grid setting you will see 0|0|240 in the display: 0 bars, 0 beats, 240 ticks.
Tracks can be tick or sample based. You can have a mixture of each type in the same session. By default audio tracks are sample based and midi and instrument tracks are tick based but this can be changed at any time and the preferences can be set to automatically create all tracks as tick based if you wish. It should be noted that on a tick based audio track the start points of the audio clips will be tick based but the clips themselves are still sample based.
Indicated by a little blue clock face graphic. Sample based tracks trigger events when the appropriate number of samples has passed. Because this is unaffected by tempo the event is fixed and will not change unless you change the sample rate of the session.
Here the tempo hasn’t been changed and there is no difference between the tick and sample based tracks.
Indicated by a green metronome. Tick based tracks will stretch or contract in response to tempo changes.
Here the tick based tracks have changed in response to the tempo being sped up. Notice the length of the audio clips has not changed but the length of the midi notes has.
Here the tempo has been lowered and as expected the tick based track events have moved and the change in length of the midi notes is very clear.
When should you use tick-based audio tracks?
Because audio is inherently sample based, audio tracks are usually sample based too. Comparing the images above you can see that on a tick based audio track, while the sample based audio clip’s position on the timeline will be altered by a tempo change, the duration of the audio clip will not change (If elastic audio is enabled clip duration will change with tempo but a discussion of elastic audio is beyond the scope of this article - Suffice it to say that Pro Tools can treat audio as wholly tick based but this is not the default behaviour). Comparing this to a tick based midi track it can be seen that the duration of the midi note itself changes in response to a tempo change. The most obvious example of a time when using tick based audio tracks is appropriate is when building drum tracks from samples of single drum hits. The duration of the samples won’t change but as they are single hits this will not introduce the problems that would be introduced if a tick based audio track containing a drum loop.
When should you use sample-based midi tracks?
While this is less obvious, a good example would be when using midi to trigger a sampler for foley or sound effects in post. Mike’s recent review of the FC999 Foley collection would be an excellent example of a midi tracks which ought to be set up referencing samples rather than ticks.
Additional points - Markers and Automation
Memory location markers are sample based by default and if you anticipate changing tempo in a session it is probably wise to set up your markers as tick based. Track automation follows the currently selected timebase and while the duration of an audio clip will not change with a tempo change, the automation will scale with the tempo change just like midi and can move out of sync with the audio.