Tempo Maps For Pre-Recorded Audio

 By John Walden

As described below, the audio file contains a rhythm guitar part with tempo variations. In the first MIDI track, quarter-note 'taps' have been recorded in time with the freely recorded guitar part. After these were edited to tidy up their timing, the second MIDI track was created, removing the MIDI notes on beats two, three and four of each bar to leave whole-note taps.

Sometimes the click can steal the life from a session, so why not start with a live performance?

Cubase SX and SL have a range of tools that allow tempo to be managed and manipulated. For example, for a short audio recording, the Beat Calculator can be used to find the original tempo of the recording and this tempo can then be adopted for the Project or inserted as a tempo change into the Tempo track. The Beat Calculator also includes a Tap Tempo function which allows you to calculate the average tempo of the section by tapping on the computer's space bar (or mouse button) during playback of pre-recorded audio. These functions are very helpful, but they are intended primarily for short sections of audio (perhaps a couple of bars) where the tempo is constant and finding an average tempo is all that's required.

Where these functions are less useful is with longer sections of pre-recorded audio where, for any number of reasons, the tempo may have varied — either because of subtle variations by the musicians while playing the song or because of sections within the arrangement where there are planned, and more dramatic, shifts in tempo. This touches upon one of the obvious pitfalls of working with any of the modern MIDI + Audio recording environments — it is so easy to just set the tempo at the start of the project, start the click track and then slavishly follow it. In reality, most music, when performed live by a group of musicians, will contain some variations in tempo. Even at a subtle level, and just like variations in dynamics, these tempo variations can add to the emotional response of the listener. A simple example would be a slight increase in tempo during a chorus relative to a verse — a live band might do this almost subconsciously, adding to the excitement of the song. However, if the same band have to follow a fixed-tempo click in the studio, they might find it difficult to capture the essence of what gives the live performance some of its character.

Tempo track mode needs to be set in the Transport Panel so SX will follow any tempo changes created.

Fortunately, SX has the necessary tools to allow a tempo 'map' to be constructed from a freely recorded performance (whether recorded as audio or MIDI or both). Indeed, it can be quite a liberating experience in the studio to forget all about the sequencer's tempo and lay down your backing or guide track 'live'. Alternatively, you could use a live recording from a rehearsal session that has the right feel as a basis for constructing a tempo map for a later studio recording of the song. Either way, once the tempo map is constructed within the SX Tempo track, the click track will follow its variations. Any subsequent MIDI parts can then be edited and quantised to the bar grid using all the usual tools, regardless of variations in musical tempo. So, just how do you become the master rather than the slave in the musician-sequencer relationship?

Quick, Slow, Quick, Quick, Slow

Despite the absence of several useful tempo tools from VST in the intial relase of SX, the current version has all the necessary tools for the job.

As described more fully below, the 'Merge Tempo from Tapping' tool (found in the 'MIDI / Functions' submenu) provides a way of using a MIDI keyboard to tap in a beat that follows the tempo of a pre-existing performance. Cubase will then take these MIDI notes and automatically construct a tempo 'map' of the performance within the Tempo track. Depending upon how accurate your tapping has been, the resulting tempo map can sometimes require a little fine-tuning, and if so, the Time Warp tool can be used to tidy up any loose ends.The 'Merge Tempo from Tapping' dialogue.

For the purposes of this example, I used a solo rhythm guitar part, recorded directly into SX via a DI. I deliberately played with some minor tempo variations during the main body of the piece and then added a 'double time' section prior to a gradual tempo decrease to provide the finish. I recorded this into a new SX Project with the default tempo set to 120bpm. Although the intention is to create a tempo map within the Tempo track, it is worth setting an original tempo that is easy to recall — if things don't go quite you'd planned, it makes it easier to delete the contents of the Tempo track and return the Project to its original state. The Project also needs to be set to 'Tempo track mode' rather than 'Fixed tempo mode' within the Transport panel.

As a brief aside, it is worth considering the kind of performance you choose to attempt this tempo-mapping process with. For example, some types of audio recording will make it easier to follow the beat than others — a full band recording is easier, because your tapping can follow the drummer as s/he (hopefully!) keeps the timing of the band locked together. The sharper the transients within the audio, the easier it is to follow the timing and locate the exact positions of beats. As in the example illustrated here, a solo guitar or a guitar/vocal performance can also work fine provided the guitar has a fairly strong rhythmic component to it. However, whatever form the live performance takes, make sure the timing itself is tight — while a duff note or two will not matter, attempts to gloss over the section where the bass player and the drummer lost the rhythmic plot are unlikely to prove very successful. While I've used an audio recording here as my 'tempo master' performance, it is perfectly possible to use a freely played MIDI performance — just don't quantise it prior to constructing the tempo map!

Push Me, Pull Me

While the method described here generally works quite well, in some cases it may prove more difficult to construct a tempo map that creates exactly the right feel. For example, the process described here often depends upon matching the sharp transients within an audio performance (by ear when tapping and by eye when editing the positions of the MIDI notes or using the Time Warp tool).

Of course, this assumes that such transients are being played exactly on the beat. This will not apply in situations where the groove is either deliberately pushed or pulled before or behind the beat to create a particular musical feel. In these cases, it can require a good deal more editing work with the Time Warp tool in order to create a tempo map that gives a comfortable musical 'fit' to the pre-recorded groove.

Doolally Tap

The next step simply requires a new MIDI track to be created and, while monitoring the existing performance, to record yourself 'tapping' in MIDI notes, in time, throughout the full length of the track. To make this easier, I usually connect the MIDI track to a suitable MIDI drum device or plug-in so I can hear myself playing along — in this particular example, I used Groove Agent.

As we will see in a moment, the 'Merge Tempo from Tapping' function can work with taps made at a number of musical intervals. For the majority of people, quarter notes (on every beat of the bar) or whole notes (just on the first beat of the bar) are likely to feel most comfortable. There are advantages and disadvantages to having more or fewer 'taps', but in most cases, quarter notes serve as a sensible starting point. However, it is worth using a different MIDI note for the first beat of the bar relative to beats two, three and four. This means that the whole notes can easily be extracted and placed in a second MIDI track if you wish to see whether a tempo map created from whole notes might provide a better match to the live performance. Whatever interval is used, it's a good idea to continue tapping out time for a couple of bars beyond the end of the pre-recorded material to avoid any sudden jumps back to the default tempo.

While most of us might like to think we can play along perfectly in time with another musical performance, the chances are that the recorded MIDI 'taps' might need a little tidying up to tighten the synchronisation between them and the original live performance. Although this step can be omitted, and any sloppiness sorted out using the Time Warp tool as described below, I find it worthwhile to spend a little time editing my playing in the MIDI track. The most effective way of doing this is to use the excellent 'Edit In Place' function (although this is only available to SX users), as this allows the alignment between the transients within the audio and the MIDI notes to be matched visually. Alternatively, providing a suitably high level of zoom-in is used, this can be done quite easily by hand — although remember to switch 'Snap' off first.

Once any editing has been completed, selecting the MIDI part that contains the 'taps' and choosing the 'MIDI / Functions / Merge Tempo from Tapping' option opens the dialogue shown in the screenshot on the previous page. There are only two settings to be made here. First, you specify the musical interval of the taps, and second, you can tick the 'Begin at Bar Start' box. As its name suggests, this forces the first note (which ought to coincide with the first beat of the first bar of the pre-recorded live track) to fall on the start of a bar, so in most cases it should be ticked.

This tempo map was created using quarter-note tapping, and while the basic pattern of tempo variation can be clearly seen, it obviously still needs some tidying up!

With these settings made, a quick click on 'OK' is all that is required and, in a blink of an eye, Cubase will have done the necessary and created a 'map' of the tempo changes within the performance and placed them into the Tempo track. For our example Project, I've shown two Tempo maps: one created based on quarter-note tapping while the other is based on whole-note tapping. As can be seen, the broad patterns are very similar but, because the process creates a tempo curve point for each note within the MIDI track, the quarter-note version looks a little busier. Given that a tempo jump is placed at each tempo curve point, in projects where there are gradual changes of tempo, quarter-note taps are probably going to produce a more appropriate result.

This second tempo map was created using whole-note tapping. The earlier bars seem a little smoother and less subsequent editing is likely to be required. However, the gradual tempo reduction between bars 47 and 57 might require work as it doesn't appear as smooth as in the quarter-note version.

The simplest way to assess the accuracy of the tempo map created, or to compare the results from different maps, is to set a drum module to follow the tempo map and let it play back alongside the original performance. Again, I used Groove Agent for this.

One-Two-Three-Four

Not all forms of music benefit from tempo variations. An obvious example would be some dance-based styles, many of which require the metronomic pulse supplied by a fixed tempo. This said, even with sequenced music written and performed only in a studio context, it can be worth added a bpm or two to the chorus section to see what effect it might have. However, it is probably best to set up this tempo structure before any live audio is recorded (such as the vocals) — although if the variations are kept subtle (a few bpm), the SX tools for time-stretching and compressing audio ought to allow such change to be accommodated after the fact.

Warp Speed

Hopefully, if all went to plan and your ability to follow a beat is up to scratch, the Tempo track resulting from the 'Merge Tempo from Tapping' function might be all that is needed. However, if there are still sections where the new tempo map doesn't sit tightly enough with the timing of the original performance, then a little judicious use of the Time Warp tool is called for.

As shown in the screenshot below, if the Time Warp tool is selected, the positions of the tempo curve points are displayed within the Project window Ruler bar. Zooming in to any problem sections allows the positions of the transients relative to the bar/beat grid to be inspected and, if required, some fine-tuning can be performed using the Time Warp tool to drag the offending bar positions directly onto the start of the transients.

The Time Warp tool can be used to perform any final, detailed tempo editing. Note how bar two seems to hit the beat with the audio exactly but the position of bar three seems a little early. The Time Warp tool can be used to drag the position of bar three to the right to coincide with the audio transient. This would also smooth the tempo value out at bar three closer to the 108/109 bpm seen in bars one and two.

Incidentally, it is perfectly possible to use the Time Warp tool to create a tempo map for pre-recorded audio from scratch. All that is required is to identify the first beat of every bar within the audio and use the Time Warp tool to drag the appropriate ruler positions into place. However, using this approach on its own is generally more time-consuming than combining it with 'Merge Tempo from Tapping' for anything longer than a couple of dozen bars.

Time Is On Your Side

There are all sorts of elements that combine to make a musical performance that engages the attention of the listener and, for many styles of music, tempo variation, whether subtle or dramatic, is an important part of this process. With the vast majority of musicians now using digital technology at the heart of their studios, the 'set and forget' mentality for song tempo is all to easy to fall into. As the example above shows, given the tempo tools within SX, it doesn't have to be this way. You don't always have to be a slave to the (fixed tempo) rhythm. Go on, do yourself a favour and record a 'live' backing track for your next song and get SX to follow you instead.

Published October 2005