Quantising Basics

THE BASICS OF QUANTISING

Subtle timing nuances play a large part in the feel of much of today's rhythm-based music. Martin Walker bangs the drum about creative use of groovy time-tweaking functions.

Music can be broken down into three parts -- melody, harmony and rhythm. Arguably, the last of these is the most important for much of today's music, and the accurate timing and placement of each beat in a riff can make a huge difference to the end result. The accomplished performer can add that almost imperceptible 'something' that transforms a pedestrian riff into a killer groove or turns a minor hit into a classic. We're not just talking about live performances here. Nowadays, just as much importance is attached to manipulating the timing of an existing performance as to playing a good groove in the first place.

Sloppy Timing

The simplest form of quantise (and the original reason for its existence in MIDI sequencers) is to move notes that are badly timed back onto the beat where they were intended to be. Devised as an aid for people with poor keyboard skills, the idea of quantisation was that however sloppy your performance during recording, every fluffed note could be pulled back exactly into line afterwards. This was a godsend to people who knew what notes they wanted, but didn't have (or didn't want to bother with) the technique necessary to play accurately and consistently in time.

However, although many people were initially impressed with this basic quantise function, perfect metronomic timing rapidly fell out of vogue, amid complaints that such music lacked soul, due to the absence of human expression. As more was understood about the mechanics of timing and musicianship, software developers added more elaborate tests for their quantising, so that only the notes that were way out of time were corrected, leaving the little nuances in place to give some life to the performance. Then it was discovered that if recordings by top-class players were analysed, their timing could be extracted and imposed on other performances. Quantising rapidly became not just a means to correct mistakes, but a creative tool in its own right.

Basic Quantise

Although the basic form of quantising moves every note played to an exact location in the bar, it is still important to understand the options. First of all, you need to set the quantise value. A quantise value of 1 will pull every note played to the nearest whole beat, a value of 2 will pull to the nearest half beat, 4 to the nearest quarter beat, and so on. The screen below shows how these three quantise values affect notes in a single free-form 4/4 bar.

To correct the timing of notes without completely destroying the feel , you need to select a value for quantise that relates to the music being played. For instance, if you have a four-to-the-bar bass drum, you need a basic quantise value of 4 to pull every note exactly on to the beat. The Over Quantise function is a more 'musical' offering from Cubase. This not only moves the notes as before, but detects chords and holds them together when moving notes, as well as noticing if you are consistently playing ahead of or behind the beat.

Fortunately, most sequencers allow you to undo any quantising, and/or to fix it in place when you are happy with the result, which makes experimentation less stressful. You can also often use automatic quantise, which moves the notes according to your chosen quantise values as they are recorded.

Just a Little Bit

Since basic quantising can rob an original performance of much of its human feel, various developers produced ways of pulling notes part way to the exact values. Cubase has Iterative Quantise which features Strength % and Don't Quantise parameters. The idea is that you set an amount, such as 50 percent, for Strength, and then any note not on an exact quantise value will be pulled 50 percent (or halfway) to the nearest hard quantise value. By setting Strength to even smaller values, you can keep applying Iterative quantise to gradually pull each note closer and closer to the exact value, then stop when you have the desired feel. The Don't Quantise function (set in ticks) ignores any notes that are close enough to the exact value to be already within the limits set by the Strength value. Only notes that are obviously out of line are moved, leaving any subtle timing imperfections alone. Analytic Quantise is a special Cubase option when you are mixing straight beats and a triplet feel, and attempts to preserve both feels when moving notes.

Groove Quantise

Once we get beyond the concept of 'correcting' timing, we enter the realm of rhythmic feels -- the way top players play certain beats in the bar consistently before or after the strict timing.

Possibly the biggest motivation in the development of groove quantise was reggae music, since many people loved the 'lazy' feel of the rhythm sections (particularly the classic pairing of Sly Dunbar and Robbie Shakespeare), who had an almost imperceptible behind-the-beat timing. Performances by such musicians were analysed in great detail, and it was discovered that some beats in the bar were always played away from the metronome position. At its most simple, in a single 4/4 bar, beats 1 and 3 might have been exactly on time, but beats 2 and 4 were slightly (and consistently) delayed.

Groove Quantise works by producing a map for a musical bar, which includes a pattern of these measured beats. The feel of a particular groove is dependent on the exact position of each of these beats. To use Groove Quantise within Cubase, you choose an appropriate map from the selection provided, and then the relevant notes in your own music are moved so that their timing exactly matches that of the groove map.

For instance, the upper (red) sequence of notes in the screen above shows a bass line played rigidly in sixteenth notes, while the lower three lines of notes show the effect of applying a few sample Groove Quantise settings. The yellow notes use the Slow Shuffle preset; the green ones use the Heavy Shuffle. Notice how the four main beats in the bar are still in exactly the same place. It is the in-between notes that have been delayed, with the Heavy Shuffle option simply shifting them further to the right. The fourth (blue) line of notes shows a real-world example. This has been matched to the audio drum loop described in the 'Rolling Your Own' section, below. The interesting thing to note in this case is that the shifts are much smaller. It shows that the timing changes made by a good live drummer will tend to be quite subtle.

Matchmakers

Although most sequencers come complete with a range of preset groove templates, you can create your own. The sort that most people want to make are for matching their MIDI music with an existing riff, either a MIDI sequence, or an audio snippet such as a drum loop taken from a sample CD. To make this easier, a few
sample CDs actually do all the hard work for you, and provide ready-made groove maps for the audio loops.

If you already have a MIDI part with the desired feel, and want to impose this feel on other parts, each phrase must be broken down into its constituent beats, and the timing of each note measured relative to its strict metronomic subdivisions. For instance, if there were four beats in each bar of music, a metronome would divide the bar into four quarter beats of exactly equal length. Measuring the actual positions of each of the four beats in a real performance allows each to be given an offset value, which can then be imposed on the corresponding beat in the other parts.

In Cubase this is easy to do by selecting the Match Quantise tool from the Arrange window toolbox, and then dragging the part with the correct feel (the source) over another part (the destination). However, you first need to set the normal Quantise box to a suitable value. This places a grid on the Match Quantise effect so that only the notes in the destination part that are close to notes in the source part will be moved, leaving the in-between ones where they are. When you drop the source part on top of the destination part, you also get the option of imposing its velocity values. If you decide to do this, there are two options. Copy will simply force the velocity values to those of the source part. However, you may already have strong accents in the original part that differ, and in this case using the Merge option will preserve them. If you are matching two different instruments it may be safer to use the No option.

Rolling Your Own

To extract the timings from an audio file, you first need to analyse it to find the start of each significant beat. This is done by scanning the file for a sudden increase in level, which normally indicates the attack start of each beat. As you might expect, most modern MIDI + Audio sequencers can do this for you automatically.

Let's start with a one-bar CD-ROM drum loop, and then apply its feel to a MIDI track. I'm using Cubase VST for this example, but you should find similar facilities in most current MIDI + Audio sequencers. After importing the drum loop as a WAV file into the Audio Pool, and then placing it in an audio track, open this up in the Audio Editor (as shown in the top half of the screen on page 102). Make sure that the contents of the window to the right of Ed Solo reads 'M-Points' and then click on the View menu, and check that Dynamic Events is highlighted.

Cubase uses M (Match) Points as Markers in an audio event, to indicate significant positions -- Get Match Points in the Do menu assigns M-Points to audio events automatically. The dialogue window has a number of settings, including Sensitivity and Attack, and although the default values will probably work well in many cases, you will probably need to increase the sensitivity to detect quieter beats such as the hi-hats between the stronger kick and snare hits. When you are happy with your settings, click the Process button in the dialogue box, and the Match Points will appear beneath the waveform. If some beats are not detected, select Get Match Points again, increase the sensitivity a bit more, and process again. If you want to be even more creative, you
can edit the Match Points using the normal Cubase tools, such as pencil and eraser.

To translate the Match Points to a quantise Groove Template, you need to select the Match Audio and Tempo option from the Do menu, which will take you to the Graphic Mastertrack editor. Here, in the Audio menu, you simply click on the M-Points to Groove option, and a new template will appear at the bottom of the existing list of Groove Quantise options. I used the Groove Template derived from a drum loop to match the timing of a MIDI bass line, as shown in the lower part of the screen on page 102. Notice how much the hi-hat beats have moved from the exact grid positions, and how the strength of each drumbeat that has been matched to a MIDI note has also been imposed on the MIDI note velocity.

Many of today's MIDI + Audio sequencers also allow you to carry out the same process in reverse, to impose the feel of a MIDI part onto an audio one. This is a similar two-stage process, the first part of which also involves the creation of Match Points. In Cubase VST, you select the desired MIDI part using the Match Quantise tool and then drop this on the audio part in question. You are first asked to confirm Dynamic Time Correction with a Yes/No response, and then exactly the same Get Match Points dialogue box appears (assuming that no Match Points have already been created).

Once the Match Points have been successfully created, the second stage happens automatically -- each segment of audio between the Match Points will be time-stretched to exactly fill the equivalent space between the MIDI notes in question. Fortunately, there is at least one level of Undo in most sequencers, as the audio data itself has to be modified to do this. In many cases you will probably get better results working a little with the Match Points, and perhaps even massaging them by hand in some cases, before applying any time-stretching.

Time To Take It Further

It's possible to lock MIDI timing to a complete live drum track, although many people prefer to sample a few bars at various points during the song, and then create their own loops to work from. However, you don't have to stick with the tempo of the original performance. Although you can get involved in time-stretching once your audio loop has had its M-Points added, another command in the Cubase Audio Do menu is Snip at M-Points which, as its name suggests, separates the audio into multiple chunks at the M-Points. Once this has been done, you can change the tempo of the entire song quite freely, with the audio staying exactly in sync, by squashing together or spacing the chunks apart.

If you are working entirely with MIDI material, it is possible to go too far, and strip out the subtle timing imperfections that can add the final icing on the cake. Some sequencers have randomising facilities available, to loosen up a track a little if it has been quantised too rigidly. Sadly, of the Steinberg range, only Cubase v4.0 for the Mac provides this feature, and although randomising is still possible using other versions, it involves working with Logical Edit, where few people dare to tread.

Finally, remember that although most people nowadays tend to use samples of drum loops in their entirety, you don't have to sacrifice audio quality to retain the feel. Simply paste that seventh-generation cassette copy of the killer groove into your sequencer, grab its Match Points, and use these to give you a MIDI groove template. Then you can map exactly the same feel on to any drum sounds (ancient or modern) that you care to use.