Take a 'half-time break' this month with Roger James Brown, as he ups the tempo and shows you how to play some time-stretching tricks with your sequencer....

Circling back to where we began this series of hints and tips for creative sequencing, let's look at 'timestretching', a piece of MIDI music to fit a sampled drum loop. Strictly speaking, you can't time-stretch a piece of MIDI music, as it isn't already fixed in time, like a sample. What we are about to do is better known by musicians as, 'playing with a halftime feel.'

For this exercise, we have a drum loop which has been sampled at 173 bpm and time-compressed by 96% to 180 bpm. The reason 180 bpm has been chosen as the final tempo has nothing to do with a dartboard, and everything to do with the guitar riff I have in mind to, 'time-stretch'. This guitar riff will probably be recognisable to you all, and was originally played at 90 bpm. It was recorded into Cubase at 90 bpm, and now we want to match it up with our drum loop. That loop being at 180 bpm, we could simply play the guitar riff at the same tempo. Instead of pitching up like a sample would, it would sound a bit manic and lose all the original feel.

What we have to do is similar to what ReCycle does with a sample loop, cutting the sample up into slices and replaying them at different tempos, without pitching up the samples. All we are going to do is double the length and placement of our notes, so the guitar riff, if played at its original 90 bpm, will sound as though it is being played at half speed. Then, once we reset the tempo to 180 bpm to match our drum sample, the riff should sound almost exactly the same as the original 90 bpm riff. Theoretically, there isn't any difference. But as Willie Nelson once crooned, it's funny how time slips away, and there are subtle differences of tone and timing between our two versions.

The procedure for 'time-stretching', a piece of MIDI music is not quite as simple as time-compression on a sampler, but it certainly is a lot quicker. Taking our original part (figure 1), we drag it out with the pencil until it is twice the length (figure 2). This is necessary, as we are about to double the note's length and placement (remember), and so we must ensure the part is long enough. Once we've done that, it's back to our old friend the Logical Editor. I've used a preset supplied with Cubase here called Half Speed, logically enough (figure 3), but you can see what it does by examining the settings. The only parameters being changed are the note length and position, both of which are being multiplied by two. Conversely, if we wished to double the tempo, we would divide these two by two.

Once we have performed this operation, if we play the piece at its original 90 bpm, that jagging feeling will soon overcome us as we listen to the notes drag themselves out. Doubling the tempo back to 180 (figure 4) restores the feel of the original, but allows us to pair this familiar riff with our new junglist drum groove. For those who are becoming a little confused at this stage, a quick peek at figures 5 and 6 should make things a little clearer. In figure 5 we see the original pattern, recorded at 90 bpm with most of the notes no longer than a sixteenth. Figure 6 shows the same piece after processing with the Logical Editor, and you will observe the lengths of the notes are doubled to eighths, while the placement is also doubled. Thus the first note, which was at bar 1, beat three and a sixteenth, is now at bar 2 and an eighth.

I've chosen this guitar riff, because it was half the speed of the compressed drum loop of course, and that makes the calculations a lot easier. If you wish to work with pieces of wildly differing tempos, you'll have to get the old calculator out, but the principles remain the same.

Listen as you learn, with Roger's demos and tutorial to accompany this month's Dream Sequences

Track 1: The MIDI files and Cubase song and arrangement files are to be found on the CD-ROM section in PC, Atari and Mac format.

Tracks 11-16: Roger gives it a whole lotta welly in his heavy metal jungle romp through an old Top of the Pops theme.