The 909 Kick drum revealed
Classic Bass Drum Sounds 1: The Roland TR909
No discussion of analogue bass-drum sounds could possibly be complete without studying the way in which Roland created the most used (and over-used) drum sounds of all time. These are, of course, the bass drums generated by the TR808 and TR909.
We'll start with the TR909, because this is the one that most closely follows the principles that we have discussed (see Figure 13 — and before anybody writes in to say that this doesn't look like Roland's schematic... you're right, I've laid it out differently).
http://img164.imageshack.us/img164/9...r909bdlem0.gif
Starting with the upper signal path, you can see that the oscillator produces a sawtooth wave whose pitch is defined by EG3, which has an instant Attack and slow Decay. The output from the oscillator then passes through a waveshaper. This removes almost all the overtones, transforming the sawtooth into something very close to a sine wave. This in turn passes through a VCA controlled by another contour generator (EG1) that provides the required AR envelope (the amplitude of EG1's Attack and its Decay rate are modified if the Accent voltage augments the Trigger).
Analogue Solutions' BD99 909 kick drum synth module.
So far, so good... so let's now turn our attention to the lower audio path. This starts with a noise generator whose output passes through a low-pass filter to remove the high frequencies. The output from this is mixed with a short pulse (essentially a click) provided by the pulse generator, and the sum of the two is contoured by a VCA controlled by EG2. Finally, a mixer combines the output from both the upper and lower signal paths to create the composite sound.
It may take a couple of moments to digest Figure 13, but once you have done so, it should be clear that Figure 1(b) and Figure 13 describe remarkably similar systems. Sure, there are detailed differences, but the fundamental ideas and patch structure are common to both.
Some synth fanatics dig deeply into the electronics of their TR909s to add additional controls for each element of the patch in Figure 13. However, if you don't fancy doing this, you could buy an Analogue Solutions BD99 (shown below left), a module that duplicates Roland's original circuitry, but provides a number of additional facilities.
For example, you can tune the basic pitch of the VCO (something that was not possible on the TR909). You can also control the amount of 'click' heard at the start of the note (the Attack level of EG2) and the Decay time for the sound (the Decay rates of EG1 and EG2). You can even apply varying amounts of Accent using the input provided, and control the pitch of the VCO using a CV.
Of course, none of this alters the basic principles we have discussed... these controls simply allow you to change some of the parameters that define the exact nature of the sound. But what a difference this can make. Many percussion instruments share common principles, so you can leave the domain of bass drums far behind, and use the BD99 to produce sounds such as toms and congas too.
http://img148.imageshack.us/img148/7...34bd99lzf6.gif
