Sampling: The 30dB Rule
Article from Sound On Sound, May 1986
Are software-based visual editing displays for samplers a help or hindrance? And what's significant about 30dB? Consultant Martin Russ reveals all...
SAMPLING: The 30dB Rule
Music technology consultant Martin Russ highlights one of the lesser-known problems of using sample waveform displays. Are those glossy brochures telling you everything? Read on to find out...
Waveform displays of sampled sounds and light-pen waveform drawing programs have always been a very appealing aspect of the expensive computer-based synthesizers: 'See the sound as you loop it', 'Draw the sound you want' etc...
Recently, similar displays have begun to appear on products with much more affordable price-tags. Whilst any tool which makes editing a sampled sound is welcome, there are one or two things to bear in mind when using such display systems, and it is some of these aspects which I intend to cover here.
Let's start by looking at Plot 1. This shows the sine wave shape so beloved of mathematicians and physicists. Musicians will be more used to hearing the 'sound' of this shape - a sine wave sounds like a very pure, simple, clean note: a sort of lifeless flute.
For reference, I will call this sine wave of +/—10 volts, the maximum or '0dB'. Most people I know seem to go a bit blank when a studio engineer starts muttering about dBs - so here's a quick potted introduction.
The range of size (measured in volts) of sound waveforms which the ear can cope with is quite enormous. On an ordinary LP track, for example, the difference in volume between the loudest sounds (large) and the quietest (small) is more than a million to one. Thus, if we said that a 1 volt peak sound wave represented our maximum (the loudest sound on the record), then the smallest sound wave would be one millionth of a volt - this is very small indeed and the numbers involved start to become very cumbersome. To get away from using these numbers we use a sort of technical shorthand - an equivalent of the 'large, medium, small, tiny' scale which people naturally use when describing objects. Decibels are simply a mathematician's way of keeping the numbers small and manageable.
The way it works, is that the largest normally used value is always assigned the label of zero decibels (0dB - pronounced 'zero dee bee'). If the value is one tenth of the largest value then it is given the label of minus twenty decibels (-20dB). If the value is one hundredth of the largest value then we use the label of minus forty decibels (-40dB) and a value of one thousandth of the largest value is given the label of minus sixty decibels (-60dB). Notice that for each division by ten, the dB value decreases by just ten. So our range of a million to one can now be expressed easily by a range of just 120 decibels. It turns out that we human beings can just about hear a change of one decibel, and that a halving of apparent volume is represented by about -8dB.
The following table shows some of the values mentioned above in an easy reference form. (Please note that decibels can also be used to measure power instead of volts, and for power the dB values are different to these.)
So, moving onto Plot 2, this shows the same 0dB sine wave as before, but also another sine wave, one thirtieth (-30dB) of the size of the original - the small wave is only just visible.
Let's try adding a similarly-sized high-pitched sine wave to the original sine wave. Plot 3 shows the result - a slightly wobbly sine wave. The ear hears sounds differently to the way we see waveform displays, and a difference of thirty decibels is not as significant to our ears - you would be able to hear the high-pitched sound quite clearly if you were to listen to the sound that this plot represents.
Talking of what things represent, if we consider the 0dB sine wave as the Fundamental note, then the high-pitched sine wave represents one of the Harmonics. Any VCF user will tell you that the harmonics are very important to the tone of a sound, since if you remove all of them all you hear is a sine wave.
You would also be able to clearly hear the noise which I have added to the sine wave in Plot 4. Here the noise is one hundredth of the size of the sine wave and is not at all obvious to the eyes when viewed on a computer monitor screen. It is probably a good idea to persuade the studio engineer to demonstrate some sound levels in terms of dBs when you are next in a studio - to get the figures the same as the ones here, ask him to tell you the level in dBV, rather than power (as I said above, the ratios are different, and I don't want you to get confused).
So, going by Plot 3 and 4, it seems that we can only just about see a Harmonic of a waveform if it is bigger than one thirtieth of the Fundamental. This is the so-called '30dB Rule' mentioned in the title. Plot 5 shows that the situation can be much worse than that - here it is just about impossible to see the noise, despite the fact that it is at a level of -30dB (one thirtieth). So it looks like we also have to be careful about how we display the sound.
If Plot 5 was showing the splice or joint in a looped sample, then we would be able to hear a glitch (click) but not see it on screen for any error in the joint which happened to be smaller than a thirtieth of the total sound level. If we were drawing a waveform with a light-pen, then the quietest harmonics which we could draw would only be of the same sort of ratio - thirty to one - to the total level of the final waveform. This sort of range is too small in many cases to be effective since the resultant sound is very sharp and bright, and usually needs filtering to tone down the high harmonics - in which case we don't hear what we have drawn, which defeats the object of drawing a waveform in the first place!
Plot 6 shows the effect that the above has on a real sound waveform - no hypothetical sine wave here! The noise in this plot is very loud - one tenth of the amplitude of the whole sound is noise (-20dB). It is possible to see the roughness (noise) in the flattish sections of the plot, but not at all on the steep slopes so beware.
The moral of this story is simple: there is no substitute for using your ears! Waveform displays are pretty and impressive, but can at present only give you a coarse impression of the sound. In contrast, your ears can tell you about both the coarse and the fine detail of the sound. So start listening!
Feature by Martin Russ
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