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Synthesis - Part 4

FILTERS

In the last tutorial we covered oscillators, envelope generators and modulators (LFOs) and the relationships between these. We also touched on the fact that one of the envelope generators is used to control the VCF (Voltage Controlled Filter). Of course, we were using a very simplistic synthesizer with 2 oscillators, 2 envelope generators, one VCA (Voltage Controlled Amplifier) and one VCF. We were also discussing subtractive synthesis and what I want to do for this month’s tutorial is to concentrate on filters in general. The reason for this is that these tutorials are for synthesis as a whole and not specific to subtractive synthesis, if it were then we would only be dealing with a couple of types of filters. So, I thought it very useful to delve into filters a little more deeply and broadly. That way you can apply this tutorial to any type of filtering, be it hardware or software, as the process is the same for both, albeit the technology might be a bit different. We also touched on the fact that when we are not dealing with analogue synthesis and are using digital synthesizers, then the terminology changes slightly, so, for VCF we would substitute DCF (Digitally Controlled Filter) and for VCA we would substitute DCA (Digitally Controlled Amplifier).

So, without any further delay:

Welcome to the wonderful world of FILTERS

Briefly explained:

A filter allows you to remove unwanted frequencies and also allows you to boost certain frequencies. Which frequencies are removed and which frequencies are left depends on the type of filter you use.

Before we can list the different types of filters and what they do, there are a few terms and definitions we need to cover. These are crucial and are used all the time so it is important that you know what these terms are and what they mean.

Last month we covered harmonics and their fundamentals so I hope you are up to date with this subject as we will be using it as we progress in this tutorial. It is also a crucial part of this part of the tutorial.

Cut-off frequency This is the point (frequency) at which the filter begins to filter (block or cut out). The filter will lower the volume of the frequencies above or below the cut-off frequency depending on the type of filter used. This ‘lowering of the volume of the frequencies,’ is called Attenuation. In the case of a low pass filter, the frequencies above the cut off are attenuated. In the case of a high pass filter, the frequencies below the cut off are attenuated. Put simply: in the case of a low pass filter, we are trying to block the (higher) frequencies above a certain point and allow the lower frequencies through. In the case of a high pass filter, the opposite is true. We try to cut out or block frequencies below a certain point and allow the higher frequencies through. On analogue synthesizers this cut-off was called the slope or gradient. The actual terminology was more accurately described as the RC (resistor/capacitor). Don’t worry about this for now. What you do need to know is that on analogue synthesizers, the filter behaves differently to modern synthesizers that use algorithms.

Analogues use circuitry and for that reason alone, it takes time for the filter to attenuate frequencies, in proportion to the distance from the cut-off point. Today’s technology allows for instant cut-off as the filter attenuation is determined by algorithms as opposed to circuits. That is why the filters off an Arp or Oscar etc, are so much more expressive and warm as they rely completely on the resistors and capacitors to, first warm up, then to work but in a gradual mode (gradual meaning sloped or curved as opposed to instant). Depending on how well a filter attenuates or the way it attenuates gives us an idea of the type of sound we will achieve with an analogue filter. You often hear someone say ‘That Roland is a warm man’ or ‘Man, is that Arp punchy’. These are statements that explain how a Roland’s filters sound or how potent the Arp’s filters are. So, the speed at which the filter attenuates is called the slope or gradient. Another point to raise now is that you will often see values on the filter knobs on analogue synthesizers that have 12dB or 24dB per octave. That basically means that each time the frequency doubles, the filter attenuates by 12dB or 24dB everything at that frequency. These are also known as 2 pole or 4 pole filters (fig 2), each pole represents 6dB of attenuation. This is how analogue circuits were built, the number of circuits being used by the filter to perform the task at hand.

Although this is a general belief, it is one that is not entirely accurate and we will come to that later, as this is tied into the topic of rolloff. I do not expect you to understand the concept of 2 or 4 pole at this stage. I am just throwing it in for tagging purposes. Since the term has been mentioned, it will stay in your mind and when we come to tackle this area, you will say ‘Yeah, I remember the dude saying something about that’. The best way, and one that I always sign off with, is to experiment. Sweep that filter. Sweep is another one of these funky words we programmers or engineers use and all it really means is grab the filter knob and twist it. Yes, I know, we like to be a bit flash, but it sounds cool….

So far, it’s all been a bit easy for you. The texts in bold is my method of tagging, so that at any point, you can come back to any parts of these tutorials and have a reference point. There is more to come on the subject of filters but first, I would like to list and describe all the different types of filters, or rather, the most common types. If you delve into the filters that Emu provide on their synthesis engines, then it could go into pages, if I had to list them all. Right now I have kept it all simple as we are still in the beginners stages and when we come to more advanced synthesis techniques, then I have to get a touch more technical. Topics I will hurt you with, with regards to filters, will be phase, harmonics and overtones, Laplace Transform, passive and active filters, resonance, isolation, self-oscillation, rolloff, and a few more mind bending surprises. He he, evil I be.

But for now, I shall be a gentle soul and just list the different types of filters and explain what they do.

Low Pass-LPF (fig1) As mentioned earlier, this filter attenuates the frequencies above the cut-off point and lets the frequencies below the cut-off point through. In other words, it allows the lower frequencies through and blocks the higher frequencies, below and above the cut-off (the frequency at which the filter begins to kick in). The low pass filter is one mutha of a filter. If you use it on a bass sound, it can give it more bottom and deep tones. If used on a pad sound, you can have the filter open and close or just sweep it and it gives that nice closing and opening effect. You can also use this filter cleverly by removing higher frequency sounds or noise that you don’t want in your sound or mix. Because it blocks out higher frequencies at the cut off you set, then it’s a great tool if you want to remove hiss from a noisy sample or, if you use it gently, you can remove tape or cassette hiss.

Fig1

Fig2

High Pass-HPF (fig3) This is the opposite of the low pass filter. This filter removes the frequencies below the cut-off and allows the frequencies above the cut-off through. Great for pad sounds, gives them some top end and generally brightens the sound. It’s also really good on vocals as it can give the vocals more brightness and you can also use it on any recordings that have a low frequency hum or sound that is dirtying the sound, although, in this instance it would be a limited tool, as you could also cut out the lower frequencies in the sound itself, but still a tool that has many uses.

Fig3  

Band Pass-BPF (fig4) This is a great filter. It attenuates frequencies below and above the cut-off and leaves the frequencies at the cut-off. It is, in effect, a low pass and a high pass together. The cool thing about this filter is that you can eliminate the lower and higher frequencies and be left with a band of frequencies that you can then use as either an effect, as in having that real mid range type of old radio sound, or use it for isolating a narrow band of frequencies in recordings that have too much low and high end. Sure, it’s now really made for that but the whole point of synthesis is to use tools because that’s what they are, tools. Breaking rules is what real synthesis is all about. Try this filter on synthesizer sounds and you will come up with some wacky sounds. It really is a useful filter and if you can run more than one at a time, and select different cut-offs for each one, then you will get even more interesting results. Interestingly enough, band pass filtering is used on formant filters that you find on so many softsynths, plugins, synthesizers and samplers. Emu are known for some of their format filters and the technology is based around band pass filters. It is also good for thinning out sounds and can be used on percussive sounds as well as creating effects type of sounds. I often get emails from programmers wanting to know how they can get that old radio effect or telephone line chat effect or even NASA space dialogue from space to Houston. Well, this is one of the tools. Use it and experiment. You will enjoy this one.

Band Reject Filter-BRF-also known as Notch

This is the exact opposite of the band pass filter. It allows frequencies below and above the cut-off and attenuates the frequencies around the cut-off point. Why is this good? Well, it eliminates a narrow band of frequencies, the frequencies around the cut-off, so, that in itself is a great tool. You can use this on all sounds and can have a distinct effect on a sound, not only in terms of eliminating the frequencies that you want eliminated, but also in terms of creating a new flavour to a sound. But its real potency is in eliminating frequencies you don’t want. Because you select the cut-off point, in essence, you are selecting the frequencies around that cut-off point and eliminating them. An invaluable tool when you want to hone in on a band of frequencies located, for example, right in the middle of a sound or recording. I sometimes use a notch filter on drum sounds that have a muddy or heavy mid section, or on sounds that have a little noise or frequency clash in the mid section of a sound.

I would like you to, as always, experiment what we have covered and make a mental note of what happens whenever you adjust the filter or the cut-off.

But most important of all, enjoy yourself.

In the next tutorial we will go deeper into filters, covering resonance, phase and other wondrous head banging delights.