EE2F2 - Music Technology

1. 9. Additive Synthesis & Digital Techniques EE2F2 - Music Technology

2. ‘Traditional’ Synthesis • Subtractive Synthesis • Covered last week in the lecture and lab • Additive Synthesis • A technique used in both analogue and digital forms • Still the subject of current research • FM Synthesis • Early digital synthesis technique • Still used in the synthesisers in cheaper soundcards • Advanced digital techniques • How it’s done today – more in the next two sessions

3. Subtractive Synthesis Limitations • Subtractive techniques can be used to synthesise a wide variety of waveforms from very simple sources • For synthesising natural sounds, it has several drawbacks • It isn’t always obvious how to synthesise the timbre (there can be several ways) • The result is usually only an approximation to the timbre you wanted • Some timbres are impossible to imitate using a simple subtractive synthesiser

4. Additive Synthesis • Recognising that all waveforms are just combinations of sine waves, an alternative synthesis technique seems obvious… • Just add together multiple sine waves • Mix them at the correct levels and you can synthesise any spectrum (i.e. create an arbitrary timbre)

5. Simple Additive Synthesis 20 Remember the oboe? 15 + 10 5 0 1 2 3 4 5 6 Taking the first six frequencies individually (fundamental plus first five harmonics) 20 15 10 5 0 1 2 3 4 5 6 + + + + + =

6. Limitations of the simple synthesiser • Just adding sine waves together gives a ‘static’ sound • Real sounds evolve with time and vary with pitch and velocity • The individual levels of the sine waves depend, therefore on pitch and velocity and they also change with time • i.e. They all have different envelopes • To synthesise this, in an additive synthesiser each sine wave goes through its own VCA controlled by its own envelope generator

7. Block Diagram mixer Output VCOs VCAs L.F.O. Trigger Env. Gens. NB. In addition to the envelope generators, an LFO is usually included, just like the subtractive synthesiser. The VCA gain is set by its E.G. and the keyboard output (and LFO)

8. Pros & Cons • Pros • Any combination of harmonics is possible • With the right balance, realistic sounds can be produced • Modern computer analysis techniques can extract the required parameters from a recorded sound • Cons • The number of harmonics is limited to the number of sine-wave generators • The complexity of the envelopes is limited • It’s difficult to program by hand • NB. All these drawbacks can be addressed using a modern, computerised equivalent

9. Traditional analogue synthesis Modern computer based synthesis Small number of partials (six or less). Large (unlimited?) number of partials. Each partial has a fixed frequency. Arbitrary frequency envelopes are possible. Amplitude of partials controlled by simple ADSR envelopes. Amplitude envelopes are arbitrarily complex. Sounds are passable synthetic versions of real instruments. Sounds are indistinguishable from the original. Current Research

10. Modern Additive Synthesis Flute example (CERL Sound Group, Illinois) Unlimited number of partials Highly realistic ‘Morphable’ Synth Flute Synth Cello Synth Cello/Flute

11. Early Digital Synthesis • Synthesisers using digital processors to generate sound emerged during the late 70s and early 80s • Common characteristics compared with modern instruments • Slow processors • Low memory • Algorithms for generating sound had to be computationally simple and undemanding of memory • The most successful entirely digital technique was FM synthesis – popularised by the Yamaha DX7 (1983)

12. Frequency Modulation Principles • A low frequency oscillator modulating a VCO produces a vibrato effect • If the modulating frequency is an audio frequency, a complex spectrum is produced • The spectrum can be calculated using Bessel functions • It depends on the magnitude of the modulation Osc. f VCO f

13. FM Synthesis - Operators • An FM synthesiser consists of a number of operators • Each operator consists of: • A digitally controlled oscillator (DCO) • An amplifier • An envelope generator • For basic FM, two operators are needed – one acting as a modulator and one as a carrier Frequency Control Input DCO Amp Env. Gen. Audio Output A single operator

14. 1 2 3 4 Additive 1 3 Paired 2 4 1 3 Stacked 2 4 FM Synthesis – Algorithms • Each operator has an (optional) input and an output • Exactly how the several operators in the synth are ‘wired’ together is not fixed • Different configurations can be used – these are called algorithms

15. Programming FM Synthesisers • Sound made by a modulator-carrier pair of operators varies dramatically with the modulation index • To program an FM synthesiser you need to: • Choose the right algorithm • Set-up the operators • Without detailed mathematical analysis, getting it right is a matter of: • Trial and error • Experience and patient experimentation • In fact, most users never bothered! • However, the range of sounds possible by varying only a few tens of parameters is (probably) unsurpassed

16. Pros & Cons • Pros • Very small number of parameters • Wide range of sounds possible • Requires very simple processing (sine wave generation and a bit of multiplication) • Unique sound is still emulated by modern synthesisers • Cons • VERY difficult to program even with computer assistance • Difficult for even a computer to figure out the best algorithm and the parameters needed to resynthesise a sound • The sounds are still unrealistic

17. Advanced Digital Techniques • Today, processing power and memory capacities have multiplied by thousands since the early digital synthesisers • Processing speeds mean that far more elaborate additive techniques are possible • Memory capacity means that digital recordings of real instruments can be stored for real-time playback • This can provide the most realistic sounding instruments (and, also the least!) using a family of techniques based on sampling

18. Summary • Subtractive synthesis • Start with a rich waveform, subtract unwanted harmonics • Not too difficult to program but will never sound realistic • Additive synthesis • Construct a timbre by adding sine waves • Can sound like the real thing, but only if enough partials can be used • Virtually useless for generating novel sounds • FM synthesis • Construct a timbre using frequency modulated sine waves • Can generate realistic or novel sounds using relatively few parameters • VERY hard to program! • Sampling • Next time!