ee2f2 - music technology 9. additive synthesis & digital techniques
TRANSCRIPT
EE2F2 - Music Technology
9. Additive Synthesis & Digital Techniques
‘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
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
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)
Simple Additive Synthesis
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Remember the oboe?
Taking the first six frequencies individually (fundamental plus first five
harmonics)
+ + + + + =
+
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
Block Diagram
Env. Gens.L.F.O.
Outputmixer
Trigger
VCOs VCAs
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)
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
Current Research
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.
Modern Additive SynthesisFlute example(CERL Sound Group, Illinois)
Unlimited number of partials
Highly realistic
‘Morphable’Synth FluteSynth CelloSynth
Cello/Flute
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)
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
VCO
Osc.
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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
DCO
AmpEnv. Gen.
A single operator
Frequency Control Input
Audio Output
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
1 2 3 4 Additive
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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
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
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
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!