FM Synthesis

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FM Synthesis(Frequency Modulation Synthesis)
In synthesisers, modulators are used (to generate a frequency, e.g. 440 Hz, concert pitch A). A tone is
generated by linking a `mother keyboard' or keyboard to a modulator to generate a pitch. Simples. The modulator
goes through another modulator, which passes through a carrier in FM synthesis. An example is a sine wave being
passed through a frequency modulator (goes up and down in pitch rapidly) and causing it to create harmonics
and interesting tones (turn your headphones/speakers right down before clicking on this link, as some of the
sounds are veryloud).
This original tone from the first modulator (concert A) passes through another Modulator, which
modulates (or changes) the pitch of the concert A (so it goes from, for example, 44 Hz to 4400 Hz (aka, 4.4 kHz,
as the kmeans 1,000).
The depth is how much the pitch goes up and down , so a large depth will have a bigger pitch range
(going from 100 Hz to 10 kHz, or 10,000 Hz), and a small depth will have a smaller pitch range (430 Hz to 450 Hz,
like a singer's vibrato almost). The rate is how fast the pitch goes up and down, and is alsomeasured in Hz.
Remember the number of Hz is how many times something happens in 1 second, and is a
measurement of frequency (like how you measure the length of something in metres or centimetres)
When you pass it through the second modulator, the original 440 Hz sounds completely different,
because you have change the pitch at a particular depthand rate.
Digital synthesis (or additive synthesis) is when you add each harmonic above the original (or
fundamental, the root note, or the note you actually hear). This is also known as additivesynthesis.
Remember that a harmonic is an overtone which is the sum of the original frequency (e.g. a 100 Hz
fundamental note will have the following harmonic series: 100 Hz (fundamental) => 200 Hz => 300 Hz => 400 Hz
=> etc...) See next page Figure 3
Sampling Synthesis
This is when you take a sample, or you record your own sound (e.g. a human voice saying something,
like Dr Pike saying `MIDI'). The sound can now be tweaked in terms of pitch, speed and any other parameter you
can think of. So if you take the `MIDI' speech and you play a middle C on a synth keyboard, it will sound at the
normal pitch (although this can be changed).
However, if you play this at a higher pitch, e.g. 3 octaves higher, the `MIDI' speech will sound like a
chipmunk. 2 octaves below the original, the speech will sound like a demonic voice. This is because the sample is
either sped up (for higher pitch) or slowed down (lower pitch). This data, for a digital sampler, would be stored on
RAM (random access memory) on a computer/digital device. Today, samplers would typically record in 16 bit
resolution and 44.1 kHz sample rate .
Sample-based synthesis
This is when you record each individual note exactly as it is played, as opposed to one (e.g., a string
section in an orchestra). Each individual sound/timbre/note would need to be recorded and edited individually.
So that when you played it back, the note (at any velocity) would sound realistic, and would avoid the chipmunk
or demonic sound you would get in normal sampling synthesis.

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Figure 1, where an approximate 7-second portion of
the sound is looped
When sampling a note, e.g. a trumpet sound, we can record a
10 second sample. But the problem is that the sample only
lasts 10 seconds. So to get around this, we loop a portion of
the wave to constantly repeat, e.g. 7 second portion of the
wave. So the sound will now continue forever...…read more


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