Yamaha PortaSound PSS-780 Operating Manual page 29

PART III
ADVANCED FUNCTIONS
DIGITAL SYNTHESIZER
Like a Yamaha DX synthesizer, the PortaSound PSS-780 uses an FM sound source — and like a digital
synthesizer, it lets you adjust various voice settings (parameters) in order to create your own original
sounds. Five of these custom voices can be stored in special memory banks.
The following short basic course on "FM Synthesis" which will help you understand this process, making
it easier for you to achieve the kind of sound you have in mind.
While this paragraph explains in detail how the voice parameters work together, their functions are
summed up in the next section, SYNTHESIZER PARAMETERS, for easy reference. You may want to read
that section first for a quick overview.
FM SYNTHESIS BASIC COURSE HIIHHTHHTHTTHTTET ETE
1) Voice Parameters and Values
The preset voices are edited (altered) by changing the values
of nine voice parameters (selectors 12c). These settings de-
termine what a voice sounds like by each adjusting a different
aspect of the tone. The controls for the DIGITAL SYN-
THESIZER section (12) are located under an illustration
(12d) that shows how these parameters are interrelated, giving
their names and numbers. It is reproduced further below for
your convenience. (The small numbers next to the parameter
numbers in the illustration are the respective value ranges
of the parameters.)
How to alter parameter values has already been explained
on p.10 in the PARAMETER CHANGE paragraph. Pressing
the parameter selectors (12c) calls up the value of each pa-
rameter on the TEMPO/PARAMETER & VALUE display
(9). The single digit left of the dot on the display indicates
the PARAMETER number/symbol. The two digit number
to the right of the dot is the current VALUE of that parameter.
@ The only difference to note here is that the parameter
selectors in the left row access two parameters each. This
is indicated by their having two numbers (e.g.[2//]) printed
on them. Repeatedly pressing any of these three selectors
switches between the two parameters shown on the selector
you pushed. The parameter number on the display will
alternate accordingly.
Once a parameter is called up on the display, you can change
its value with the — and + VALUE buttons (10).
The following explains how the parameters influence the
instrument sound. While reading this information, you will
be making various settings so you can actually hear how a
voice is created step by step.
26
2) A Bit of FM Theory
a) Modulation
Select voice 99, the SINE WAVE. Play the keyboard and listen
to the tone. Press a single note, and it sounds like an electronic
signal. Play a chord, and it is more similar to a cheap electric
organ.
As the voice name says, this is the sound of a pure sine wave.
It is the shape of such waves that determines the character
of a sound — a violin and clarinet sound different because
they produce a different wave shape. A sine wave looks like
this:
All FM sounds are based on such sine waves —- one sine wave
is used to modulate, or interact with, a second one, thus
changing the sound quality. In fact, FM is short for "Fre-
quency Modulation", which is the sine wave interaction
method your PortaSound uses to produce all the realistic
sounds you have already tried out.
Depending on the frequency ratio of the FM sine waves, the
resulting wave form will differ. The following two examples
show what happens when a sine wave with a frequency of
250 Hz modulates another sine wave of the same frequency
(ratio t:1), and when it modulates a 750 Hz sine wave (ratio
1:3).
Sine wave Sine wave Saw-tooth wave
Pro to am ARS
250 Hz 250 Hz
; , Square wave
Sine wave Sine wave 9
en Aa ici as
250 Hz 750 Hz