Functional Description - Spectral Audio ProTone Service Manual

Functional Description

2
The basic principle of the ProTone corresponds to the tried and tested analog synthesizer principle with VCO,
VCF, VCA, envelope generator and LFO. This principle is called subtractive synthesis.
In order that the ProTone is suitable for MIDI, an additional MIDI to CV converter is necessary which converts
the MIDI signals into analog signals with which the VCOs, the filter, as well as the envelope generator are
controlled.
The raw material for the sound is offered by the VCOs (Voltage Controlled Oscillator) in the form of a sawtooth
or rectangular signal as well as the noise generator (for effects such as wind and thunder sounds). The width
of the rectangular signal from VCO1 can be changed with the PW regulator (2). The narrower the rectangular
signal (regulator turned to the right), the 'sharper' the sound. VCO2 is synchronised from VCO1 with the SYNC
switch (6). The tone will be interesting in this case when the TUNE regulator (5) of VCO2 is turned (or by
modulating with the LFO VCO 2), which results in a typical "Sync-Sound".
By means of the external VCO input, other sounds can also serve as raw material. The ring modulator
multiplies the signals of the two VCOs which markedly amplifies the beat (frequency difference).
Note: When the SYNC switch (6) is switched on, no beats are developed and the ring modulator has
therefore no effect.
The SLIDE regulator (4) determines the time balance from one note to another and is valid for both VCOs.
Afterwards, the signal flows through the voltage controlled filter (VCF=Voltage Controlled Filter), within which
certain frequency ranges are suppressed. The lowpass filter allows low frequencies to pass and suppresses
the high ones, the highpass filter lets high frequencies through and suppresses the low ones. The frequency
from which the signals are suppressed is called the limit or cut-off frequency. With the ProTone, this is formed
from various sources:
1. CUTOFF frequency regulator (11)
2. ENV MOD regulator (29) (influence of the envelope on the cut-off frequency)
3. LFO
4. ACCENT regulator (14)
5. KEYFOLLOW switch (17).
With the KEYFOLLOW switch (17), the cut-off frequency increases on higher notes so that audible frequency
bands always remain the same. The resonance forms a feedback of the output to the input of the filter and
causes an amplification of the frequencies around the cut-off frequency.
The ACCENT regulator (14) sets the share of the second envelope and affects the cut-off frequency as well as
the volume. It is only active when the ACCENT indicator (15) lights, i.e., when the accent function is switched
on via the MIDI controller 65. The accent function is always active when the unit is switched on. By means of
the MIDI controller 65, it is now possible, as with the TB 303, to give individual notes an accent (value 127) or
to take an accent away (value 0). For this purpose, the corresponding control value must be sent in the
sequencer, timed either before or with the note.
Before the signal leaves the ProTone, it arrives at the VCA (Voltage Controlled Amplifier) which is available in
the ProTone in stereo form. Here the input signal is multiplied with a control signal. In this way, the volume can
be influenced by a control signal which comes from the envelope generator and the MIDI converter. The
volume of the ProTone is controlled with the GROUND LEVEL regulator (30) as well as via the MIDI controller
EXT (7). Additional to this level is the short-term increase through the accent function. The panorama (volume
relationship of the left-hand to the right-hand signal) can be set with the PAN regulator (31).
The LFO is particularly developed with the ProTone. Using its signal, the pitch from VCO1 and VCO2, the
pulse width of the rectangular signal from VCO1, the cut-off frequency of the filter as well as the panorama can
be modulated with the LFO ON ... switches (24).
With switches (21) and (22) selection can be made between 9 different signal forms. By means of the external
LFO input on the rear of the housing, it is also possible to use any other signal for modulation. To do this,
switch (22) must be turned completely to the right. A microphone, etc, must be connected via a pre-amplifier
as this is a high level input. When the external inputs of the ProTone are used, the ProTone must
simultaneously receive a note command (the same MIDI channel) in order that something is audible.
The envelope generator determines the chronological sequence of the volume after receiving a note
command. The MIDI to CV converter then gives out a gate signal (+5V) with which the sequence of the
envelope generator begins. The gate signal remains at +5V until the 'note off' command comes, i.e., the key is
Service Manual ProTone Page 5 of 20 © 1996-2001 Spectral Audio