Left hand slurring technique is implemented simply
by sending a key-on event with a very low velocity value.
In this case the gain of the loop filter is not modified.
Additional pitch information is sent from ENP as a
scaler using two adjacent MIDI messages (this provides us
with a resolution of 14 bits). The maximum depth (or
max-depth) of a vibrato to be applied around the current
main frequency value is calculated as follows. If the score
does not contain for the current note any specific vibrato
expressions (i.e. we want only to play a "straight" note),
the max-depth value depends on whether the current fret is
0 (i.e. an open string) or not. If it is 0 the max-depth is
equal to 0. For higher fret values the max-depth is
calculated by adding more and more vibrato (the amount
is though always very moderate) as the fret value gets
higher. This is done in order to simulate the fact that for
higher frets the string is more loose which in turn makes
it more difficult for the player to keep the pitch stable. If,
however, the score contains vibrato expressions, the
vibrato max-depth is calculated depending on the name of
the vibrato expression. Vibrato expressions are named
with the string "vb" with an extension (a number from 1
to 9) indicating the max-depth of the vibrato. Thus to
simulate a very slight vibrato one can use "vbl", a
moderate vibrato is given with "vb5", an extreme vibrato
with "vb9", and so on. The speed of the vibrato is
normally kept static (typically around 5-6 Hz). The overall
depth, however, is controlled by an envelope - scaled to
the current max-depth value - with an ascendingdescending function with two humps, in order to avoid a
mechanical effect when applying a vibrato.
Forte vs. piano playing is simulated by changing the
gain of the excitation sample. Also the system adjusts the
cut-off frequency of a lowpass filter that filters the
excitation sample (forte playing has higher cut-off values,
piano lower ones). In forte playing the pitch is affected by
starting with a slightly sharp pitch which is gradually
lowered to the normal pitch value (an alternative way to
simulate this idea is discussed in Tolonen et al. 2000).
The pizzicato effect (where the player damps the
strings with the right hand) is accomplished by lowering
slightly the gain and the cut-off frequency of the loop
filter of the current string. Although this produces
reasonable results it would probably improve the pizzicato
effect if one would use special excitation signals for this
purpose.
The harmonics used in the classical guitar repertoire
are accomplished so that the player, while plucking a
string with the right hand, damps for a short time the
string with the left hand. After this the left-hand fingers
are lifted rapidly allowing the string to ring freely. This
effect produces a very distinct "bell" like sound. The
harmonics effect is simulated in ENP by setting the pluck
position value so that it matches the current string length.
Thus, if we want to produce a harmonic that is one octave
higher than the open string (i.e. the player damps the
string at the 12th fret) the pluck position value is 0.5.
Although we do not simulate the actual complex physical
behavior, our approach produces fairly good results.
6. Conclusions
We have presented in this paper how a classical guitar
model can be implemented in a general-purpose synthesis
environment. We also discussed how various playing
techniques are realized using an enriched notation package.
Future plans include for instance the improvement of the
automated analysis system for extracting excitation
signals and control data (some recent developments are
reported in Erkut et al. 2000). Also it would be
interesting to change the current MIDI control system to a
more flexible synthesis protocol such as Open Sound
Control (OSC, Wright and Freed 1997).
Acknowledgements
This work has been supported by the Academy of Finland
in project "Sounding Score - Modeling of Musical
Instruments, Virtual Musical Instruments and their
Control".
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