Subject: Re: [linux-audio-dev] more on portamento and legato ...
From: Tom Pincince (stillone_AT_snowcrest.net)
Date: Mon Sep 25 2000 - 02:08:41 EEST
>Legato Pedal:
>When on, this causes a legato effect between notes, which is usually
achieved by skipping the attack
>portion of the VCA's envelope. Use of this controller allows a
keyboard
>player to better simulate the phrasing of wind and brass players, who
often
>play several notes with a single tonguing, or simulate guitar pull-offs
and
>hammer-ons (ie, where secondary notes are not picked). If a
MultiTimbral
>device, then each Part usually has its own Legato Pedal setting.
This definition is almost correct. Musically it simply means playing a
note smoothly relative to the previous note, with no
silence between notes and no accentuation of the attack or release of
the notes, so that a continuous sound with variations in
pitch only is produced. The opposite effect is staccato, where the
attack and release of each note is punctuated and there is
a short silence between notes. For synthesis, this effect is applied to
monophonic voices by not retriggering the amplitude
envelope. The old note simply stops abruptly and the new one inherits
the old envelope. This is different from "skipping the
attack" because, for envelopes with long attack and decay times, the new
note might come in while the old note was still in
the attack or decay portion of the envelope. In this case the new note
would begin in the location of the envelope precisely
where the old note was, the instant that the new note is played.
Skipping the attack implies jumping immediately to the
sustain level, which will produce an abrupt level change if the previous
note has not yet reached the sustain level. While
legato may be indicated with a pedal, it can also be controlled with
playing technique. A synth in legato mode knows that it
should not retrigger the envelope for a new note if the previous note
has not been released. If all previous notes have been
released the synth knows that it should trigger a new envelope for the
new note. The new envelope may be triggered from
the beginning, or the attack level may start at the release level of the
previous note if the previous note has not yet faded to
silence even though it has been released. This choice of attacks is
represented by two modes of mono synthesis, and is independent of legato
mode.
>What does this mean in terms of sample playback engine ?
>(we are dealing with samples of real instruments, not with elementary
waves
>like in synths)
It means complexity and an enormous opportunity for subtlety and
refinement. Large sample size means that looping is not necessary for
instruments with a long natural decay, like piano. However one of the
fun things about samplers is creating new sounds from combinations of
samples. The way to address this is probably to use a format that
allows loop points to be designated for all samples, and a patch
parameter that allows the loop to be used or ignored.
>first, it seems to me that this only makes sense in monophonic
>instruments (correct me if I'm wrong).
Yes. Legato performance on polyphonic instruments is accomplished
entirely through playing technique and does not depend
on the mode of synthesis in any way afaik.
>Does "skipping the attack portion of the VCA's envelope" in the case of
a
>sampler mean that we shoud skip the attack volume envelope, and play
the raw
>sample from the beginning, or skip the attack part of the sample too ?
>(eg jumping directly to the sustain section, but this may not be
possible
>without clicks (a fast fadein might solve this problem, but I'm not
sure if I'm
>thinking in the wrong direction here)
I recommend trying to treat position within a sample and position within
an amplitude envelope identically, and play the new sample from the same
position in its soundfile as the old one was. If this results in
clicks, then abandon this technique and just jump to the start of the
loop, but stick with the old amplitude envelope. As a practical matter,
legato almost always results in jumping to the sustain section, because
the envelope almost always reaches the sustain level, and the sample
almost always reaches the loop point, before the second or third note of
a legato line has been played.
Regarding clicks and fades, there are no fades in legato playing for
acoustic monophonic instruments. Pitch change is
immediate. To avoid clicks, I recommend changing pitch or sample at the
first available zero crossing. While I don't have a sampler, (only a
sample playback synth) I seem to recall that samplers do allow for cross
fades at the loop splice, so it might work for legato also.
With portamento, which is a continuous slide between notes, my insights
are derived from observing the method used by my
UltraProteus. If I play the highest note on my keyboard, then play the
lowest note, the synth first triggers the sample mapped
to the highest note. The instant I play the lowest note, the synth
stops the first sample and triggers the sample mapped to the
lowest note and pitch shifts it up 5 octaves. It immediately descends
in pitch until it stabilizes at the unshifted lowest sample.
No middle samples are accessed. If I play a third note before the
portamento has reached the end of the second note, the second note is
stopped, and the sample mapped to the third note is played and pitch
shifted to the exact point that the second note had reached. If the
portamento time is long enough, the quality of the sound can be heard
while it is gliding, and it is BAD. When used in a reasonable way this
is not so evident because the sound is changing so fast that you can't
determine sound quality.
Tom
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