Hello,
Yamahas BC3a was legendary, but there always alternatives was on the market and there have always been many DIY projects with BreathControls.
Common to all is the basic function to convert the breath pressure signal to MIDI messages. And with each of them - and any other other kind of controller - with a Pa-arranger you can use upper 1,2,3 at the same time with Midi-In-setting to "Acc2".
Overview Breath Controller --->
http://www.digital-notes.de/uebersicht- ... ontroller/
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In order to be able to make the sound of a Pa keyboard more lively with a breath controller, the oscillators of the multisamples must also take into account the use of this controller, regardless of which command it sends.
With CC # 2 most sounds playing "Sustain" or "Legato" for brass and wood base instruments will be used for all kinds of swells, fortepiano,
sforzando etc (Standard AmpSetting:Amp Modulation Veloctiy Intensity -99, AMS Joystick -Y, Intensity +99).
If the converter (of airflow to a signal for CC) is too slow for staccato or repetition, this should be taken into account for blowing noises in the sound editor, because than it is more suitable the breath control control may send CC # 11 (Expression). So for staccato passages, a continuous flow of air and a constant volume are created and notes are triggered via the velocity of the keyboard.
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With some Breath Controls the airflow can be regulated mechanically using a rotatable valve, so you need less air for a flute than with a tuba.
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For Strings with Breath Control you can get dynamic sounds with realistic results. Commercial Breath Controls converts input to MIDI-CC values which you then can map to control expression, dynamics, filters and basically anything that you can map MIDI CC to.
In case you plan a DIY-breath-control (maybe with an Arduino) it seems to be better to use the inverse signal for CC#2, to make better use of the dynamic range and thus to achieve gradients up to a complete fade-out of the sounds (setting: Amp Modulation Veloctiy Intensity 0, AMS Joystick -Y, Intensity -99). Otherwise you have to be careful, especially when playing solo strings, that under a certain minimum of air pressure no more values may be sent, so that delicate fade-outs cannot easily be realized.