Conference napalm::commusic_v1

"Piezo" probably refers to a crystal that alters its electrical characteristics
when it's physically stressed (i.e. pounded, vibrated, squeezed, etc.).  They're
used, for example, in stress sensors and scales like the kind you might have
seen in a supermarket (where the teller puts veggies on a scale and a 
measurement is sent directly to the register, but the scale doesn't seem to 
move).  There are even cigarette lighters that use piezo's.  When struck, the
crystal generates the spark that lights the flame.

							Steve

Ahh, so a pad is effectively a capacitor that changes capacitance when struck.
Makes sense.  The octapad accepts Simmons pads so Simmons must have made the
technology public.  I will call Simmons and find out.

Thanks,

Todd.

    No, not quite - the pad emits a small voltage pulse when struck.
    Piezoelectric transducers change stress to voltage.  There are other
    kinds of pressure transducers, like capacitive, but I'm pretty sure
    the transducers in almost all drum pads are voltage output.
    
    The technology has been around for a long time - it's not proprietary
    to Simmons.  Most of the transducers produce similar voltages, and
    a resistor pad can be used to adjust things suitably.  Most
    incompatibilities between pads and boxes is level related, rather
    than sensor mode related.  I have triggered my Simmons head from
    a small, cheap microphone.  Any voltage pulse source will work.
    E.g., you could probably trigger the Simmons from the rim click out
    of another drum machine.  Hint hint.  Get a drum machine with
    separate audio outs and a MIDI in, run the audio outs for five selected
    sounds to the Simmons trigger ins, maybe with some way to fool
    around with levels and EQ, and you've got a MIDI to Simmons adaptor.
    
    Note that the voltage pulse from a pad is pretty "dirty", i.e.,
    it has a lot of "irrelevant" ups and downs.  Basically what you want
    to do is integrate the voltage over time and that's what you set your
    velocity from.  You could also look for a voltage peak and base
    velocity on that.  Alternatively, with a MIDI to Simmons convertor,
    you want to find out how the Simmons determines amplitude and tailor
    your pulse outputs suitably.

>    kinds of pressure transducers, like capacitive, but I'm pretty sure
>    the transducers in almost all drum pads are voltage output.
    
How can a passive element like a pad create a voltage?  Or are you saying
that the box is active, and the pad creates a variable potential between 
the two conductors going into the box via change in resistance.?

Todd.

    What Tom's saying is that the pads are not passive - you hit a piezo-
    electric transducer and it *generates* voltage.  The voltage comes
    from the mechanical energy of the impact (you don't get something
    for nothing).  They are not like resistors (or capacitors or
    inductors).  I'm not sure I'd call them active (like transistors);
    they're more like batteries?

    len.
    

    
    But.................this voltage may need processing
    to be "big" enough, have enough drive, derive a trigger
    from, etc.
    
    Nice thing is you get a proportionaly larger signal for
    more stimulus (ya hit it harder)...*almost* for free.
    
    The pad still may have batteries and circuitry, like I said.
    
    Ron
    

    
    Piezos are transducers, like loudspeakers. They convert one type
    of energy to another. Like a conventional loudspeaker, they can
    convert electrical energy to sound energy, or the other way around.
    Electrically, though, they look like a capacitor, they store electric
    fields.
    
    CdH
    
    
    

    re .7 - most pads do not have any batteries or onboard circuitry.
    The pulse conditioning is done in the input stages of the box.
    Another nice feature of piezoelectric transducers - no batteries
    required.  They can put out substantial voltages (approaching whole
    volts) if you stress them enough.
    
    len.
    

Geez, if I go home and put a cigarette in the pad jack and strike the pad
as hard as I can, will it light the cigarette? ;^)

Todd.

    Only if it's properly grounded.  ;^)
    
    len.
    

    
    Piezo tweeters will handle around 35 volts before exploding.
        
    TOm, what's a rank-3 tenor????
    
    CdH
    
    
    

    Aw shucks, I thought a rank-3 tenor was some guy with a high voice
    who'd made level three (like a black belt or something).  I wasn't
    sure if rank-1 was better than rank-3 (like privates and sergeants?),
    or vice versa.  I had a little trouble figuring out what tenors
    had to do with piezoelectric transducers, but I figured Tom was
    being his usual "just enough off the wall to keep it interesting"
    and it had something to do with pitch and stress.
    
    Now I find out it's just ordinary tensor stuff.  (Don't worry, I don't
    understand tensors, I just know they exist.)

    len.
    

    
    WHAT???? you don't understand tensors??? I..I...don't believe it.
    
    Len, my opinion of you has fallen to new lows.
    
    8^) 8^) 8^) 8^)
    
    CdH
    
    

Ok.  Suppose it is piezo and has the ability to create a potential.

What would be a good circuit to use to sense this potential and fire
a logic level signal?  Comparitor?

Todd.

I just realized I don't want a logic level signal (except for pulse
detection) because I want velocity sensing.  Thus I need some sort of 
voltage amplifier circuit feeding an A/D converter.  And I have five 
pads, so I would need either 5 A/D's or one A/D multiplexed in time 
(only if the A/D was fast enough).

Lesee, I don't think there would be enough time to do what Len suggested
regarding an integration of the pulse for velocity info.  This may cause
a noticable delay between the pad strike and the generation of the MIDI
info.  I think I would just be able to sample the peak of the pulse with 
the A/D (ie only one measurement) after detecting its existence by using 
a schmitt trigger to generate an interrupt ala Tom's idea.

Does this sound right?

Todd.

    Just a sniglet:
    
    Piezoelectric sensors will also produce a voltage output proportional
    to a magnetic field. (New guitar pickup?) I tried it already and
    it worked, the chip I used was a Hall Effect chip.
    
    Pretend you have a Piezo Crystal shaped like a miniature rectangle
    laying on your kitchen table with the long side running north and
    south.  If you apply a Magnetic field from the ceiling through the
    chip to the floor you'll get a voltage output proportional to the
    magnetic field on the east and west sides of the crystal.
           
             
    They're also used in Ribbon microphones (pressure change ---> Vout)
    
    
    If anyone wants any info. just say so.  I've got lots.                      
                                                                        
    
    Anybody got a midi spec out there I can get a copy of?
    
    Mike

    You can get a MIDI spec from the International MIDI Association.
    I don't remember their address, but it is elsewhere in this conference.
    Velocity has 127 values, but you don't need to implement them all.
    If you don't provide all values you should provide bias and sensitivity
    parameters for each pad.  These can be in the digital part of the
    interface.
        John Sauter

    The way the JLCooper DrumSlave works is to scan the inputs looking
    for something nonzero - it does this at logic rates (I don't remember
    how fast, but it's like at least 5000 times a second around all
    inputs, and there are about 12 of them - that's only 60kHz, or about
    16 usec per input).  I don't know how it does velocity mapping.
    You can integrate with a capacitor and measure the charge by
    dischargingthrough a known resistance and measuring the time for
    the voltage to cross a threshold.  This can all be done fast enough
    to not introduce noticeable delays.  However, a DrumSlave driving
    a TR-707 from Simmons pads did seem to me to be more "sluggish"
    than the Octapad, even through its external pad inputs.  I don't
    know how the Octapad does it, but it's cheaper than a DrumSlave
    and more flexible.  Incidentally my DrumSlave is for sale, for the
    best offer I can get.  Try me.  It listed for $600, but I'll settle
    for a *lot* less.
    
    len.