[sdiy] MIDI velocity

[Richie Burnett]

Thanks guys.  If it's possible to get acceptable performance with an 8051 
then I should be able to achieve something similar or better with a PIC18F 
series microcontroller.  Tom, your comments about 1ms resolution on the 
Prophet T8 are also encouraging.  This is only for a DIY fun project, so I 
don't need to aim for perfection, but need to get some grasp on the 
magnitude of the task.  I'll get the firmware working as a non 
velocity-sensitive keyboard first, then see what I can achieve in terms of 
implementing velocity sensitivity.  Even if I don't get the velocity 
sensitivity working for a while, at least the hysteresis afforded by having 
separate upper and lower switch contacts should do a nice job of de-bouncing 
the keypresses for me!

-Richie,

-----Original Message----- 
From: Ullrich Peter
Sent: Tuesday, April 05, 2016 2:39 PM
To: Richie Burnett ; synth-diy DIY
Subject: Re: [sdiy] MIDI velocity

Hi Richie!

>1. Is it reasonable to expect that I could use a low-end micro (e.g. PIC) 
>to scan a 61-key Fatar velocity sensitive keyboard with sufficient velocity 
>resolution to work well?  Or does >it need dedicated hardware to measure 
>the time between top and bottom switch closures for every key in parallel 
>with sufficient resolution.  For instance the old E510 is >quoted as having 
>128us resolution, and it sounds challenging to write a program to poll all
>61 keys nearly 8000 times a second?  Obviously I can use a free-running 
>timer, and subtract time stamps obtained from the top and bottom switch 
>closures for each key, but it >still sounds tight.  I couldn't find any 
>online examples of PIC based DIY velocity sensitive keyboards, and most 
>commercial keyboards seem to have an ASIC taking care of timing >the switch 
>closures and reporting results to a general purpose micro to form the MIDI 
>packets.

If a simple midi channel 0 only design is enough then you could use the 
keyboard chip from the Crumar bit one. It is a simple 8751 chip that outputs 
midi data and it is velocity sensitive. But you can only use it if it's a 
DIY project - so no commercial project... I have the binary on one of my 
PCs - so if you can program these chips this could be a solution for you.
The keyboard schematics can be found on page 3 of this service manual:
http://manuals.fdiskc.com/tree/Crumar/Crumar%20Bit%20One%20Service%20Manual.pdf

>3. How should a receiving synth set it's voice's volume depending on the 
>velocity byte received?  I tried to think through the physics of someone 
>hitting a piano key with a >specific velocity, what the energy is from
>0.5mv^2 etc, and how that should influence the amplitude of the resulting 
>sound wave.  It seems that most synths either have a linear response or an
>x^2
>response curve to velocity, but many offer a selection of different 
>responses.

As all instruments have their own interpretion some master keyboards have 
different velocity curves they send out.
I designed a velocity converter that in principles has some tables with 
different curves in it to convert from one velocity "feeling" to another.

Ciao
Peter

http://www.ullrich.at.tt        (My homepage)
http://synpro.heimat.eu/datadial.htm    (Synth addon)
http://rc100-usb.at.tt/     (Roland sampler tool)






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