[sdiy] MIDI velocity

Tony Kalomiris tokalo at videotron.ca
Mon Apr 11 15:02:24 CEST 2016


The Jupiter-6 master cpu scans 61 keys, the control panel, handles MIDI interrupts, 
and communicates with the slave voice micros at the MIDI baud rate, including firing arpegiated note on/off messages
 at full speed without a glitch..well not quite, 
it does miss attacks on very short notes in quick succession coming from MIDI (remedied in later firmware versions)
Of course it's not a velocity sensitive keyboard, the subject of this thread. 
That poor overtaxed little 12 Mhz 8051.

The clock doubling is a good idea, but it would create havoc I think, linking a modern 'faster' device to an incompatible older slower MIDI device , unless a new instruction was added to signal normal or double speed.
This reminded me of how cleverly the Roland MC-202 got over the 24 ppqn resolution- with a hardware clock doubler (XOR gate), to get 48 ppqn. 
Of course can't do that cheap trick with baud rate.
I seem to recall a Sequential sampler or something that doubled speed with other Sequential gear for SDS , sample dump. Correct me if I am wrong.

Tony K

On 2016-04-10, at 2:33 PM, Tom Wiltshire wrote:

> 
> On 10 Apr 2016, at 19:09, "Richie Burnett" <rburnett at richieburnett.co.uk> wrote:
> 
>>> Did you do the keyscanner as a separate processor then, Paul? Doesn't that
>>> re-introduce latency when you have to communicate with the main uP?
>> 
>> The latency won't be much if you use a high baud rate for the communications
>> between the keyscanner and the main micro.  That's what I'm going to do.
>> 
>> The low data rate of MIDI is a real bottleneck for getting shot of all the
>> note events that can potentially be generated in each small time interval of
>> a fast key scanning process.
> 
> I definitely agree about MIDI being the real bottleneck. You have to wait like literally *milliseconds* for anything to happen! <teenage eyeroll>
> 
> I suppose if you're scanning at (say) 4KHz/250uS, then sending multibyte messages at a few MHz isn't going to hold anything up. For example, three bytes: 24 bits x 1usec = 24usecs, so we've time for ten such messages before our next scan. So SPI or even a fast serial connection between the keyscanner and the main uP looks feasible.
> 
> (Incidentally, I never really understood why the MMA didn't just up the data rate of the MIDI spec back in the 90s sometime. We could have coped with "two speed" MIDI. Modern synths could have had a baud rate selector so they could talk to whatever. We manage with USB1, USB2, etc. It doesn't need a whole new spec, just the same, but faster.)
> 
> Tom
> 
> 
> 
> 
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