objects\gpio\in\analog

[DIY DSP]

Hi!  I have a question about analog.axo:

The krate code is:

%out%= adcvalues[%channel%]<<15;

And the values, when I send them to a disp/i object, range from 0-127.

So how does a value get left-shifted by 15 bits, then have a value from 0-127?

Thank you very much,
DIYDSP!

[Evgeniy Chernyh]

as i understand, it's fixed point representation of int.

четверг, 23 апреля 2015 г., 8:42:48 UTC+5 пользователь DIY DSP написал:

[Johannes Taelman]

the long answer:

The "gpio/in/analog" object has a fractional output.

Fractional data (blue for control rate, red for audio rate) are integers indeed.

To the user, fractionals are represented as -64.0 to 64.0 range. This allows for 1 unit per semitone scaling. If it had been -1.0 to 1.0 users would need to deal with 0.00

Internally, the -64 to 64 range uses -0x08000000 to 0x07FFFFFF. Note that this is not using the upper 4 bits of the 32 bit integer. The upper 4 bits are "arithmetic headroom". Without arithmetic headroom, two fractionals cannot be added or subtracted with 32bit integer operations without the risk of overflow. With 4 bits "arithmetic headroom", 16 fractionals can be added without overflow.

The "disp/hex" exposes this internal representation.

The ADC is setup for 12 bit right-aligned conversion results. Left-shifting this by 15 places converts this into the 0 to 64 "user unit" range. A fractional output on the "gpio/in/analog" object abstracts the actual resolution of the conversion.

Connecting a blue outlet (fractional) to the green (integer) inlet of "disp/i" implies converting (truncating) to -64..64 range.

More details: 64.0 is not legal, 63.999999523... is the legal maximum.

When MIDI-mapped 63.5 is the practical maximum. In future, a midi-mapped parameter should be constrained to values that can be transmitted too over midi - so that sequencer-recorder MIDI reproduces the same result. Non midi mapped parameters should be able to reach 63.999...
A trick the Nord Modular did is substituting midi value 127 with 127.99... to reach a more complete range, far closer to "preserving unity" when multiplying with a full scale midi value. It is a dirty trick to remap the midi range of 0-127 to 0.0...64.0 bipolar or -64..64 for bipolar. The alternative is scaling midi with 128.0/127.0 but that deviates from 1 unit/semitone scaling.
I filed issue #27 on github to keep track of this future change.

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