# [PD] very compressed chip sounds

Mathieu Bouchard matju at artengine.ca
Sun Oct 16 03:19:47 CEST 2011

```Le 2011-10-15 à 17:21:00, glerm soares a écrit :

> & sorry about the naive question - but can someone here try to explain
> shortly what this line does? This ">>" means some kind of bitshifts ?

Yeah... but to be clear, it does convert to plain int (truncating), shifts
the integer, and converts back to float, which is both slow and often less
useful than ldexp. But there *is* a real use for a truncating bitshifter
in pd.

about C or C++, asking for a float equivalent of the << and >> operators.
At least five people began by assuming that I wanted something weird that
has no possible use, and they started flaming me for it (real angry). In
the end, when I got them to figure out what a useful equivalent of << and
>> could be, they didn't know. Then I probably scanned in <math.h> for all
the functions I didn't know and finally found ldexp. I think that in pd,
it's only available in [expr] and in GF.

so, the difference is that 31>>3 == 3 whereas ldexp(31,3)==3.875, with
fractionary bits kept.

BTW, in floats, when bitshifting, the bits don't actually shift, and
instead, the exponent field increases or decreases, because that's how the
float format is multi-scale : it has a builtin concept of << and >> at its
core (even though hardly anyone ever knows what ldexp is !)

> I think some curious question could be - if there is a an [>>~] operator
> in pd what it does? Moves a bit for the next cycle of the of the sample
> block ?

What happens with nearly all simple math operators that have a class in pd
for floats, and another similarly-named class for signals, is that the
latter does the job of the former on every float that is found inside each
block. This happens without interactions between floats of different
instants.

So, [>>~] does, for each time t inside each block, take input x[t], apply
[>>], put into output y[t], that's all. It's all the same pattern as [+]
vs [+~], and [*] vs [*~], etc.

The big exception is stuff like [cos~], that has a different scale factor
because Miller said so.

> Can you indicate some patches to clarify this theory?

No... actually, I don't know much of a use for [>>~] in particular...
whereas [expr~ ldexp(\$v1,...)] might be optimised way(s) to do certain
cases of [*~]. I use >> a lot, but never in a signal context.

With people solving control-style problems using signal-style solutions
(e.g. Barknecht's experiments), I can see more use of typically non-signal
stuff being done with signal anyways, and this would explain the existence
of [>>~].

But frankly, I think that the existence of [>>~] is simply for
consistency, for completing the pattern of correspondence between float
ops and signal ops. In a certain sense, pd is simpler when it is more
complete, because there are less exceptions in its design. You know what I
mean ?

______________________________________________________________________
| Mathieu BOUCHARD ----- téléphone : +1.514.383.3801 ----- Montréal, QC
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