#+TITLE: Phasor* OverviewThe =phasor= (not to be confused with the phaser audioeffect) is a name used to describe a classic signalgenerator that produces a periodic ramp signal normalizedto be in range 0-1, set to move at a specified frequency.Phasors are typically used in the context of table-lookuposcillators such as @!(ref "oscf")!@. The term "phasor" comes from the engineering and physicsworld, used to describe the complex value of a sinusoid(phase, and amplitude). While not a complex value, thephasor in this computer music context could be the phasecomponent of the phasor. The adoption of the phasor term in the computer music worldcan be traced back to one of the oldest opcodes in Csound,but the author believes it may go back even further to theMUSICN family of computer music languages. It is noteworthythat the word phasor is six letters, which was thecharacter limit for opcodes in both MUSICN languages andthe older versions of Csound. In this context, it is areally nice name to describe this.* Tangled FilesTangles to =phasor.h= and =phasor.c=. If =SK_PHASOR_PRIV= is defined, it exposes the struct=sk_phasor=, otherwise it is opaque. #+NAME: phasor.h#+BEGIN_SRC c :tangle phasor.h#ifndef SK_PHASOR_H#define SK_PHASOR_H #ifndef SKFLT#define SKFLT float#endif <<typedefs>><<funcdefs>>#ifdef SK_PHASOR_PRIV<<structs>>#endif#endif#+END_SRC #+NAME: phasor.c#+BEGIN_SRC c :tangle phasor.c#define SK_PHASOR_PRIV#include "phasor.h"<<funcs>>#+END_SRC* Struct and initializationThe state of phasor is wrapped in a structcalled =sk_phasor=, and  initialized with =sk_phasor_init=.You'll need the sampling rate, and initial phase. #+NAME: typedefs#+BEGIN_SRC ctypedef struct sk_phasor sk_phasor;#+END_SRC In =sk_phasor=, =freq= stores the frequency, =phs= storesthe previous phasor value, and =onedsr= is the inverse valueof the sampling rate, which is used in order to shave offa traditionally more expensive divide operation. #+NAME: structs#+BEGIN_SRC cstruct sk_phasor {    SKFLT freq;    SKFLT phs;    SKFLT onedsr;};#+END_SRC #+NAME: funcdefs#+BEGIN_SRC cvoid sk_phasor_init(sk_phasor *ph, int sr, SKFLT iphs);#+END_SRC #+NAME: funcs#+BEGIN_SRC cvoid sk_phasor_init(sk_phasor *ph, int sr, SKFLT iphs){    ph->phs = iphs;    ph->onedsr = 1.0 / sr;    sk_phasor_freq(ph, 440);}#+END_SRC* Setting the FrequencyThe frequency of the phasor can be set with=sk_phasor_freq=. #+NAME: funcdefs#+BEGIN_SRC cvoid sk_phasor_freq(sk_phasor *ph, SKFLT freq);#+END_SRC #+NAME: funcs#+BEGIN_SRC cvoid sk_phasor_freq(sk_phasor *ph, SKFLT freq){    ph->freq = freq;}#+END_SRC* ComputationA single sample of audio is computed with=sk_phasor_tick=. #+NAME: funcdefs#+BEGIN_SRC cSKFLT sk_phasor_tick(sk_phasor *ph);#+END_SRC The output of the phasor is the current internal phasestate before being incremented. The behavior of the phasor is to increment its internalphase by an increment amount, determined by =freq*onedsr=.It is then wrapped to be in range 0 and 1. Instead ofusing something =fmod= for the wrapping, conditionalsand arithmetic are used. In addition to being simpler,this is often a faster approach. #+NAME: funcs#+BEGIN_SRC cSKFLT sk_phasor_tick(sk_phasor *ph){    SKFLT phs;    SKFLT incr;    SKFLT out;     phs = ph->phs;    incr = ph->freq * ph->onedsr;     out = phs;     phs += incr;     if (phs >= 1.0) {        phs -= 1.0;    } else if (phs < 0.0) {        phs += 1.0;    }     ph->phs = phs;     return out;}#+END_SRC* Resetting The PhaseThe phase of the phasor can be reset witha particular value in =sk_phasor_reset=. If the thevalue is less than 0, it will use a default value of 0. #+NAME: funcdefs#+BEGIN_SRC cvoid sk_phasor_reset(sk_phasor *phs, SKFLT val);#+END_SRC #+NAME: funcs#+BEGIN_SRC cvoid sk_phasor_reset(sk_phasor *phs, SKFLT val){    if (val >= 0) phs->phs = val;    else phs->phs = 0;}#+END_SRC