Metro

Overview

The metro (short for metronome) algorithm implements a pretty common kind of signal generator that you see in all the computer music environments.

Metro will produce a series of single-sample impulses at a specified rate. I personally like to call these impulses ticks, and the signal they produce a clock. A clock signal such as the one produced by metro can be used to drive things like sequencers and envelope generators (at the time of writing, these don't exist in sndkit yet, but it might be worth checking out Soundpipe modules like tenv, tseq, tenvx, tgate, and trand.).

Tangled Files

As per usual, this tangles out to header/C combo metro.c and metro.h. SK_METRO_PRIV exposes the sk_metro struct.

<<metro.h>>=

#ifndef SK_METRO_H
#define SK_METRO_H

#ifndef SKFLT
#define SKFLT float
#endif

<<typedefs>>
<<funcdefs>>

#ifdef SK_METRO_PRIV
<<structs>>
#endif
#endif

<<metro.c>>=

#define SK_METRO_PRIV
#include "metro.h"
<<funcs>>

Struct Initialization

Struct is called sk_metro. Initialized with sk_metro_init.

<<typedefs>>=

typedef struct sk_metro sk_metro;

<<funcdefs>>=

void sk_metro_init(sk_metro *m, int sr);

<<funcs>>=

void sk_metro_init(sk_metro *m, int sr)
{
    <<init>>
}

Struct Contents

<<structs>>=

struct sk_metro {
    SKFLT freq;
    SKFLT phs;
    int init;
    SKFLT onedsr;
};

freq is the metronome frequency.

An arbitrary default value of 2 is used.

<<init>>=

sk_metro_freq(m, 2.0);

phs is an internal phasor signal keeping track of time.

<<init>>=

m->phs = 0;

init is a flag set at startup to guarantee an initial tick on the first sample.

<<init>>=

m->init = 1;

onedsr is a constant that is 1 / sr, or one divided by the sample rate.

<<init>>=

m->onedsr = 1.0 / sr;

Setting The Frequency

Set the frequency of metro with sk_metro_freq.

<<funcdefs>>=

void sk_metro_freq(sk_metro *m, SKFLT freq);

<<funcs>>=

void sk_metro_freq(sk_metro *m, SKFLT freq)
{
    m->freq = freq;
}

Compute

Compute a sample of audio with sk_metro_tick.

<<funcdefs>>=

SKFLT sk_metro_tick(sk_metro *m);

metro requires so few lines of code it's almost not worth saying anything all about it.

The init flag is used to guarantee an initial tick at the beginning.

The phs value acts as a normalized incrementor. When it reaches 1, it wraps around back to zero. The increment amount is calculated from the frequency parameter.

<<funcs>>=

SKFLT sk_metro_tick(sk_metro *m)
{
    SKFLT out;
    SKFLT phs;
    SKFLT freq;

    phs = m->phs;
    freq = m->freq;

    out = 0;

    if (m->init) {
        m->init = 0;
        out = 1.0;
    } else {
        phs += freq * m->onedsr;

        if (phs >= 1) {
            out = 1.0;
            phs -= 1.0;
        }
    }

    m->phs = phs;
    return out;
}