GENfarey
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## GENfarey

"farey" — Fills a table with the Farey Sequence Fn of the integer n.

## Description

A Farey Sequence Fn of order n is a list of fractions in their lowest terms between 0 and 1 and in ascending order. Their denominators do not exceed n. This means a fraction a/b belongs to Fn if 0 ≤ a ≤ b ≤ n. The numerator and denominator of each fraction are always coprime. 0 and 1 are included in Fn as the fractions 0/1 and 1/1. For example F5 = {0/1, 1/5, 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 4/5, 1/1} Some properties of the Farey Sequence:

- If a/b and c/d are two successive terms of Fn, then bc - ad = 1.
- If a/b, c/d, e/f are three successive terms of Fn, then: c/d = (a+e) / (b+f). In this case c/d is called the mediant fraction between a/b and e/f.
- If n > 1, then no two successive terms ofFn have the same denominator.

The length of any Farey Sequence Fn is determined by |Fn| = 1 + SUM over n (phi(m)) where phi(m) is Euler's totient function, which gives the number of integers ≤ m that are coprime to m.

Some values for the length of Fn given n:

n | Fn
--- | ---
1 | 2
2 | 3
3 | 5
4 | 7
5 | 11
6 | 13
7 | 19
8 | 23
9 | 29
10 | 33
11 | 43
12 | 47
13 | 59
14 | 65
15 | 73
16 | 81
17 | 97
18 | 103
19 | 121
20 | 129

## Syntax

    f # time size "farey" fareynum mode

## Initialization

size -- number of points in the table. Must be a power of 2 or power-of-2 plus 1 (see f statement).

fareynum -- the integer n for generating Farey Sequence Fn

mode -- integer to trigger a specific output to be written into the table:

- 0 -- outputs floating point numbers representing the elements of Fn.

- 1 -- outputs delta values of successive elements of Fn, useful for generating note durations for example.

- 2 -- outputs only the denominators of the integer ratios, useful for indexing other tables or instruments for example.

- 3 -- same as mode 2 but with normalised output.

- 4 -- same as mode 0 but with 1 added to each number, useful for generating tables for tuning opcodes, for example cps2pch.

## Examples

    f1 0	-23 "farey" 8 0

Generates generates Farey Sequence F8. The table contains all 23 elements of F8 as floating point numbers.

    f1 0 -18 "farey" 7 1

This generates Farey Sequence F7. The table contains 18 delta values of F7, i.e. the difference between ri+1 - ri, where r is the ith element of Fn.

    f1 0	-43 "farey" 11 2

This generates Farey Sequence F11. The table contains the denominators of all 43 fractions in F11.

    f1 0	-43 "farey" 11 3

This generates Farey Sequence F11. The table contains the denominators of all 43 fractions in F11, each of those divided by 11, i.e. normalised.

    f1 0	-18 "farey" 7 4

This generates Farey Sequence F7. The table contains all fractions of F7, same as mode 0, but this time '1' is added to each table element.

Example 1226. A simple example of the GENfarey routine.

See the sections Real-time Audio and Command Line Flags for more information on using command line flags.

    <CsoundSynthesizer>
    <CsOptions>
    </CsOptions>
    <CsInstruments>

    sr=44100
    ksmps=10
    nchnls=1

    instr 4
          kndx init 0 ; read out elements of F_8 one by one and print to file
          if (kndx < 23) then
          	 kelem tab kndx, 1
          	 fprintks "farey8table.txt", "%2.6f\\n", kelem
          	 kndx = kndx+1
          endif
    endin
    </CsInstruments>
    <CsScore>
    ; initialise integer for Farey Sequence F_8
    f1 0 -23 "farey" 8 0
          ; if mode=0 then the table stores all elements of the Farey Sequence
          ; as fractions in the range [0,1]
    i4	0     1
    e
    </CsScore>
    </CsoundSynthesizer>

Here is a complete example of the GENfarey routine. It uses the files genfarey-2.csd.

Example 1227. Another example of the GENfarey routine.

See the sections Real-time Audio and Command Line Flags for more information on using command line flags.

    <CsoundSynthesizer>
    <CsOptions>
    ; Select audio/midi flags here according to platform
    -odac     ;;;realtime audio out
    ;-iadc    ;;;uncomment -iadc for RT audio input as well
    ; For Non-realtime ouput leave only the line below:
    ; -o genfarey.wav -W ;;; for file output any platform
    </CsOptions>
    <CsInstruments>
    sr = 44100
    ksmps = 32
    nchnls = 2
    0dbfs  = 1

    ; GENfarey creates table gidelta.
    ; The table contains the delta values of Farey Sequence 7 (p5=7).
    ; They are used as Inter Onset Intervals (IOIs) or event durations.
    ; If p6 is set to 1 for IOI output then the length of the table (p3=-18) is -(|F_7| - 1)
    ; Remember that a negative sign is for non-power-of-2 table lengths.
    ; The negative sign in front of the GEN number prevents post-normalisation of its values.

    gidelta ftgen 0,0,-18,"farey",7,1

    ; Use GENfarey with p6 set to 2 to generate the denominators of fractions of F_7
    ; this is used in this example as factors to create a series of pitches:
    gimult ftgen 0,0,-18,"farey",7,2

    ;-------- loop and trigger instrument 901 using a Farey Sequence polyrhythm
    	  instr 1
    kindx init 0
    kindx2 init 0
    ktrigger init 0
    ktime_unit init p6
    kstart init p4
    kloop init p5
    kinitndx init 0
    kfn_times init gidelta
    knote init 60
    kbasenote init p8
    ifundam init p7
    ktrigger seqtime ktime_unit, kstart, kloop, kinitndx, kfn_times
      if (ktrigger > 0 ) then
         kpitch = cpspch(ifundam)
         kmult tab kindx2, gimult
         kpitch = kpitch * kmult
         knote = kbasenote + kmult
         event "i", 901,   0,   .4, .10, kpitch, kpitch * .9, 0.4,  5,   .75, .8,  1.0, .15, .0,  .125, .125, .25, .5,  1.0, .0, .0,  .0,  .0,  .125, .25, .25, .25, knote
         kindx = kindx + 1
         kindx = kindx % kloop
         kindx2 = kindx2 + 1
         kindx2 = kindx2 % kloop
      endif
    endin

    ;------ basic 2 Operators FM algorithm ----------------
    	instr 901
    inotedur	=		p3
    imaxamp		=		p4 ;ampdb(p4)
    icarrfreq	=		p5
    imodfreq	=		p6
    ilowndx		=		p7
    indxdiff	=		p8-p7
    knote	        =		p27
    aampenv		linseg	p9, p14*p3, p10, p15*p3, p11, p16*p3, p12, p17*p3, p13
    adevenv		linseg	p18, p23*p3, p19, p24*p3, p20, p25*p3, p21, p26*p3, p22
    amodosc		oscili	(ilowndx+indxdiff*adevenv)*imodfreq, imodfreq, 10
    acarosc		oscili	imaxamp*aampenv, icarrfreq+amodosc, 10
    		outs		acarosc, acarosc
    endin
    </CsInstruments>
    <CsScore>
    f10 0 4096 10 1	;sine wave
    ; p4 kstart  := index offset into the Farey Sequence
    ; p5 kloop   := end index into Farey Seq.
    ; p6 timefac := time in seconds for one loop to complete
    ; p7 fundam  := fundamental of the FM instrument
    ; p8 basenote:= root pitch of the midi voice output
    ; note that pitch structures of the midi file output are not equivalent to the
    ; ones used for the FM real-time synthesis.

    ;	start		dur		kstart	kloop   timefac	fundam. basenote
    i1	0.0		44		0 	18	2	6.05	60
    i1	4		30		0 	18	3	7.05	72
    i1	34		12		9 	18	3	7.05	72
    i1	10		12		0 	18	1.5	8	84
    i1	22		12		0 	9	1.5	8	84
    i1	15		16		0	18	1	5	48
    i1	22		20		5	17	1.7	4	36

    i1	46		20		3 	11	2.5	7.04	71
    i1	51		20		5 	13	2.5	7.06	72

    i1	73.5		1.5		11	18	1.5	5.05	48
    i1	75		1		12	18	1	6.03	58
    e
    </CsScore>
    </CsoundSynthesizer>

These are the diagrams of the waveforms of the GENfarey routines, as used in the example:

gidelta ftgen 100,0,-18,"farey",7,1 - delta values of Farey Sequence 7

gimult ftgen 101,0,-18,"farey",7,2 - generate the denominators of fractions of F_7

## Credits

Author: Georg Boenn
University of Glamorgan
2010

New in Csound version 5.13

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