View source for APU period table

[[Category:APU]]
[[APU Pulse]] and [[APU Triangle]] use "period" values to set the pitch of the note.
But some people might not know the [[wikipedia:piano key frequencies|piano key frequencies]] or how to convert them to periods for the NES.
Fortunately, this has been done for you.

== Lookup table ==
Here's a lookup table from note numbers to the values to write to the pulse and triangle period registers.
For the triangle channel, the first value corresponds to the lowest key on a standard piano (an A).
The pulse waves sound one octave higher.
<pre>
; NTSC period table generated by mktables.py
.export periodTableLo, periodTableHi
.segment "RODATA"
periodTableLo:
  .byt $f1,$7f,$13,$ad,$4d,$f3,$9d,$4c,$00,$b8,$74,$34
  .byt $f8,$bf,$89,$56,$26,$f9,$ce,$a6,$80,$5c,$3a,$1a
  .byt $fb,$df,$c4,$ab,$93,$7c,$67,$52,$3f,$2d,$1c,$0c
  .byt $fd,$ef,$e1,$d5,$c9,$bd,$b3,$a9,$9f,$96,$8e,$86
  .byt $7e,$77,$70,$6a,$64,$5e,$59,$54,$4f,$4b,$46,$42
  .byt $3f,$3b,$38,$34,$31,$2f,$2c,$29,$27,$25,$23,$21
  .byt $1f,$1d,$1b,$1a,$18,$17,$15,$14
periodTableHi:
  .byt $07,$07,$07,$06,$06,$05,$05,$05,$05,$04,$04,$04
  .byt $03,$03,$03,$03,$03,$02,$02,$02,$02,$02,$02,$02
  .byt $01,$01,$01,$01,$01,$01,$01,$01,$01,$01,$01,$01
  .byt $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
  .byt $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
  .byt $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
  .byt $00,$00,$00,$00,$00,$00,$00,$00
</pre>

== Table generator ==
This Python program generated the above lookup table.
You can use it to make a table for a PAL NES, which has a different CPU [[Cycle_reference_chart#Clock_rates|clock rate]].
<pre>
#!/usr/bin/env python3
#
# Lookup table generator for note periods
# Copyright 2010, 2020 Damian Yerrick
#
# Copying and distribution of this file, with or without
# modification, are permitted in any medium without royalty
# provided the copyright notice and this notice are preserved.
# This file is offered as-is, without any warranty.
#
assert str is not bytes
import sys

lowestFreq = 55.0
ntscOctaveBase = 39375000.0/(22 * 16 * lowestFreq)
palOctaveBase = 266017125.0/(10 * 16 * 16 * lowestFreq)
maxNote = 80

def makePeriodTable(filename, pal=False):
    semitone = 2.0**(1./12)
    octaveBase = palOctaveBase if pal else ntscOctaveBase
    relFreqs = [(1 << (i // 12)) * semitone**(i % 12)
                for i in range(maxNote)]
    periods = [int(round(octaveBase / freq)) - 1 for freq in relFreqs]
    systemName = "PAL" if pal else "NTSC"
    with open(filename, 'wt') as outfp:
        outfp.write("""; %s period table generated by mktables.py
.export periodTableLo, periodTableHi
.segment "RODATA"
periodTableLo:\n"""
                    % systemName)
        for i in range(0, maxNote, 12):
            outfp.write('  .byt '
                        + ','.join('$%02x' % (i % 256)
                                   for i in periods[i:i + 12])
                        + '\n')
        outfp.write('periodTableHi:\n')
        for i in range(0, maxNote, 12):
            outfp.write('  .byt '
                        + ','.join('$%02x' % (i >> 8)
                                   for i in periods[i:i + 12])
                        + '\n')

def makePALPeriodTable(filename):
    return makePeriodTable(filename, pal=True)

tableNames = {
    'period': makePeriodTable,
    'palperiod': makePALPeriodTable
}

def main(argv):
    if len(argv) >= 2 and argv[1] in ('/?', '-?', '-h', '--help'):
        print("usage: %s TABLENAME FILENAME" % argv[0])
        print("known tables:", ' '.join(sorted(tableNames)))
    elif len(argv) < 3:
        print("mktables: too few arguments; try %s --help" % argv[0])
    elif argv[1] in tableNames:
        tableNames[argv[1]](argv[2])
    else:
        print("mktables: no such table %s; try %s --help" % (argv[1], argv[0]))

if __name__=='__main__':
    main(sys.argv)
</pre>
The following are exercises for the reader:
* Adapt to [[FDS audio]] and other mapper sound chips by changing <code>ntscOctaveBase</code> and the formula for <code>periods</code>
** Hint: <code>fdsOctaveBase = lowestFreq * 65536 * 64 * 22.0 / 39375000.0</code> and <code>octaveBase * freq</code>
* Adapt to other [[wikipedia:musical tuning|musical tuning]] systems by changing the formula for <code>relFreqs</code>
* Adapt to assemblers other than ca65

== See also ==
*[[Celius_NTSC_table|Celius' NTSC table]]
*[[Celius_PAL_table| Celius' PAL table]]