THE EQUAL-TEMPERED MUSICAL SCALE
Table 5-7 gives a close approximation of the equal-tempered scale over one octave when
the sample size is 16 bytes. The "Period" column gives the period count you enter into the
period register. The length register AUDxLEN should be set to 8 (16 bytes = 8 words). The
sample should represent one cycle of the waveform.
Table 5-7: Equal-tempered Octave for a 16 Byte Sample
NTSC PAL Ideal Actual NTSC Actual PAL
Period Period Note Frequency Frequency Frequency
254 252 A 880.0 880.8 879.7
240 238 A# 932.3 932.2 931.4
226 224 B 987.8 989.9 989.6
214 212 C 1046.5 1045.4 1045.7
202 200 C# 1108.7 1107.5 1108.4
190 189 D 1174.7 1177.5 1172.9
180 178 D# 1244.5 1242.9 1245.4
170 168 E 1318.5 1316.0 1319.5
160 159 F 1396.9 1398.3 1394.2
151 150 F# 1480.0 1481.6 1477.9
143 141 G 1568.0 1564.5 1572.2
135 133 G# 1661.2 1657.2 1666.8
The Table above shows the period values to use with a 16 byte sample to make tones in
the second octave above middle C. To generate the tones in the lower octaves, there are
two methods you can use, doubling the period value or doubling the sample size.
When you double the period, the time between each sample is doubled so the sample
takes twice as long to play. This means the frequency of the tone generated is cut in half
which gives you the next lowest octave. Thus, if you play a C with a period value of 214,
then playing the same sample with a period value of 428 will play a C in the next lower
octave.
Likewise, when you double the sample size, it will take twice as long to play back the
whole sample and the frequency of the tone generated will be in the next lowest octave.
Thus, if you have an 8 byte sample and a 16 byte sample of the same waveform played at
the same speed, the 16 byte sample will be an octave lower.
- 154 Audio Hardware -
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