Commodore Amiga A2000 - Figure 5-5 Frequency Domain Plot of Low-Pass Filter; Figure 5-6 Noise-Free Output (no Aliasing Distortion)

^
/|\
|
|
0 db |____
| \
| \ Filter response
| \
-30 db |_______\_________________________________\
| | | | | | /
05 10 15 20 25 30
KHz
Filter passes all frequencies below about 5KHz
Figure 5-5: Frequency Domain Plot of Low-Pass Filter
Figure 5-6 shows that it is permissible to use a 12 KHz sampling rate to produce a 4 KHz
waveform. Both of the beat frequencies are outside the range of the filter, as shown in
these calculations:
12+4= 16KHz
12-4= 8KHz
^ Filter response
/|\
| 12 KHz sampling frequency
| |
0 db |____ |
| \ Diff. | Sum
| \ | | |
| 4| \ | | |
-30 db |____|__\_|___|_____|_____________________\
/ | | | | | | /
/ 05 10 15 20 25 30
/ KHz
/
Desired output frequency
Figure 5-6: Noise-free Output (No Aliasing Distortion)
You can see in Figure 5-7 that is unacceptable to use a 10 KHz sampling rate to produce a
4 KHz waveform. One of the beat frequencies (10 - 4) is within the range of the filter,
allowing some of that undesirable frequency to show up in the audio output.
- Audio Hardware 151 -