Commodore Amiga A2000 - Low-Pass Filter; Figure 5-7 some Aliasing Distortion

^ Filter response
/|\
| 10 KHz sampling frequency
| |
0 db |____ |
| \Diff. | Sum
| \ | | |
| 4| \ | | |
-30 db |____|__\|__|____|______________________\
/ | | | | | | /
/ 05 10 15 20 25 30
/ KHz
/
Desired output frequency
Figure 5-7: Some Aliasing Distortion
All of this gives rise to the following equation, showing that the sampling frequency must
exceed the output frequency by at least 7 KHz, so that the beat frequency will be above
the cut-off range of the anti-aliasing filter:
Minimum sampling rate = highest frequency component + 7 KHz
The frequency component of the equation is stated as "highest frequency component"
because you may be producing a complex waveform with multiple frequency elements,
rather than a pure sine wave.
LOW-PASS FILTER
The system includes a low-pass filter that eliminates aliasing distortion as described
above. This filter becomes active around 4 KHz and gradually begins to attenuate (cut off)
the signal. Generally, you cannot clearly hear frequencies higher than 7 KHz. Therefore,
you get the most complete frequency response in the frequency range of 0 - 7 KHz. If you
are making frequencies from 0 to 7 KHz, you should select a sampling rate no less
than 14 KHz, which corresponds to a sampling period in the range 124 to 256.
At a sampling period around 320, you begin to lose the higher frequency values between
0 KHz and 7 KHz, as shown in Table 5-6.
- 152 Audio Hardware -