5 / Producing High-quality Sound / Low-Pass Filter

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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.
 Table 5-6: Sampling Rate and Frequency Relationship
 Sampling Sampling Maximum Output
 Period Rate (KHz) Frequency (KHz)
 -------- ---------- ---------------
 Maximum sampling rate 124 29 7
 Minimum sampling rate 256 14 7
 for 7 KHz output
 Sampling rate too low 320 11 4
 for 7 KHz output

In A2000's with 2 layer motherboards and later A500 models there is a
control bit that allows the audio output to bypass the low pass filter.
This control bit is the same output bit of the 8520 CIA that controls
the brightness of the red "power" LED (CIA A $BFE001 - Bit 1: /LED).
Bypassing the filter allows for improved sound in some applications, but
an external filter with an appropriate cutoff frequency may be required.

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