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.
[Back to Amiga Developer Docs]