Commodore Amiga A1000, Hardware Reference Manual

Page: 162 / 380

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 A2000s with 2 layer motherboards and later AS00 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. Bypassing the
filter allows for improved sound in some applications, but an external filter with an
appropriate cut-off frequency may be required.
USING DIRECT (NON-DMA) AUDIO OUTPUT
It is possible to create sound by writing audio data one word at a time to the audio output
addresses, instead of setting up a list of audio data in memory. This method of controlling
the output is more processor-intensive and is therefore not recommended.
To use direct audio output, do not enable the DMA for the audio channel you wish to use;
this changes the timing of the interrupts. The normal interrupt occurs after a data address
has been read; in direct audio output, the interrupt occurs after one data word has been
output.
Unlike in the DMA-controlled automatic data output, in direct audio output, if you do not
write a new set of data to the output addresses before two sampling intervals have
elapsed, the audio output will cease changing. The last value remains as an output of the
digital-to-analog converter.
The volume and period registers are set as usual.
- Audio Hardware 153 -