Creating The Waveform Data - Commodore Amiga Hardware Reference Manual

CREATING THE WAVEFORM DATA
The waveform used as an example in this section is a simple sine wave, which produces a
pure tone. To conserve memory, you normally define only one full cycle of a waveform
in memory. For a steady, unchanging sound, the values at the waveform's beginning
and ending points and the trend or slope of the data at the beginning and end should be
closely related. This ensures that a continuous repetition of the waveform sounds like a
continuous stream of sound.
Sound data is organized as a set of eight-bit data items; each item is a sample from the
waveform. Each data word retrieved for the audio channel consists of two samples.
Sample values can range from -128 to +127.
As an example, the data set shown below produces a close approximation to a sine wave.
Note that the data is stored in byte address order with the first digitized amplitude
value at the lowest byte address. the second at the next byte address, and so on. Also,
note that the first byte of data must start at a word-address boundary. This is because
the audio DMA retrieves one word (16 bits) at a time and uses the sample it reads as
two bytes of data.
To use audio channel 0, write the address of "audiodata" into AUDOLC, where the
audio data is organized as shown below. For simplicity, "AUDxLC" in the table below
stands for the combination of the two actual location registers (AUDxLCH and
AUDxLCL). For the audio DMA channels to be able to retrieve the data, the data
address to which AUDOLC points must be located in the low 512K bytes of RAM.
Audio Hardware 137