Playing The Waveform - Commodore Amiga Hardware Reference Manual

To output the series of eight samples at 1 KHz (1,000 cycles per second), each full cycle
is output in 1/1000th of a second. Therefore, each individual value must be retrieved in
1/8th of that time. This translates to 1,000 microseconds per waveform or 125
microseconds per sample. To correctly produce this waveform, the period value should
be
125 microseconds/sample
Period value
0.279365 microseconds/interval
447 timing intervals/sample
To set the period register, you must write the period value into the register AUDxPER,
where "x" is the number of the channel you are using. For example, the following
instruction
sh~ws
how to write a period value of 447 into the period register for chan-
nelO.
SETAUDOPERIOD: MOVE.W #447, AUDOPER
To produce high-quality sound, avoiding aliasing distortion, you should observe the limi-
tations on period values that are discussed in the section below called "Producing Qual-
ity Sound."
For the relationship between period and musical pitch, see the section at the end of the
chapter, which contains a listing of the equal-tempered musical scale.
PLAYING THE WAVEFORM
After you have defined the audio data location, length, volume and period, you can play
the waveform by starting the DMA for that audio channel. This starts the output of
sound. Once started, the DMA continues until you specifically stop it. Thus, the
waveform is played over and over again, producing the steady tone. The system uses
the value in the location registers each time it replays the waveform.
To start the channel, you write a 1 into the AUDxEN bit of the DMA control register
named DMACON. To start the DMA, you write a 1 into the DMAEN bit of DMACON.
All these bits and their meanings are shown in table 5-3.
Audio Hardware 143