CHAPTER 17

SOUND

     Generating sound can be very simple.  For simple sounds there are four
     audio channels, each controlled by two control registers.
     
     GENERATING SOUNDS
     
     To generate a sound in channel 1, put the frequency and volume codes
     into the frequency and control registers.  The frequency register for
     channel 1, AUDF1 [$D200 (53760)] can have any number from 0 to $FF
     (255).  0 causes the highest frequency; 255 causes the lowest.  The
     volume/noise (control) register for channel 1, AUDC1 [$D201 (53761)]
     is more complicated.
     
                 Audio channel control (volume/noise) register
     
             7 6 5 4 3 2 1 0
            -----------------
     AUDCx  | noise | volume|
            -----------------
             1 6 3 1 8 4 2 1
             2 4 2 6
             8
     
     The noise bits can have various values.  The best way to learn to use
     them is by experimentation.  The technical details of the polynomial
     counters which generate the noise has little bearing on what is heard.
     The two special values of interest are: $1 (volume+16 in decimal),
     which causes a DC voltage proportional to the volume bits and; $A
     (volume+160), which causes a pure tone (square wave).  The volume bits
     select the relative volume, 0=off.  Therefore, the number, $A8 (168
     [8+160]) in AUDC1, will cause the frequency selected by AUDF1 to be a
     pure tone of medium volume.
     
     In BASIC the dirty work is done fore you.  The SOUND command will do
     all the calculations for you.  The Sound command format is shown
     below.
     
                        The BASIC sound command format
     
      SOUND channel,frequency,noise,volume
     
     The channel numbers is 0 to 3 instead of 1 to 4. The frequency, 0 to
     255, is put into the frequency register.  The noise is put into the
     high bits of the channel control register with volume in the low bits.
     Therefore...
     
      SOUND 0,125,10,8

     will produce a pure tone of medium frequency and volume in channel 0
     (called channel 1 in assembly language).
     
     ADVANCED SOUND
     
     The Audio Control register, AUDCTL [$D208 (53768)], (not to be
     confused with the four audio channel control registers), adds more
     control for assembly language programmers.  Again, to go into
     technical details will be less productive than experimentation.
     
                     The audio control register. (AUDCTL)
     
              7 6 5 4 3 2 1 0
             -----------------
      AUDCTL | | | | | | | | |
             -----------------
              1 6 3 1 8 4 2 1
              2 4 2 6
              8
     
     
          7    0 = 17 bit polynomial noise
               1 =  9 bit below polynomial noise
          6    0 =  clock channel 1 with 64 KHz
               1 =  clock channel 1 with 1.79 MHz
          5    0 =  clock channel 3 with 64 KHz
               1 =  clock channel 3 with 1.79 MHz
          4    0 =  clock channel 2 with 64 KHz
               1 =  clock channel 2 with channel 1
          3    0 =  clock channel 4 with 64 KHz
               1 =  clock channel 4 with channel 3
          2    1 =  insert logical high-pass filter in
                     channel 1, clocked by channel 3
          1    1 =  insert logical high-pass filter in
                     channel 2, clocked by channel 4
          0    0 =  64 KHz main clock
               1 =  16 KHz main clock
     
     All bits of AUDCTL are normally zero.  The BASIC sound command causes
     it to be reset to zero.
     
     By clocking one channel with another, the range can be increased. 
     This essentially allows two channels with twice the range as each of
     the four normal channels.  This is called 16 bit sound.
     
     To calculate exact frequencies, use the following formulas.  The exact
     clock frequencies are also given if more accuracy is needed.  The
     clock frequencies are acquired by dividing the signal from the TV
     color-burst crystal.  This crystal has a frequency of 3.579545 MHz.
     
                              Clock frequencies:
     
          1.7897725   MHz    (color-burst/2)
     
            63.920446 Khz    (color-burst/56)
     
            15.699759 KHz    (color-burst/228)
     
                                   Formulas:
     
     
      For 1.79 MHz
     
     
                      clock                    clock
               f = ------------         f = ------------
                   2(AUDFn + 7)             2(AUDFn + 4)
     
     
                     16 bit                    8 bit
     
      AUDFn is the number in the audio frequency register.
     
     
      For 16 KHz and 64 KHz
     
     
                      clock
               f = ------------
                   2(AUDFn + 1)
     
     
     AUDIO TIMER INTERRUPTS
     
     When the audio timers count down to zero they generate IRQ interrupts
     (if enabled).  The timers can be reset by writing any number to STIMER
     [D209 (53769)].
     
     THE CONSOLE SPEAKER
     
     The console speaker is where key clicks and the cassette signals come
     from.  On XL and XE models this speaker is heard through the TV
     speaker.  It is operated by toggling bit 3 of CONSOL [$D01F (53279). 
     This bit always reads 0 but it is actually set to 1 during vertical
     blank.
     
     
                   Useful data base variables and OS equates
     
     
     CONSOL $D01F          (53279): bit 3 controls console speaker
     AUDF1  $D200          (53760): Audio frequency 1
     AUDC1  $D201          (53761): audio control 1
     AUDF2  $D202          (53762): Audio frequency 2
     AUDC2  $D203          (53763): audio control 2
     AUDF3  $D204          (53764): Audio frequency 3
     AUDC3  $D205          (53765): audio control 3
     AUDF4  $D206          (53766): Audio frequency 4
     AUDC4  $D207          (53767): audio control 4
     AUDCTL $D208          (53768): general audio control
     STIMER $D209          (53769): audio timer reset