VIC 20 SOUND

FOUR SPEAKERS AND 5 OCTAVES

 The VIC has 3 tone registers (for music), and one white noise generator (for
sound effects). The tone generators cover 3 octaves each but are staggered
slightly so you can actually reach a total of 5 separate octaves.
 The VIC's speakers and volume control are stored in specific memory 
locations which can be accessed and controlled by reading and writing to
these locations. Whenever you write to these locations, you activate that 
tone generator, or the volume control.
 When programming sound - especially music - it is often helpful to think of
these various sound controls as "speakers", and the volume setting as a
standard "volume" control.
 Here, briefly, is a list of memory locations relating to sound:

 $900E  36878  VOLUME SETTING
 $900A  36874  SPEAKER 1 - LOWEST - BASS
 $900B  36875  SPEAKER 2 - MIDDLE - ALTO
 $900C  36876  SPEAKER 3 - HIGHEST -SOPRANO
 $900D  36877  SPEAKER 4 - NOISE

 There are 15 volume settings. These volume settings are controlled by the
bottom four bits of memory location 36878. The top four bits set the 
auxiliary colour for multicolour graphics and thus have nothing to do with
sound. A value of zero in the bottom four bits turns off the volume. A value
of fifteen in the bottom four bits gives the loudest volume.

 The tone registers and noise register all behave in the same way when it
comes to programming them. The top bit turns the speaker on or off and the
bottom 7 bits adjust the frequency produced. A one in the top bit turns the
speaker on and a zero turns it off. The frequency produced by the value 
in the lower 7 bits is determined using the following formulas:

                 VIC Chip Frequency Formulae
 Frequency Output (F) = Clock/(255 - N)         NTSC            PAL
 Number in Table (N) = 255 - (Clock/F)       (N.America)     (European)
            VIC 20 Voice 1 (36874):  Clock =    3995            4329
            VIC 20 Voice 2 (36875):  Clock =    7990            8659
            VIC 20 Voice 3 (36876):  Clock =   15980           17320
            VIC 20 Voice 4 (36877):  Clock =   31960           34640

 NOTE: When N is equal to 255, the frequency made is not higher than the
 frequency produced by the N being equal to 254, but is infact lower than
 the frequency produced by the value 128. To find the exact frequency
 produced, make N = 127 rather than 255.


NOISE REGISTER/WHITE NOISE GENERATOR

 The noise register is different from the tone registers. It is generally
used for special sound effects like explosions. The noise register produces
a blank noise sound like that on your television set when you fall asleep 
late at night. It has the same 3 octave range as the tone generators 
described above and its frequency changes with each value as given by the
table and formulae above.