With no audio input signal applied, the AudioA/B
inputs are biased at the mid-point of the triangular
wave, and the duty cycle at the output of the
comparators is nominally 50%. As the AudioA/B signal
ascends towards the peak level, the crossing points
with the (higher frequency) triangular wave also
ascend. The comparator monitoring these signals
exhibits a corresponding increase in output duty cycle.
Similarly, as the AudioA/B signal descends, the duty
cycle is correspondingly reduced. Thus the audio input
Pulse Width Modulates the comparator outputs. This
principle is illustrated in Figures 3a, b, c and d. The
comparator outputs are buffered and used to drive the
OutA and OutB outputs. These in turn drive the speaker
load (with the audio information contained in the PWM
signal) via the off chip output bridge and single stage
L-C filter network.
The ramp amplitude is approximately 1V. The AudioA,
AudioB, TriangleA and TriangleB inputs are internally
biased to a DC voltage of approximately VCC/5. The
mid - point DC level of the OscA and OscB triangular
outputs is around 2V. The triangular wave at the Cosc
pin traverses between about 2.7Vand 3.8V and the dist
pin exhibits a roughly square wave from about 1.4V to
2V. (The above voltages may vary in practice and are
included for guidance only).
ZXCD1010
ISSUE 4 - AUGUST 2007
5
Audio A/B
Triangle A/B
PWM Comparator
O/P
Audio A/B
Triangle A/B
Comparator O/P
(Duty Cycle = 50%)
O/P
Audio A/B
Triangle A/B
Comparator O/P
(Duty Cycle = 75%)
Audio A/B
Triangle A/B
Comparator O/P
(Duty Cycle = 25%)
Figures 3a,3b,3c and 3d
The audio input Pulse Width Modulates the comparator output
