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).

ZXCD1000

ISSUE 2 - APRIL 2002

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%)

Figure 3b.

Figure 3a.

Figures 3a,3b,3c and 3d

The audio input Pulse Width Modulates the comparator output.

Figure 3c.

Figure 3d.