AN2394

Audio playback

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Where:

OCBR: Output Compare B register value

Prescaler: Timer clock prescaler

The value set in the OCAR determines the duty-cycle:

Where:

OCAR: Output Compare A register value

Figure 2.

Typical PWM signal waveform

2.2

Audio playback

Before the audio data can be played, the header must be parsed so that the sampling rate of

the data and its length can be determined.

The task of reproducing audio is achieved by using sampled data (data contained in the

.WAV file) to vary the duty cycle of the PWM signal who’s period is held at a constant value.

The duty cycle represents the ratio of the pulse width to the pulse period, i.e. it represents

the ratio of the OCAR value to the OCBR one.

As the duty cycle varies according to the values of the sampled data and this data is coded

in 8 bits (with values from 0 to 255), the OCBR value must be set to 255 and the OCAR

value fills with the sampled data values (the OCAR value varies from 0 to 255). In this

manner the duty cycle varies from 0% to 100%.

The OCAR value is updated at intervals specified by the SysTimer period which is equal to

the sample rate of the .WAV file.

The PWM output signal is then fed to a simple RC low-pass filter to generate an output

voltage directly proportional to the average time spent in the HIGH state (i.e., 50% duty

cycle is equal to 1.65V when VDD = 3.3V). When this is averaged over time, we get a

reasonable reproduction of the original analog signal.

There are several design considerations involved in the selection of the cut-off frequency of

the low-pass filter. Primarily, the filter cut-off frequency must be much lower than the PWM

frequency to reduce the noise generated by PWM switching. A PWM frequency of 10 times

the cut-off frequency is a generally sufficient. However, the cut-off frequency of the filter is

critical to the bandwidth of the output audio.

In addition, it is recommended, on the basis of the Nyquist Criterion, that the cut-off

frequency of any filter used be set at half the frequency of the sample playback rate. Since in

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