Prior art digital PWM modulators generate a pulse width modulated signal with a carrier frequency of minimum twice the signal frequency to comply with the Nyquist criteria. The objective for such a system implementation is to achieve noise and distortion artifacts lower than quantization noise of the sampled signals, which is a challenging task when operating with sampled audio signals of 24 bit resolution. The result is a complex and expensive system.
In the art well-known systems are known e.g. from the document WO 97/37433. Such a system is illustrated in FIG. 1, and comprises a sample rate converter 1, an oversampler 2, a noise shaper 4 and an error correcting algorithm unit 3.
This prior art approach suffers from several drawbacks.
Firstly, the carrier frequency is dependent on the sample frequency, making the use of a sample rate converter 1 necessary. Especially as audio media comes in many different formats, sample rate conversion becomes mandatory. The sample rate converter 1 can be complex to implement due to high order filtering. Quantization errors and aliasing will be introduced leading to reduced dynamic ranges.
Further, in order to increase the dynamic range, an error correcting algorithm 3 and a noise shaper 4 must be comprised in the design leading to unnecessary complexity and cost.
Further, the noise shaper 3 is limited in bandwidth, as the maximum loop gain to suppress the quantization noise is constrained by the stability of the noise shaping loop. Therefore, the noise shaper 4 does still not achieve a satisfactory dynamic range.
Analog PWM modulators where an analog input signal is converted into a PWM signal is also known from prior art systems. In order to obtain a stable system and shape the control loop characteristics, such prior art systems often require additional lead-, lag, lead-lag or lag-lead compensators in the control structure.
Prior art modulator systems based on non-oscillating triangular modulation are greatly reduced in loop gain by necessary demands of phase-margins for the loop to stay stabile. Self-oscillating systems, as described e.g. in WO 02/25357, do not need a phase margin and will have higher loop gain to suppress error and noise components. However, self-oscillating modulators have not yet been implemented successfully in the digital domain.