Low Carrier Frequency Noise-Shaper for Digital Amplifier
Noise-shaper has not been a severe concern in all-digital amplifier which is to convert PCM data to PWM digitally, because other factors - clock jitter, power supply regulation and power stage nonlinearity - are more dominant to get an audiophile quality SNR. Simple noise-shaper with order of a few is good enough for most applications if over sampling ratio is 8. For more power sensitive applications, PWM carrier frequency or over sampling ratio can be reduced to enhance power efficiency by lowering the switching loss of power stage which is one of the most important sources of power loss. This can make, however, high frequency noise to be smeared into audio band from the lower over sampling ratio and, therefore, more crafty design of noise-shaper is required to overcome the artifacts. Low over sampling ratio noise shaper can be designed by spreading the zeros of Noise Transfer Function optimally with parameters of sampling frequency, over sampling ratio, requantization resolution, modulation index and noise-shaper order. This paper shows how the budget of over sampled band can be used up to meet SNR requirement. SNR and THD+N with this optimized noise shaper are good enough for the most of digital audio sources with only 4 times or even 2 times over sampling ratio.
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