The Axign technology of the digital feedback loop is a first-in-class technology with its full audio bandwidth feedback loop taken from behind the output filters and converted in ultra-high speeds. It solves all audio performance related issues from traditional class-D amplifiers and brings a product to market that enables most loudspeakers to perform to their best and enables the reproduction of the original recording to a new benchmark level for consumer audio amplifiers.
A traditional application area for Class-D amplifiers is high-power amplification for professional use. Efficiency, weight, and reliability are determining factors for these use cases. The new Class-D amplifiers extend their traditional strengths to high-end audio applications. The best performance can be reached using Post Filter Feedback. Compensation of the output filter can be done more precisely in the digital domain. With careful design of a digital compensation filter, it is possible to create a single loop with full global feedback. In that case the full loop gain is used to suppress non-linearities and frequency dependent load behavior in the LC output filter. Digital implementation of a loop-filter in combination with feedback after an output filter does require an ADC with a high-resolution in the audio band in combination with a low latency to avoid degradation of the loop stability. Furthermore, the high-frequency residue of the switching signal at the input of the ADC should also not degrade its performance. Such combination of requirements can only be met by using either a costly over-designed general-purpose ADC, or by using a low-latency ADCs (LL-ADC) which was purposely designed to meet these criteria.
The overall signal-processing path consists of a digital input interface, volume control, an interpolation stage, the digital loop-filter and a PWM controller. The analog signal measured at the loudspeaker terminals (1) is digitized by our low latency ADC (2). The LL-ADC creates a single-bit output with a sample rate typically in the order of 25 MHz for wide bandwidth and low latency. The high-pass shelving filter in its input-output transfer greatly simplifies compensation of the LC output filter. The 1-bit stream is passed on to the digital signal processor (3). Here the signal is compared with the digital input (4). The difference between the input signal and the actual signal measured at the loudspeaker will be used to generate a new signal that compensates for the errors, non-linearities and noise in the following path to the loudspeaker. In the PWM controller (5) the PWM signal is made to direct the power stage. The reconstruction filter is used as a low-pass filter to remove the high frequency content of the PWM signal. Finally, the loudspeaker will transform the clean analog signal to music we love to hear.
In the image below a reference is made to competing solutions of analog-in and digital-in standard class-D solutions and the DDFA solution with feedback before the output filter. It is clear from this table that the Axign solutions enables the highest performance of your audio amplifier.