Technical & Editorial

AVR - Audio Video Receiver - Build Quality: Part I


Digital Reconstruction Filter

Data sampling systems require reconstruction filters to eliminate out-of-band spectra that are artifacts (e.g., aliasing) of the sampling process. Modern audio DAC ICs use a digital filter to remove most of the off- band components. By doing so, the analog filter at the output of the DAC can be a simple circuit. The more complex the digital filter, the better its ability to reject the out-of-band components from the sampling process.

A more complex filter also has less frequency response ripple in its pass band. With high-resolution material sampled at 96k samples / second and above, the DAC filter may provide different, digitally selectable, filter roll-off rates. A slow mode improves the impulse response in the time domain by reducing the slope of the transition band. If the slow mode is used for a CD with 44.1 samples / second the response will start to rolloff at 10kHz and will be down -3dB at 20kHz which is obviously audible. For sampling rates above 96k samples / second slow mode is flat past 20kHz. In most cases, these options are documented in the datasheets for the DACs. However, ESS does not disclose this publically.

Information on digital reconstruction filtering process is found in Part II of his article

Some designers perceive an aural effect with different digital reconstruction filters. The paper below does extensive testing of different digital reconstruction filters and concludes different approaches do not change the sound.

The next link, below, is from Wolfson Semiconductor. It was presented at an AES conference, but never in the AES journal which has a more demanding review process. Wolfson draws different conclusions than the paper above.

The potential changes in sound quality of digital filters and the quasi- periodic tonal behavior of some DACs may explain an AVR designer's justification of selecting DAC ICs that do not maximize SNR and THD for a given chip price.