- Support for 4x20 LCD Display and large number display
- Brightness and contrast adjustment with remote
- (OPUS/Wolfson WM8741) DAC volume control: remote and rotary encoder
- (OPUS/Wolfson WM8741) DAC random filter selection 1 to 5 with remote
- (OPUS/Wolfson WM8741) DAC upsampling selection (L, M, H -this is the OSR setting)
- I2C level shifting (5V to 3.3V)
- Optimized power-up sequence

Monday, May 11, 2009

Apodizing vs Non-Apodizing

I've interposed the filter response diagrams of the WM8741 DAC found in the data sheet. Indeed, the Apodizing filter has a sharper roll-off than the "soft-knee" filter. Both filters are minimum phase, (thus eliminating the pre-ringing), but the Apodizing removes aliasing distortion and the soft-knee reduces post ringing. Conceptually, Ayre is a proponent of the minimum phase soft-knee filter whereas Meridian is a proponent of the minimum phase apodizing filter. Which one is "better". I think Wolfson was wise to include different filters as there is no consensus about which filter is "better.

Here is a comment from the Meridian Forums by user scottb:
The discussion of eliminating artifacts from the original A/D conversion is not a function of apodising filters per se, but rather a function of the specific filter parameters chosen. Peter Craven, when he originally proposed minimum phase filters, also proposed that the filter should roll off very steeply after 20 khz, more steeply even than the usual filters applied at the A/D and D/A conversion. That way, any ringing from the A/D conversion process would be filtered out, albeit at the expense of considerable post-ringing and phase shift from the apodising filter itself - which shows up in JA's measurements. [Note: This is from Stereophile]

By contrast, the Ayre white paper I've mentioned frequently before claims that their implementation sounds better with quite gentle filter slopes, which will obviously allow more ringing from A/D conversion to get through, as well as more aliasing distortion, but will create much less post-ringing from the minimum phase filter. That's a pretty interesting disagreement between the ears of two of the best digital audio designers in the business, and JA's evaluation of the new Ayre MP filter will bear watching.

BTW, you must upsample to do apodising, or any digital anti-aliasing filtering, because the filter knee is so close to the Nyquist frequency. Whether you speak of the filtering being applied after upsampling, or as part of the upsampling process, is really a matter of semantics. From the perspective of upsampling the original digital signal, you must upsample in order to filter, and you must filter if you upsample.

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