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WM8741 Digital Filters
One of the reasons I embarked to use Arduino to control the WM8741 DAC was because certain digital filters are only available thought the s/w interface. I've done some additional reading about these filters. The DAC has the following 5 selectable filters. The names of these filters are mentioned in a white paper and in the data sheet of the new WM8742. (The 8742, however seems to be a cost-efficient version of the older WM8741 which is spec'ed as having better signal to noise ratio). These filters are:
- Linear phase ‘soft knee filter’
- Minimum phase ‘soft knee filter’
- Linear phase brickwall filter
- Minimum phase apodising filter
- Linear phase apodising filter
Filter #3, the linear phase brickwall filter is the traditional/historical filter for digital audio. It is a steep filter right after 20 KHz and has been shown to produce a lot of pre and post ringing from an inpulse response. Wolfson indicates that this filter is to be used with other filters (from other components such as DSP chips) in the digital audio path. Filter #1 is the "slow roll off" filter that more modern DACs and CD players have used to remove some of the "digital hash" that have been observed in the past. The slow roll off filter reduces both the pre and post ringing of an impulse response. These two filters brickwall or fast roll-off and soft knee or slow roll-off have been the mainstay of digital audio reproduction for all these years.
Filter #2 is a new kind of digital filter. Whereas in the past audio engineers have insisted in phase linearity (meaning all frequencies have equal phase or delay), More recent research have shown that a "minimum phase" filter sacrifices some of the phase linearity (adds some phase distortion) for better time response. Specifically, minimum phase filters minimizes the pre-ringing of an impulse response. Audio researchers have argued that pre-ringing is an un-natural effect and therefore the ear is more sensitive to this kind of distortion. They have also argued that phase distortion is not very audible. This filter also incorporates soft-knee or slow roll-off and this reduces post ringing as well. The properties of this filter are similar to the "MP filter" found in Ayres latest CD player.
Here is a diagram depicting the impulse response in a linear phase filter and a minimum phase filter.
According to the Ayre white paper, adding the properties of slow roll-off filter to the minimum phase filter will decrease the post ringing, resulting in something like this:
Note that the axis are different, but in relation ot the first diagram, the second diagram shows no pre-ringing and much reduced post-ringing.
Conceptually, the Wolfson Minimum Phase soft-knee filter is the equivalent of Ayre's MP listen filter. Obviously the implementation is different and the relative quality of these filters have not been evaluated.
Filter 4 and 5 are only available through the s/w interface. "Apodising" ("Apodizing" in American English) filters have been equated to minimum phase filters and minimum phase filters with slow roll-off in the literature. But as the name of the filters in the Wofson DAC suggests, "apodizing" is an additional filter technique to that provided by minimum phase soft-knee.
The use of slow roll-off filters allows some of the higher frequency (beyond the Nyquist frequency) energy to be reflected back into the audio band. This is known as "aliasing" an is a source of distortion. An apodizing filter according to the Wolfson white paper, is one where the filter fully attenuates by Fs/2 (the Nyquist frequency) and thus they start attenuating earlier than Fs/2 often sacrificing flat requency response to 20KHz.
Filter #4 appears to combine the best qualities of these filters. "Minimum phase apodizing" signifies that pre-ringing is eliminiated, post-ringing reduced and aliasing distortion eliminated. As the Wolfson white paper indicates, no one filter is the perfect filter but a designer hopes to makes the best trade offs by using multiple filters.
Conceptually Filter #4, minimum phase apodizing filter, is what Meridian is using in their latest CD player. Obviously the implementation is different and the relative quality of these filters have not been evaluated.