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All FIR filters are fatally flawed.

Frequency and impulse response plots provide little information regarding a DAC’s performance with music. Frequency response uses steady-state sine waves and very few DACs have trouble with that kind of signal. But, if you add an amplitude envelope the landscape changes.

Here is a tone with a linear up/down ramp processed by DAC Y (non-oversampled.)


Here is the same signal processed by DAC A (8x oversampled, linear phase FIR, delta-sigma modulation.)


The ringing is evident even though the signal slowly ramps up/down from/to zero. Worse yet is the dynamic compression. This is common to all FIR filters. FIR filters essentially replace each sample point with a weighted average of the surrounding samples.

An impulse is noise and a DAC’s response to noise is not very informative. However, if you add a frequency component to the impulse all kinds of unpleasant things show up.

Here is a two-cycle sine wave burst processed by DAC Y.


Here is the same signal processed by DAC A.


Here is the same signal processed by DAC W (8x oversampled, minimum phase FIR, delta-sigma modulation.)


Unlike the natural decay of a note, the ringing is enharmonic. What’s more, the reproduced signal is also off-pitch.

In all cases the signal frequency is at least seven octaves below the Nyquist frequency.


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