What is a 6th order digital filter roll-off?

[greg7]

One of my DACs uses a 6th order (Butterworth?) digital filter. What would this correspond to (apodizing, linear phase slow roll off, minimum phase slow roll off, etc.)? And what would the impact be on the impulse response?

 

[MarcelvdG]

A sixth-order Butterworth low-pass filter is a minimum-phase filter with relatively slow roll-off (about 36 dB/octave, at least for frequencies above cut-off and well below Nyquist). Its impulse response has post- and no pre-ringing. Are you sure it is a digital filter?

 

[Mark Tillotson]

The question mark means all we know is its a 6th order digital filter, presumably IIR. So 6 poles / zeroes.

Perhaps a link to the relevant datasheet could help figure this out.

 

[MarcelvdG]

My question is there because sixth-order IIR filters are unusual in audio DACs, so I wonder if it isn't an analogue reconstruction filter.

 

[Markw4]

Fixed IIR filters sometimes follow the programmable FIR filters. That's how ESS dacs work.

 

[MarcelvdG]

Is that the signal transfer function of the sigma-delta modulator or really an interpolation stage between FIR-filter and sigma-delta modulator?

In either case, the answer to the original question should then be that the FIR mainly determines the roll-off and impulse response and the IIR gives suppression at far ultrasonic frequencies and adds a little post ringing.

 

[Markw4]

Please see below for excerpt from ESS datasheet...

 

[MarcelvdG]

Probably it's an interpolation stage then, as otherwise they should warn you for worsened noise shaping when the sample rate is too low.