Notch Filter

Just when I thought things couldn't get better, they did with the welcomed Notch filter Integra Research incorporated into this processor.

The Notch filter is adjustable from 20Hz to 300Hz in 1Hz increments, from 0 to -15dB in 1 dB increments.

The Q of the filter is adjustable from .05 to .25. I would have liked to have seen a Qmax of at least .5 to deal with broad bumps such as what my room suffers from at my primary listening position. I have a bass bump centered around 36Hz from 25 to 40Hz in width that could stand to use a bit of attenuation. In order to effectively do this, a system Q of at least .42 is needed. I did my best using the somewhat limited Notch filter of the RDC 7.1 and effectively removed most of the excessive energy from 31.5Hz to 40.5Hz.

It's important to note, if your speaker system is wideband and linear, you probably don't desire flat in-room frequency response, but more of a shelved response at the listening position with a gradual increase in bass as frequency decreases below 80Hz and a decrease in high frequency energy above 6kHz in order to sound pleasing to the human ear.

Editorial Note on Notch Filters
A notch filter (aka. band-reject filters) is a special type of filter that attenuates a specific frequency band only while allowing all other frequencies to pass through. This is particularly useful for removing excessive bass energy in a room do to modal peaking.

In room, on axis measurement at the listening position
Using LMS I found the primary room mode that added a huge bump in the lower bass region. I adjusted the notch filter to minimize this-10dB from 25Hz to 40Hz.

I did a quick check in a remote seating location to verify the notch I added didn't do more harm than good for other listeners.

As you can see, the notch filter smoothed out the bass response even at other listening positions making the overall bass response +-10dB from 20Hz to 1kHz as opposed to +-20db without the notch filter.

Unfortunately there are no free lunches. When I ran my FFT analyzer into the RDC-7.1 with the notch filter engaged, I heard a very low level driver resonance present in all of the channels which became more predominant the lower in frequency the notch filter was set to. When I popped in a DVD I could hear the same resonance mode (though extremely faint) created by the notch filter in my rear channels if I put my head up to the midrange driver. In most cases this is inaudible, but it lead me to further investigate this issue with my trusty Audio Precision SYS 2722 Audio Analyzer.

What I found was the Notch filter was being applied globally to all speakers and NOT just the subwoofer channel. In my opinion, this is a major oversight as the only place the Notch filter sound be applied is to the speaker system capable of exciting room modes to benefit from it. In this case that is the subwoofer channel, not the channels we just applied bass management too!

Regarding the Notch Filter and reasons to NOT apply it globally:

• Applying a notch filter to the main speaker channels has the capability to mess up transient response and upper order harmonics, particularly in the center channel when listening to male vocals. This is especially true if you are not using FIR filters which the RDC-7.1 is not.

• The notch filter is only relevant at the sub to listening position that it was calibrated for. Each speaker has its own unique position in the room and subsequently plays into the room differently - creating its own nulls and peaks. Thus the global notch can actually be degrading the overall sound quality of the room.

Regarding Notch noise issue: The noise gets worse as you lower the frequency. I found it most prominent when I set the Notch Filter between 20-24Hz. It is independent of Q or amplitude adjustments. When I set it to 20Hz, I can clearly hear resonance in all channels. To test this, I simply inputted bandwidth limited pink noise (20Hz to 250Hz) to an analog input and engage Stereo or All Channels mode. I suspect what I was hearing was a resonance/ringing issue of the filter response or noise induced somewhere in the associated circuitry.

Pictured here is an electrical frequency response plot of the Center Channel (with no bass management applied) and the notch filter set for 36Hz -10dB from 31.5Hz to 40.5Hz. Note, I shut off the bass management of the center channel to illustrate the shape of the notch filter. With bass management applied, the notch filter is still present with the added roll off at whatever the center channel crossover is set to.

I informed Integra Research about this and they are investigating a firmware upgrade to allow user selectability over which channels (if any) other than the subwoofer channel utilize the notch filter. In the meantime, I stress caution when using this filter as it currently operates. Applying a notch for the sub channel is only relevant for the bass modal issue it is correcting relative to the sub(s) and listening area. The other speakers in the system, located at different positions create their own modal and nodal responses which usually don't coincide with the corrected response of the subwoofer.