I found that really nice and invaluable discussions have been going-on starting last Sunday on DSP integration of subwoofers with main-SP's woofers mainly focusing on group delay (GD) of subwoofers.
Let me summarize the major factors affecting the cause of the GD and DSP correction/tuning thereof as follows;
1. Most upstream DSP software and its functionalities
i.e. selection of XO Fq, XO filter type and slopes, phase/polarity, relative gains, EQ(s) if applied,
etc.,
2. Ported
or sealed in subwoofer design,
3. Active (directly driven by dedicated amplifier)
and/or passive LCR low-pass (high-cut) network,
or DSP-type low-pass filter (AD for DSP then DA), in subwoofer,
4. Ported
or sealed in main SP design for woofers,
5. Active (directly driven by dedicated amplifier),
or passive LCR network with high-pass (and of course low-pass), for the woofers.
Edit:
6. Individual room acoustic environments.
The selections and combinations of these factors greatly vary in our individual audio setups, and
therefore the optimal (compromise) DSP configurations/corrections and tunings would also very much vary/dependent on each of our setups.
I essentially agree with the following nice comments;
The problem is pushing LF down with a boost requires a strong High Pass "protection" filter which is what causes the crazy high Group Delay on a sub like this. Audibility is not proven but 100 ms is bordering on crazy and almost has to be audible (10 ms is limit for lip sync).
Group Delay is frequency dependent and is caused by DSP filters (In the case of a small DSP sub much of it by the "high pass protection filter) not by the DAC and typically it rises rapidly toward lower frequencies.
Sealed subs have less and flatter GD. Another interesting feature of GD is that, it's harmless when it's flat no matter how large, GD with a slope is the real problem.
There are also plenty of well designed ported subs with less than 20 ms of GD. To me the problems come in when you "push" things to make specs look better (high SPL at 20 Hz) at the expense of distortion and GD.
Having noticed all the above factors and comments, I assume it would be worthwhile and "of-reference-for-you" sharing (again) my setup and tunings in my PC-DSP-based multichannel multi-SP-driver multi-amplifier fully active stereo audio rig (ref.
here #931 for the details).
a. I use simple and reliable DSP
"EKIO" (IIR filters in cascade 2nd order direct form II biquad in 64 bit floating point) in upstream digital domain in PC.
b. My L&R subwoofers are large-rigid-heavy
(48 kg) YAMAHA YST-SW1000 (ref.
here) with
30-cm cone driver in
ported design and in YST-Helmholtz resonance mechanism, and also have built-in passive -24 dB/Oct LP variable filter as well as with its built-in dedicated powerful amplifier; even with these rather old-fashion configurations, I found (ref.
here) that
YST-SW1000 is well within the above
@levimax's comment of
"There are also plenty of well designed ported subs with less than 20 ms of GD." I found the GD of YST-SW1000 in 20 Hz - 50 Hz zone was around 16 msec against woofers (and midranges, tweeters super-tweeters) (ref. here #493 and #494).
c. My main SP's
woofer, 30-cm YAMAHA JA-3058 is in heavy(
39 kg)-rigid
sealed cabinet (ref.
here), and now dedicatedly and directly driven (passive LCR has been eliminated) by powerful and excellent-damping-factor amplifier YAMAHA A-S3000.
Prior to fully deciding the DSP integration for my
ported subwoofer YST-SW1000 and
sealed woofer JA-3058, I thought that
it should be critically important to know/observe the transient behaviors (responses to rectangular tone-burst input/excitation) of YST-SW1000 and JA-3058 around the possible low-Fq XO zone, as I shared in my posts
#495,
#497,
#503,
#507 on my project thread.
I fortunately (and unexpectedly) found that the
woofer JA-3058 in the heavy-rigid-sealed cabinet driven directly by dedicated amplifier (with no passive CLR network) still have excellent transient behavior with little distortion even in 50 Hz - 65 Hz zone.
On the other hand, the subwoofer
YST-SW1000 showed unexpectedly excellent transient behavior in 32 Hz zone, but it showed some
"distortions" in 50 - 65 Hz zone (I may better to say
"muddiness" here) by a little bit prolonged after-shock of the subwoofer cone and/or slow low-gain lower-Fq Helmholtz resonance of the inner air of the so-designed ported cabinet.
The 3D color Gain-Fq-Time spectrum given by ADOBE Audition 3.0.1 was very much effective in semi-objectively measure/observe the tightness and cleanliness of the sound energy distribution in 3D Gain-Fq-Time space. These spectral observations were much helpful for me to decide the XO FQ and the filter slopes thereof for XO between
YST-SW1000 and
JA-3058.
The whole of above-described measurements and observations have led me to
my compromise optimal DSP and other configurations, not only the 16.0 msec GD setting for woofers to synchronize with subwoofers but also selection of XO Fq and filter slopes, in my own audio rig as shown in detail in the diagram under the below spoiler cover.
You would please note that, in my setup, the DSP LP filter for
subwoofer YST-SW1000 is 50 Hz (-24 dB/Oct), and furthermore the built-in passive LP in
YST-SW1000 is set at 55 Hz (-24 dB/Oct). DSP HP filter for
woofer JA-3058 is set at 55 Hz (-12 dB/Oct). These mean
YTS-SW1000 and
JA-3058 sing well together (or very smooth transition) in 40 Hz - 60 Hz zone.
Edit: on May 18
Let me emphasize again that it should be always very important and indispensable not only objective and semi-objective "measurements and tunings" but also the utilization of persistent/consistent and suitable "audio reference/sampler music playlist" (in my case please refer here), throughout our audio tunings as well as room acoustic treatments for our final intensive subjective assessments.