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Many subs are 'DSP optimised' at the factory these days... should they be?

8K views 104 replies 28 participants last post by  MichaelJW 
#1 ·
It's been a while since I've been in the market for subwoofers, and I've noticed many high end models have ruler-flat responses. Looking at the specs, they're all using some form of factory DSP optimisation.

When I bring one (or more) of these home, I'll inevitably apply secondary DSP again based on how they uniquely respond to my room (using MiniDSP in my case).

These ruler-flat graphs look great in the marketing materials, but here's the question:

1) wouldn't we have more headroom available to make in-room adjustments if the sub hadn't already been DSP'd?

If so...

2) for people who DSP themselves, wouldn't it be better if the factory DSP could be bypassed or was omitted entirely?

Curious about how others look at this so I can refine my thinking.
 
#84 ·
Here are 3 different impulse responses that I have pulled from the library. They are not labeled as to what subwoofers produced them. If anyone reading along wants to play a game, simply pick an impulse from the 3 and briefly state why that one is better or more desirable to you. I'd be interested to see which of them gets picked and why.

For the exercise assume that the subwoofers producing these are all evenly matched. Assume that the: Output headroom, distortion, size, shape, weight, looks, price, etc, etc, whatever aspects that are important to you when shopping for a sub, are all equal. Unlikely I know, but needed for this thought experiment.

Please choose one of the 3 impulse responses that are attached.

Subwoofer Impulse Response A

Subwoofer Impulse Response B

Subwoofer Impulse Response C
 

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#87 ·
Intuitively, I would pick A, as well, although I can't really justify that choice. What would be helpful at the end of this exercise would be not only the reveal, which is sure to confound our expectations, but some explanation of what we are actually looking for in an impulse response. I think I followed Mark's post on why we have to know what frequencies (upper frequency extension) we are looking at to make the response meaningful. But, I think that everyone would still benefit from whatever explanation you have time to write.

Regards,
Mike
 
#89 ·
I like B for its low spike and short settle time.
 
#90 ·
Based on what I've read (for example at REW Impulse Responses)

I'd say B for "In a perfect system of infinite bandwidth with totally absorbent boundaries, the impulse response would look like a single spike at time 0 and nothing anywhere else". (quoted from the page linked above)
 
#91 ·
Does this help in picking Josh's choices? :)

>>>>>>>>>>>>>>>>
http://www.stereophile.com/features/100/index.html#yepjP58jiAMbE8ed.99
Fig.8 shows a perfect impulse response. The voltage instantaneously rises from the graph's time axis, remains at a fixed DC level for a small period of time, then instantaneously drops back to the time axis. The width of this rectangular pulse is inversely proportional to the frequency bandwidth of the signal. If the impulse were infinitely narrow on this graph's time axis, it would require a system with infinite bandwidth—"from DC to light," was how one of my college professors used to describe it—to be reproduced with its shape intact.





Fig.8 Perfect impulse response.
 
#92 ·
Here is a recent post by Brian @Rythmik relevant to this discussion.

Servo is based on remote sensing. So the DCR of the driver is not even in the close-form equation that determines the output. So the compensation is instantaneous and its compensation range is determined by the loop gain. On the other hand, DSP adjustment is at specific time instances or intervals (for instance every second). In addition, it is a "control" meaning it has to know what the resistance of voice coil is before it can control. In the servo, there is no need to know the DCR. So the DSP control loop is: measure -> control -> measure -> control -> measure -> control.... In servo, it is ...............................

Servo does not know if the driver or amp is at its limit.


BTW, Enrico quoted me for a comment that I made a few years back. I'd like to explain a bit more. The unit-to-unit variations are about T/S parameter variations. Very often we take the T/S parameters off the manufacturer's website and think those apply to all units. But in reality as many have done it before, if someone buys 4 units of the same driver model and measures the T/S paremeters, he actually gets 4 sets of different T/S parameters and each set changes over time and changes between small signal vs large signal. Thilo (TC Sounds) told me a story that he delivered drivers to one of his customers and customer rejected those drivers based on T/S paramets out of spec. It turns out, the difference is due to the fact that they measured at different signal levels. After that, Thilo measure parameters at the signal level specified by the customer.



Now back to the "thermal memory" effect. Here is a plot that I use to study thermal memory in non-servo subs. Here I use "two signal levels" to simplify discussion so that we can see output changes much easier. In real world, the signal level is constantly changing. As you can see the output of the large signal is gradually decreasing due to temperature rise in voice coil in what I call "heat-up" period. After we switch to the small signal, at about 3.1sec, the opposite happens that the output gradually increases, in what I call "cool-down" period. We call it memory effect because it is not repeatable. If we run a waveform like above in a round robin fashion, the output of the large signal is different with each repetition because the cool-down period may not be sufficient to bring it back to room temperature. If we take the correlation of the output variation vs the input signal, it is a delayed correlation as temperatutre change takes time to build up and go away. For instance at the moment when the signal changes from large to small, does it go back to room temperature? No it does not. In fact, at that very moment (3.1S), it has the largest temperature variation. But where is the center of the large signal? It is at 2.8S which is the center of the large signal period. That is why I called it memory effect. It is not just a static thermal compression. The folks with DSP say they solve the problem. But in fact, they misunderstand the problem.
Brian, it is a bit unfair to simply say your competitors misunderstand the problem. They likely understand and perhaps you mean they cannot fully address it.

I learn more and more each time insiders such as yourself post. I'm quoting you here: http://www.avsforum.com/forum/showthread.php?t=2754489

It would be great for you to participate in the discussion which has been largely polite and productive.
I'm hoping Brian can clarify his quoted criticism of subs w/SMPS while his Servo subs are not know to excel in mid-bass.
 
#95 ·
Here are a few more examples...


The first 5 are from an SVS PB13U in 20Hz tuning. These are from a simple adjustment of the low pass filter and that is all. The response that correspond to these are shown. You can see what a huge difference it makes.


The last 3 are from a Funk Audio FW18.0. One is with no EQ preset engaged and the other is with the HT preset. Both response shapes are shown. Again you can see that this makes a huge difference.


Impulse response is a useful tool in the right circumstances and most measurements are derived from it but as far as comparing speakers directly as a performance metric it's not useful in my opinion. It's the other various graph types derived from it.
 

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#96 ·
For the exercise assume that the subwoofers producing these are all evenly matched. Assume that the: Output headroom, distortion, size, shape, weight, looks, price, etc, etc, whatever aspects that are important to you when shopping for a sub, are all equal. Unlikely I know, but needed for this thought experiment.

Lol, yea I'd say they are pretty evenly matched. It's the SAME DRIVER. HAH. Pretty unlikely, well played, @Ricci :)

Fascinating that the impulse frequency bandwidth made such a big difference. This is the bandwidth of the SIGNAL, right? Any explanation for this? Also, how does one measure impulse responses?

Impulse response is a useful tool in the right circumstances and most measurements are derived from it but as far as comparing speakers directly as a performance metric it's not useful in my opinion. It's the other various graph types derived from it.
What are the other various graph types derived from Impulse Response?

I understand now why comparing IRs and FRs between two subs are no where near comparable. FR can give decent impressions of sub's performance, IR can too, but only if the measurement parameters are basically identical.
 
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