Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

PC Based Computer music servers, crossovers, and equalization

sensorless DSP cone excursion limiter
sensorless DSP cone excursion limiter
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 30th November 2016, 04:16 PM   #11
bcodemz is offline bcodemz  Canada
diyAudio Member
 
Join Date: Feb 2014
I really need to visit this forum more.

Charlie this is of extreme interest for me. This is a sliding high pass limiter right?
  Reply With Quote
Old 1st December 2016, 03:28 AM   #12
CharlieLaub is offline CharlieLaub  United States
diyAudio Member
 
Join Date: Mar 2007
Location: Michigan
Quote:
Originally Posted by bcodemz View Post
I really need to visit this forum more.

Charlie this is of extreme interest for me. This is a sliding high pass limiter right?
I'm not super familiar with that term, but I think that it one way you could describe it. The limiting function is like a high pass filter, and with increasing limiting action the filter corner frequency is increased. The code modulates the corner frequency along with the music signal so that some quantity of interest (predicted cone excursion in this case) is kept below user-defined ceiling value.
__________________
Visit my Audio Web Page <<--CLICK TO LEARN MORE-->> Get my LADSPA plugins
  Reply With Quote
Old 1st December 2016, 08:09 AM   #13
bcodemz is offline bcodemz  Canada
diyAudio Member
 
Join Date: Feb 2014
Quote:
Originally Posted by CharlieLaub View Post
I'm not super familiar with that term, but I think that it one way you could describe it. The limiting function is like a high pass filter, and with increasing limiting action the filter corner frequency is increased. The code modulates the corner frequency along with the music signal so that some quantity of interest (predicted cone excursion in this case) is kept below user-defined ceiling value.
Yes, that's exactly what they are.

Have you ever considered doing the inverse to compensate for thermal compression? This is pretty significant for woofers that have large amounts of bass boost that would quickly have a lot of power heating the voice coils.

Without a temperature sensor accurate feedback isn't possible, but an approximation should still do quite a good job. For example, since excursion can be modeled for a given signal, then power requirement can be modelled too. An approximation can be made on the heat buildup and heat dissipation rate to approximate temperature on the voice coil. Then a certain amount of boost can be applied depending on the temperature of the voice coil.

Last edited by bcodemz; 1st December 2016 at 08:22 AM.
  Reply With Quote
Old 1st December 2016, 09:25 AM   #14
soundbloke is offline soundbloke  United Kingdom
diyAudio Member
 
Join Date: May 2005
Location: England
David Birt's self-balancing bridge described in an AES publication and two (now expired) patents might be what you are looking for... I have discussed some aspects of this much overlooked stroke of genius in two other threads, namely "Better dome midrange design than ATC?" and "ATC SCM200 ASL Pro" that you may also care to read.

Birt's self-balancing bridge provides accurate temperature sensing and excursion sensing WITHOUT any additional sensor required for the driver. The output can be used to drive limiters as required (although strictly this becomes a compressor for the coil temperature case) regardless of whether it is used for motional feedback as is/was its intended purpose.

As I said, a stroke of genius that to date remains conspicuous by its absence...
  Reply With Quote
Old 1st December 2016, 03:21 PM   #15
CharlieLaub is offline CharlieLaub  United States
diyAudio Member
 
Join Date: Mar 2007
Location: Michigan
Quote:
Originally Posted by soundbloke View Post
David Birt's self-balancing bridge described in an AES publication and two (now expired) patents might be what you are looking for... I have discussed some aspects of this much overlooked stroke of genius in two other threads, namely "Better dome midrange design than ATC?" and "ATC SCM200 ASL Pro" that you may also care to read.

Birt's self-balancing bridge provides accurate temperature sensing and excursion sensing WITHOUT any additional sensor required for the driver. The output can be used to drive limiters as required (although strictly this becomes a compressor for the coil temperature case) regardless of whether it is used for motional feedback as is/was its intended purpose.

As I said, a stroke of genius that to date remains conspicuous by its absence...
I never claimed to do anything "new" here...

Thanks for the tip on the Birt stuff. Any chance you have a link or reference for that?

My work is to develop a limiter, specifically a look-ahead limiter running in software on inexpensive ARM boards. The other types (e.g. analog implementations) have major flaws IMO. I have already completed another lookahead limiter that limits the signal level. I am modifying it to be more flexible in that it will be able to limit the cone excursion in a more direct way.
__________________
Visit my Audio Web Page <<--CLICK TO LEARN MORE-->> Get my LADSPA plugins
  Reply With Quote
Old 1st December 2016, 04:30 PM   #16
soundbloke is offline soundbloke  United Kingdom
diyAudio Member
 
Join Date: May 2005
Location: England
Quote:
The other types (e.g. analog implementations) have major flaws IMO
Would you care to enlighten us what these major flaws are?

I have implemented the exact same limiter/compressor using a modified form of Birt's self-balancing bridge in whole analogue and DSP-driven versions. There is no compromise - audible or otherwise - of which I am aware with either approach.

Limiter design does take some care in the analogue domain but audible artefacts can be overcome - and reduced to a level below that of the intended limiter operation. Compressors are somewhat easier and can be implemented via opto-coupled devices.

In DSP there is, however, a big disadvantage, namely that hard limiting even at the lower end of a passband can produce aliased components folding back into the passband. To prevent such distortion for a tweeter requires operating at at least twice the baseband sampling rate.

I would also recommend that if using excursion, thermal or power compression in an active set-up that the compression is implemented prior to the crossover (although possibly post any excess phase part). This does not compromise the compressor action but does avoid the audible changes to the overall frequency balance that will occur with specific band compression.
  Reply With Quote
Old 1st December 2016, 05:30 PM   #17
CharlieLaub is offline CharlieLaub  United States
diyAudio Member
 
Join Date: Mar 2007
Location: Michigan
Quote:
Originally Posted by soundbloke View Post
Would you care to enlighten us what these major flaws are?
Sure. For example, a pure limiter (e.g. without fudging by adding some compression near the limiting threshold) only knows two things: (a) the signal is below the limit threshold, or (b) the signal is above the limit threshold. Let's say we feed it a signal that will exceed the limit threshold. The limiter say to itself:
...signal below threshold...
...signal below threshold...
...signal below threshold...
...signal below threshold...
Oops! Signal has exceeded the threshold! Limit it!
Oops! Signal has exceeded the threshold! Limit it!
Oops! Signal has exceeded the threshold! Limit it!
Oops! Signal has exceeded the threshold! Limit it!
...signal below threshold...
...signal below threshold...
...signal below threshold...
(etc).
The above is what a hard limiter would (analog or otherwise). Very poor!

Ah, says some wise person, let's tweak this a bit to improve the limiting behavoir and avoid all those nasty side effects! Enter myriad schemes that try to smooth out the limiting behavior, usually by adding some compression around the limit threshold. This has the result of limiting the signal before it needs any limiting. Also, by reducing the "hardness" of the limiting, you can't enforce a ceiling but instead have to put up with an occasional "above the limiting threshold" blip. When the action (no matter what circuit you are using) is applied at the same instant that the governor monitors the circuit, there really is no way around these problems. You can only try to mitigate them to make the downsides less objectionable. IMHO this is why limiters have a very bad rap.

With DSP you get a new dimension that you can use: delay. No longer does the limiting algorithm need to take action on the signal immediately. It can monitor the signal and then "decide" what should be done IN ADVANCE! Yes, that's right, it has "advance" warning of a peak! This is because the delay that is introduced (which also produces latency) allows some time for the algorithm to make smooth and gradual changes BEFORE the peak occurs and before the data must be delivered by the limiter to the rest of the signal chain. The delay allows the algorithm to "look ahead" in time, and so is called a look-ahead limiter. The limiting action can be frequency dependent, if perhaps there is some knowledge that the signal peak is due to some sub-band of the full audio spectrum. The algorithm can smoothly modulate the audio signal around the peak to JUST accommodate it so that the peak is exactly reduced below some threshold. The downside is that you cannot do this for one band in a multi-band system without adding in the latency of the band that is being limited. Some pro units try to implement this as a "multiband" limiter for the full audio spectrum, but this again is just a series of compromises because, in general, this kind of limiter knows nothing about why the peaks is exceeding the threshold and all the frequencies within each band are treated the same way. In my case I am designing the limiter specifically for a subwoofer, and have some information about where and why the peaks must be limited. This helps to tailor the action so as to be as blameless as possible, only compressing that part of the signal that is causing the peak in the first place and only doing it when (in time) and where (in frequency) it is needed.

I really don't know anything about your balanced bridge approach, but I am convinced that a customized DSP look ahead limiter is better than any other approach for this application.
__________________
Visit my Audio Web Page <<--CLICK TO LEARN MORE-->> Get my LADSPA plugins
  Reply With Quote
Old 1st December 2016, 10:39 PM   #18
soundbloke is offline soundbloke  United Kingdom
diyAudio Member
 
Join Date: May 2005
Location: England
Quote:
IMHO this is why limiters have a very bad rap
In my experience this is not the case. There is a distinction that first needs to be drawn between limiters employed to effect such as multi-band limiters in a studio rack and those employed for loudspeaker protection. The major failing of limiters employed for protection is in the release and not the attack provided the 'knee' is soft enough. FET limiters applied with caution are capable of being practically inaudible in such applications.

From here two corollaries are evident: Firstly the audible effects of such limiters are confined to the 'post' response and the ability to shape a 'pre' response by implementing delays is superfluous; Secondly, displacement limiting is by its nature apparent at the low frequency end of the passband (or at the low frequency end of a sub-woofer band in your application) where a simple FET limiter that is good enough at high frequencies is then more than adequate here.

A further point of relevance here is that a sub-woofer channel is likely active and therefore processed separately plus band-limited by its very nature. Any higher harmonics due to an overly sharp knee are not then likely to be audible in any case. Indeed the major advantage of 'look ahead limiters' is to prevent the aliasing I described in my previous post and this will simply not be relevant in a sub-woofer application with a FET limiter.

Quote:
The limiting action can be frequency dependent... only compressing that part of the signal that is causing the peak in the first place... a customized DSP look ahead limiter is better than any other approach
This requirement is simply a linearly filtered detector (for your uncalibrated model at least). As stated previously, the best method is to use a bridge to extract both the coil displacement and coil temperature. And as also stated I have done this in both the analogue and DSP domains where the latter offers some advantages - but audibility is not one of them. Look-ahead limiting is not a requirement for realising a "blameless" displacement limiter but accounting for thermal compression and a more accurate model are.

Last edited by soundbloke; 1st December 2016 at 10:46 PM.
  Reply With Quote
Old 1st December 2016, 10:44 PM   #19
soundbloke is offline soundbloke  United Kingdom
diyAudio Member
 
Join Date: May 2005
Location: England
Quote:
The above is what a hard limiter would ?do? (analog or otherwise)
This description is not valid BTW since the channel is band limited.
  Reply With Quote
Old 1st December 2016, 10:53 PM   #20
soundbloke is offline soundbloke  United Kingdom
diyAudio Member
 
Join Date: May 2005
Location: England
It might also be advantageous to consult the many publications by Klippel who describes a limiter such as you are trying to implement in amongst a much larger suite of applications.
  Reply With Quote

Reply


sensorless DSP cone excursion limiterHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Frequency dependent limiter to avoid one excursion above x max hahfran Multi-Way 9 12th April 2016 05:31 PM
Cone Excursion johnc124 Subwoofers 7 10th April 2009 02:45 AM
Question about cone excursion. perpetual Subwoofers 15 5th May 2005 01:45 PM
calculating cone excursion? Jim85IROC Multi-Way 11 3rd April 2004 08:41 AM
Peak cone excursion Vikash Multi-Way 8 9th July 2003 06:21 PM


New To Site? Need Help?

All times are GMT. The time now is 08:45 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki