Servo-feedback Subwoofer - diyAudio
Go Back   Home > Forums > Source & Line > Digital Source

Digital Source Digital Players and Recorders: CD , SACD , Tape, Memory Card, etc.

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 29th August 2005, 11:30 PM   #1
diyAudio Member
 
Join Date: Feb 2004
Location: Illinois
Default Servo-feedback Subwoofer

Hi all. Another year, another EE project. I always seem to levitate towards projects that involve audio since audio it is a hobby of mine. =)

So, I've been thinking of creating a servo controlled subwoofer. Implementation of the feedback circuit is presumably going to be done via a 16bit integer based motorola DSP (don't remember which off the top of my head). I was wondering if any of you digital guys and gals had papers, articles, or anything related to the theory behind servo controlled stuff (doesn't have to be about subs for the related theory to become apparent) done in the digital domain, meaning no differential amps, analog circuits, or the like. Google is usually my friend but all I can seem to come up with is servo controlled sub advertisements. I will continue looking as well as updating the post with progress and eventually code and circuit layouts for the final project (if it so turns out well).


Last year I made an automated equalization unit like the behringer DEQ2496's autoeq feature on the same DSP. Suffice it to say it was a major PITA b/c of the very limited precision the 16bit int platform but by george it worked and earned me a nice grade. It did get a little noisy after heavy equalization b/c of quantization errors but it was the best that could be done with limited equipment. Oh yeah, I might also be able to land my hands on a more powerful 32bit fp DSP is I dig around a bit and ask nicely.
  Reply With Quote
Old 31st August 2005, 07:12 AM   #2
diyAudio Member
 
Join Date: Feb 2004
Location: Illinois
No nibbles yet huh?

Does no one do this stuff besides the big boys (meaning no DIY guys versus actual companies).
  Reply With Quote
Old 31st August 2005, 07:38 AM   #3
seoman is offline seoman  Netherlands
diyAudio Member
 
Join Date: Apr 2004
Location: Nijmegen
I'm wondering how you plan to get a decent feedback from the speaker.

Somme do 't with piezo, but thats far from lineair.

If you could use some optical/sonical/ultrasound distance measurement you could leave the specs of the driver unchanged.

Regards and good fun with your project

Simon
  Reply With Quote
Old 31st August 2005, 09:04 AM   #4
diyAudio Member
 
Francis_Vaughan's Avatar
 
Join Date: Feb 2005
Location: Adelaide
Feedback control of a speaker is a tricky issue. You probably want to ask on the speaker groups anyway. There have been quite a few attempts, but they are often flawed by what I tend to feel is a misapprehension about what it is you can do.

Typically the feedback sensor is an accelerometer, and so the first thing people do is to integrate the input to get velocity, and then sometimes even try again in the hope they get position. Perhaps in the unfortunate hope that the input voltage to the speaker's amplifier is supposed to translate to cone position. Then they try to create a feedback loop. The whole thing is pretty much doomed, ending in a marginally stable mess that often yields very little improvement. Draconian bandwidth limiting of the input helps stability and may yield something useful.

An alternative approach I have been hoping to try is a feed-forward approach. Use the sensor, not as a feedback input device, but as the input to characterise speaker, building a parameterised model of the speaker. Then using this model, dynamically perturb the signal to correct for the modelled errors.

You could certainly cope with Bl transfer non-linearities - both static limits from the magnetic gap geometry and dynamic errors due to flux modulation from the voice coil., thermal non-linearities (you could build a paramaterised dynamic model of the voice coil temperature and thus correct for dynamic resistance changes.)

You would of course also need to know the ideal Thiel Small derived behaviour of the speaker in the particular enclosure. You could also look to adding artificial alignment changes (usually bass boost - but they needn't be) and also - since you have a quite complete model of the driver - you could perform dynamic level limitation - and never drive the speaker outside of its real physical (i.e. excursion and thermal) limits.

Such correction could be useful across a much wider bandwidth than a simple feedback system, and I would bet generate vastly better results.

My two cents anyway
  Reply With Quote
Old 31st August 2005, 10:51 AM   #5
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
The feedforward thing is quite nice and shown to be reliable since it is done often within active systems although not usually done in an adaptive fashion like suggested by Francis.
I don't know how much THD reduction could be achieved by such an approach. But response correction can definitely be done. The advantage of the adaptive system would be the automatic incorporation of manufacturing tolerances and ageing.
There is even a system around that takes the room interaction into account as well (although the manufacturer is not my favourite one ):

http://www.bang-olufsen.com/web2/sys...=ls&ProdID=544

Regarding servo control:
Another alternative to position- and motion- sensors is the measuring of the actual audio output.

http://www.meyersound.com/products/s...x-10/index.htm

Regards

Charles
  Reply With Quote
Old 1st September 2005, 08:34 AM   #6
using_e is offline using_e  Austria
diyAudio Member
 
Join Date: Jan 2005
Location: Vienna
A few years ago I wanted to build a servo feedback sub and therefore gathered all the infos about I could get. It took quite some time to dig everything up but in the end there was enough info to start with. Unfortunately the only limiting factor for me is always time and so I put the project on ice since I wanted to realise other more important projects first. I still have all these infos and documents but don't know what I can make available here as some of the articles are from electronics magazines and there are sure some copyright issus to take care of. Maybe in the afternoon I have some time to look into it what I can spare.
Anyone interested in?
  Reply With Quote
Old 1st September 2005, 10:53 AM   #7
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
I once owned the original of the following interesting article (in German only unfortunately) about the microphone based servo control of a German BM speaker:

http://www.johannes-krings.com/funkschau2

The subject has already been discussed on this forum about two years ago.

Regards

Charles
  Reply With Quote
Old 1st September 2005, 10:54 AM   #8
forr is offline forr  France
diyAudio Member
 
forr's Avatar
 
Join Date: Dec 2004
Location: Next door
Hi Using_e
Yes, I am very interested in every thing related to servo-loudspeakers. I've collected quite a lot of documentation (many patents) but still looking for more. Feedback stability is never fully treated.

~~~~~ Forr

  Reply With Quote
Old 1st September 2005, 09:25 PM   #9
diyAudio Member
 
Join Date: Feb 2004
Location: Illinois
I as well am interested using_e and will be willing to share my progress as the project (hopefully) continues. If you don't want to post, send me an email instead (dswiston AT uiuc.edu)

Tomorrow I will be discussing the idea with a professor of mine. The feedforward idea also sounds interesting so I will bring that up. BTW, this particular professor is the creater of the continuum fingerboard so I'm hoping to get some good info out of him.

Link if anyone is interested:
http://www.hakenaudio.com/Continuum/

Thanks for the ideas/suggestions/info guys. If anyone has more to share, please do.
  Reply With Quote
Old 3rd September 2005, 01:55 PM   #10
using_e is offline using_e  Austria
diyAudio Member
 
Join Date: Jan 2005
Location: Vienna
Ok, I've now scanned, enhanced, converted and optimized all the articles I found up to now. They are now available as PDF files. Just note that they have been converted in the best possible quality so if you cannot read a word or a part of an article I cannot do anything about it because I don't have this article in a better quality either. So please don't mail and ask for hires scans or so because they are already up in the best quality possible.

They have been converted to Acrobat 7 format to get the smallest file size possible so please don't mail and inform me that the files are corrupted. They aren't just use Acrobat Reader 7 to open the files and you will be just fine. I've tested them all and they work.

Files will be up on the following link for a few hours only. I think it's clear why. If someone misses the first phase just drop me a message and we will find a way.

Download Link


Enjoy
using_e
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Ultra sound transducer for servo feedback subwoofer? e96mlo Subwoofers 28 11th July 2013 05:59 PM
Help with DC Servo for Current Feedback Opamp klewis Chip Amps 31 4th June 2010 06:18 PM
DC blocking cap in feedback path or DC servo? roender Solid State 27 19th September 2007 10:40 AM
Where to put DC feedback for a servo integrator rtarbell Solid State 2 26th August 2006 04:00 AM
Bandpass box with servo feedback??? Circlomanen Subwoofers 0 16th August 2005 05:30 PM


New To Site? Need Help?

All times are GMT. The time now is 10:18 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2