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Old 16th August 2012, 10:20 AM   #11
DF96 is offline DF96  England
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Quote:
Originally Posted by CopperTop
It requires accurate prediction of the signal that, when added to the input signal, will minimise the output error. Which isn't necessarily the same thing, I would suggest.
I think you will find that this is mathematically equivalent to what jcx said. You need to get the predistortion right. That means careful calibration. This will never be exact. The result could be lots of low level high order distortion. If you think of valve and SS as being at opposite ends of a line of distortion 'flavour', this would be beyond SS!

Quote:
Originally Posted by CopperTop
Presumably even a partial correction would allow you to reduce the amount of NFB required to achieve the same distortion level, so it sounds like an idea worth investigating if you hate the idea of NFB.
Possibly less NFB, but probably higher frequency NFB. Incomplete predistortion will have the same effect as re-entrant distortion from NFB: proportionally more higher order terms. It solves the problem already more easily solved by NFB, while introducing the same problem as NFB.

As I said, people only use predistortion where NFB cannot be used.
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Old 16th August 2012, 11:14 AM   #12
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Originally Posted by DF96 View Post
I think you will find that this is mathematically equivalent to what jcx said.
Not sure about that. If I can measure the error accurately (between actual output and scaled up version of input signal), it still doesn't tell me what I must add to the input in order to correct it... To do that, I effectively have to have accurate knowledge of the characteristics of the output stage. This is the sort of thing that is done when linearising a system in industry using an inverse neural network model of the 'plant' (or so I believe).
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Old 16th August 2012, 03:00 PM   #13
JZatopa is offline JZatopa  United States
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Quote:
Originally Posted by CopperTop View Post
I started a thread a few months back on the use of pre-distortion based on neural network-type structures, but was given short shrift!

A neural network, or similar structure, can be trained to create, effectively, a multi-dimensional lookup table that is 'indexed' by a vector (an array comprising multiple values) at its input. In theory, this would allow it to produce an output based on a combination of past and future samples.

At the start of training (using a combination of test tones, noise, real music and synthetic 'difficult' signals perhaps), the signal would be pre-distorted randomly or maybe a straight 1:1, and the amplifier's output into the particular speaker monitored for accuracy. Gradually, the pre-distortion would be adjusted in the direction of better accuracy. Once trained, the network would be frozen and used in 'runtime mode' only.

The amplifier would be genuinely feedback-less at runtime, but an objection someone raised before was that it was, in effect, simply deferred feedback (from the training). However, the training does more than simply feed back an instantaneous amplitude error to the input: the 'error' can be any measurement we wish, including total least squares error or peak error over some time period etc. At runtime, the amplifier can see into the future so, for example, the system can deliberately allow (introduce) some error that could otherwise have eliminated, if it is going to allow the amplifier to 'prepare' better for a much more troublesome signal further ahead- this is the sort of thing that can fall out of the training, automatically.

Factors such as transistor and speaker coil temperature could be fed into the network as more inputs, but neural networks become more practical the more you eliminate 'dimensions', so we might want to run the output transistors at a fixed temperature, for example.

It may be a completely impractical idea, but I think it would be possible in theory.
I postulate that current technology available is of a level that could utilize predistortion to improve an amplifiers response further then what is currently possible with negative feedback. It would need regular recalibration but I dont see why you couldnt use test tones initially and then the music signal itself later. Hardware wise, couldnt you use a raspberry pi, a dac and an adc? It is really the software that is beyond my skills.

Exactly how many parameters are able to be improved by pre distortion? Frequency response and phase are obvious but van it be used to improve tramsienr response or imd?
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Old 16th August 2012, 04:26 PM   #14
DF96 is offline DF96  England
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Quote:
Originally Posted by CopperTop
Not sure about that.
I assumed jcx meant error in the sense of imperfection in the in-out function, not the raw output-'desired output'. If you know what the forward function is then in principle you can calculate the inverse function and use it for predistortion.

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Originally Posted by DJNUBZ
I postulate that current technology available is of a level that could utilize predistortion to improve an amplifiers response further then what is currently possible with negative feedback.
I postulate that the best you are ever likely to achieve will be worse, and more expensive, than properly-used NFB. Audio is an ideal application for NFB, because forward time delays are negligibly small when compared with bandwidth and equipment size is such that the lumped quasi-static (i.e. non wave) approximation of EM is valid.
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Old 16th August 2012, 04:50 PM   #15
JZatopa is offline JZatopa  United States
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Quote:
Originally Posted by DF96 View Post
I assumed jcx meant error in the sense of imperfection in the in-out function, not the raw output-'desired output'. If you know what the forward function is then in principle you can calculate the inverse function and use it for predistortion.


I postulate that the best you are ever likely to achieve will be worse, and more expensive, than properly-used NFB. Audio is an ideal application for NFB, because forward time delays are negligibly small when compared with bandwidth and equipment size is such that the lumped quasi-static (i.e. non wave) approximation of EM is valid.
Well that is entirely possible as well, in fact without any testing we are both equally as right. Are there any papers on this field of work that might shed some more light?

Negative feedback is relatively simple, low cost and works but it is about 90 year old technology. Now that digital is at a point where it is "relatively" simple, low costs and works, shouldn't we test this? I imagine that a predistortion system in a desgin where the ADC,DAC and processor are all part of the amplifier could offer benefits that negative feedback isn't capable of doing. This system wouldn't have to be constantly measuring either, it could take samples over time and adjust accordingly to account for part wear over time. Sort of like digital room correction except that it is internal to the amp and only serves to maximize the performance of the amp and nothing else. If I am thinking clearly (I haven't had any coffee yet today) I don't see why you couldn't use such a technology to flatten response and lower THD to vanishingly low levels. You could also correct for the phase issues that seem to be inherent in tube amplifers? There is a company making a class D amplifier with very low distortion that employes hawksford's forward error correction. I don't see how this wouldn't be taking everything to the next level above that.

Could you build a proof of concept using DRC? Instead of using a mic in a room, you take the signal off of the amp?
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Old 16th August 2012, 05:31 PM   #16
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All this digital correction stuff suffers from change in the system from the time calibration is performed. Why not use a simple time accurate analog solution?

A high frequency low amplitude pilot signal gets summed into the amplifier input and the pilot gets measured at the output of the system (an amplifier here, speakers would be too slow). The output pilot level is used to adjust the gain of the system by varying the tail current in a differential stage so that the pilot output level remains constant. The pilot signal gets removed from the final output by either a filter, adding a nulling signal, or just ignoring it, since it is above the audible frequencies. This has the advantage over conventional NFB in that a fixed low gain level (an R attenuator) path is all that is needed. No stability problems here.
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Old 16th August 2012, 05:42 PM   #17
jcx is offline jcx  United States
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control theory is a lot broader than audio amplifier design - but often the extensions of the theory were driven by problems encountered controlling machines, chemical processes, many industrial applications where the limitations of the "plant", sensors are major impediments, changes expensive or physically impossible and lots of effort is justified on the control side


in audio amplifiers the "plant" would correspond to audio gain stage, output transistors depending on your viewpoint

but audio output transistors are so much faster than the required "working bandwidth" for audio signals,
amp error sensing (feedback network, input diff pair) can have more S/N than the source recording we want to reproduce, is far more linear than any audio transducer

together these mean we can pretty much do as much as can be done by any method with negative feedback alone



dynamic speakers are physical performance limited "plants", sensing adds expense, economical sensors have accuracy, noise limits - so only limited negative feedback can be applied - and that really only for subs/woofers


for lots of info on dynamic speaker driver distortions, measurements, design cruise the Klippel site:

Home

specifically for a summary view of loudspeaker distortion correction try:

http://cogsys.imm.dtu.dk/nonlincomp/Klippel.pdf

and its often worthwhile poking around sites Google directs you to - to see if related material is in the higher level directories:

http://cogsys.imm.dtu.dk/nonlincomp/

Last edited by jcx; 16th August 2012 at 06:03 PM.
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Old 16th August 2012, 05:58 PM   #18
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I am slightly confused, because the learned replies suggest that there is no question concerning feedback, and that amplifiers are understood completely. I'm happy to accept that, but I thought there was supposed to be a controversy or something..? I could have sworn that I have seen threads with thousands of posts concerning the mysteries of why some amplifiers measure well apparently, but sound mediocre, with suggestions that it may be transient instability, thermal distortion etc. exacerbated by feedback, and beyond conventional analysis.
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Old 16th August 2012, 06:31 PM   #19
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I was reading this . It barks up the same tree I think ? I agree with the writer . Shame as it looks so good on paper and on the scope .

http://ken-gilbert.com/images/pdf/harmonic.pdf
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Old 16th August 2012, 07:03 PM   #20
DF96 is offline DF96  England
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Quote:
Originally Posted by CopperTop
and beyond conventional analysis.
People who don't understand something will sometimes claim that it is beyond conventional analysis. That way they can either feel more comfortable in their ignorance by believing that nobody else understands it either, or they can leave the field supposedly open for their unconventional 'explanation'.
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