why don't we use predistortion more in the audio world? - diyAudio
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Old 15th August 2012, 05:02 AM   #1
JZatopa is offline JZatopa  United States
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Default why don't we use predistortion more in the audio world?

So I started thinking about melding old and new technology today and started wondering why we don't use pre-distortion more in home audio. Has anyone played with this before? If we were to measure a tube amplifier's THD, phase, frequency response, etc. couldn't we create an inverse filter which would improve the performance of the amplifier? Could this work to replace negative feedback in our amplifiers for that "zero negative feedback" sound without any of the drawbacks? Could this be taken a step further to measure an amps performance when connected to a particular set of speakers so that you take into account the complex interactions between the speaker and the amplifier?
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Old 15th August 2012, 05:31 AM   #2
jcx is offline jcx  United States
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predistortion requires accurate knowledge of the error you want to correct

even measured at one time you haven't captured all of the longer term variables eg tubes age, thermal history moves bias points, the power supply sags...

the technological advantage is to negative feedback - why it is universally used is that it can continue to correct for unknown, varying errors with no more "intelligence" than a difference measuring stage, excess loop gain and a feedback network

only in the weird world of audiophiles, among the subset that refuses psychoacoustic controls when making listening comparisons, does negative feedback have any poor connotation
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Old 15th August 2012, 05:54 AM   #3
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I believe the answer to this question is - predistortion can only work for static non-linearities. Most distortions of interest in amplifiers aren't static, they're frequency dependent.
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Old 15th August 2012, 06:15 AM   #4
jcx is offline jcx  United States
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certainly more difficult but Volterra, Wiener series can capture/describe frequency dependent nonlinearities

a few papers have been written about using them in loudspeaker correction

I suspect Dr Geddes would claim the parts of loudspeaker distortion you could correct with predistortion aren't particularly audible
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Old 15th August 2012, 06:37 AM   #5
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I once read about a multitrack studio tape recorder that had a predistortion circuit in the recording circuitry to reduce the distortion in the final played back signal. I later read an opinion of it on rec.audio.pro, the preferred sound was with the predistortion circuit turned off.

It's remarkable that someone could even manage to do a predistortion circuit back then. Nowadays it would be handled with floating-point DSP code. I've read where it's easy enough to cancel the floor bounce from a speaker. Most other problems are more subtle and harder to measure, much less knowing how to pre-correct them.
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Old 15th August 2012, 11:58 AM   #6
DF96 is offline DF96  England
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Predistortion would require some form of adaptive recalibration from time to time, as components age or mains voltage varies. Negative feedback solves the same problem and inherently adjusts to most reasonable circuit changes.

Predistortion is most useful in situations where NFB is difficult to apply, such as RF PAs - where there sometimes really are time delay issues. There the main issue is not exact fidelity, but avoiding spectral regrowth which can interfere with adjacent channels. If you have to satisfy, say, a -50dBc spec then you don't care what IM rubbish etc. you throw into the adjacent channel as long as it stays below -50dBc.
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Old 15th August 2012, 04:08 PM   #7
jcx is offline jcx  United States
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Old 15th August 2012, 06:13 PM   #8
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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.
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Old 15th August 2012, 10:55 PM   #9
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Quote:
Originally Posted by jcx View Post
predistortion requires accurate knowledge of the error you want to correct
Ecxactly.

Now we can close the thread.

Edit: if to mean "Predistortions of frequency response", i.e. Equalization, it is used widely. Earlier I used 1/3 octave LC notch filters, now I relie on digital Audissey in my home theater.
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Last edited by Wavebourn; 15th August 2012 at 10:58 PM.
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Old 16th August 2012, 07:49 AM   #10
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Quote:
Originally Posted by jcx View Post
predistortion requires accurate knowledge of the error you want to correct
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.

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. People around here are always fiddling about with bias currents etc. in order to do something similar, without accurate knowledge of the error they're trying to correct.

There does seem to be an inconsistency between the importance some people attach to an idea like zero feedback, and their willingness to dismiss any solution that is slightly 'hybrid'. If we were to suggest a system that required a quarterly, computerised, 1 hour re-calibration sequence to be performed, it would be dismissed out of hand as a joke, too inconvenient. If, on the other hand, we were to suggest that the audio storage medium be of a format that could easily be damaged by normal handling and required 'cleaning' upon every play, no one would bat an eyelid.
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