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MQA and the "Pre Ring - Post Ring" Hoax

There's been a lot of misinformed babble on various audio forums about impulse response, digital filters, "time errors", "time correction", "time blurring", and similar pseudo science clap trap to convince audiophiles that suddenly in the year 2018, there's something drastically wrong with digital PCM audio - some 45 years after this landmark technology was developed by Philips Electronics engineers. Newsflash folks - it's a scam.

First, let's take a close look at what an impulse or discontinuity signal really is. The wikipedia definition actually is pretty accurate thanks to a variety of informed contributors from around the globe. It is a infinite aperiodic summation of sinusoidal waves combined to produce what looks like a spike (typically voltage for our purposes) in a signal. Does such a thing ever occur in nature or more importantly in our case - music? Absolutely not. In fact, the only things close to it are the voltage spikes that occur when a switch contact is thrown or an amplifier output stage clips because supply voltage to reproduce the incoming signal waveform has been exceeded. So if this freak of nature signal representation doesn't exist in nature or music, of what good is it in measuring the accuracy of audio equipment? The answer might surprise you.

In fact, impulse response, or an audio system's response to an impulse signal, is one of the most useful and accurate representations in existence of such a system's linearity and precision - or its fidelity to an original signal that is fed to it.  A lot of  focus has been placed on the pre and post ringing of these "discontinuity signals"  but what you have to understand is that the ripple artifacts are nothing more than an analog system's (all electronics is analog -digital is just a special subset of analog) limitation in attempting to construct the impulse or discontinuity signal waveform. They are a result of the impact produced by the energy storage devices themselves in creating the signal. To create a large energy peak, you need large storage devices. The larget the capacitor for example, the longer in time it takes for it to absorb and discharge electric field energy. This is the same with inductors. One type stores electric field energy - the other magnetic. Smaller value capacitors can react to voltage changes very quickly but are limited in the peak value of energy that can be stored and dissipated. But if you combine a large number of high value and low value devices in a circuit and apply a voltage spike, you wind up with the kind of oscillations you see in an impulse response graph. Small capacitors for example, rapidly reach their charge capacity and can discharge into larger capacitors that are much more slowly building up charge in the transition from no input voltage to full spike value. This "sloshing around", if you will, or oscillation is what happens in circuits built to provide extreme voltage attenuations. In a linear, time invariant system, any rapid change in frequency response or time response - has these characteristics.
So effectively the entire debate about ringing in digital audio is a misnomer - a hoax. The impulse response ripple is not something that happens in real world sounds or in a properly designed audio reproduction chain. Ever since digital oversampling was developed in consumer products in the early 1980s, there has been no need for steep analog filter circuits with their attendant ringing. The problem very simply DOES NOT EXIST. The ringing generated  artificially in an impulse signal is useful in that it provides a very high frequency stimulus to linear audio systems as  a means of measuring high frequency and transient response. IT IN NO WAY BY ITSELF, REPRESENTS THE TIME DOMAIN BEHAVIOR OF THE AUDIO REPRODUCTION CHAIN. An accurate audio reproduction system should fully render the impulse signal in all its pre and post ring glory without alteration. Any audio system that eliminates or significantly alters this pre/post ringing present in the signal that is fed to it is not truly "high fidelity" and is thus bandwidth limited.
cj1965
cj1965 OP
124 posts
 
" Your point that preringing and postringing do not exist is not generally accepted as fact among the people who design and build audio equipment. If you want to present that opinion to us, fine, but to say it's fact and that anyone who doesn't agree is ignorant is not helpful to your cause. I'm just suggesting that you lower the intensity of your posts a little. We don't need an MQA war here. As I said, there are forums where such a war would be welcome." -  tomcy6

Please cite some examples of "people who design and build audio equipment" who claim that music reproduction involves "pre and post ring" signals. I use impulse tests on a daily basis. I would like to see evidence of ultra broad band high intensity, ultra short duration peaks in music or any other signal source to back up what you appear to be saying. If you read the thread carefully, you will find that no one is saying pre and post impulse ripple doesn't exist. What you will find if you actually read the beginning of the thread is that these phenomenon only exist in circuits that utilize extreme attenuation of signals. Filter theory is very old. There are no special new filters that violate the laws of physics. Every steep filter produces oscillations that can easily be calculated for mathematically in the transfer function and represented in a Bode plot displaying stability or lack thereof. The problem with the entire foolish MQA promotional exercise is that a handful of "Industry people" are using impulse response data to infer that such signals are routinely present in music and their special characteristics (pre/post ripple) must somehow be dealt with. This is a complete farce that exposes widespread ignorance in the "audiophile" community - or at least that subset of the community that embraces ridiculous postulations about impulse signals that aren't true for the majority of music data fed to digital circuits.

Again, if you know anyone in the industry who can demonstrate that pre and post impulse ripple is encountered in music playback, I for one would like to see evidence of it. The anecdotal " I know some people" isn't sufficient. Evidence is required. If you want further citations for what an impulse signal is, I can provide links and you can actually generate them yourself using readily available software and a modest equipment setup. You can contrast the signals you produce with that of sharp pulses produced by a square wave generator - another signal type that doesn't exist in the real world of music but is used for equipment testing nonetheless.

There's an old saying about "a little information" being dangerous. That is precisely what is going on with MQA and signal ringing. The folks that are being misguided know just enough about the subject matter with which to make fools of themselves. Take the time to at least generate some impulse response signals yourself before you go on an internet forum to "educate" others about it. I have done it. You can too.
hgeifman
2,361 posts
 
As posted above, there has lots of positive and negative comments on Audiogon about MQA coded albums.  Of course, everyone is entitled to their own opinion that I enjoy reading.  I listen to Tidal MQA coded albums since I feel they sound slightly better (sometimes) than regular albums.  The music seems clearer and with less background noise.

I do NOT LIKE the Audio Manufacturers having to pay for the MQA license and noticed that some support this extra cost and many others do not.  In addition, I know that some people “believe that supporting MQA means handing over the entire recording industry to an external standards organization”.  If this statement is true (is it?), it is also a concern to me.

I own the Aurender N10 Music Server (it decodes the first MQA layer) and has 10,000+ MQA Coded albums from Tidal that I enjoy.  However, I would NOT PAY extra for MQA coded albums.   I also know that some Manufacturers want nothing to do with the MQA coding process.

I continue to believe we have too many audio formats that make it very confusing for people new to audio and maybe everyone else (such as who does the unfolding of the different MQA layers?).   Some DAC's support MQA and many others do not and will NOT.   There is much confusion, concern and debate around MQA coded albums, so I am waiting to see what happens.  


tomcy6's avatar
tomcy6
2,638 posts
 
Charles Hansen of Ayre Acoustics (now deceased, R.I.P.) and many other audio manufacturers use Minimum Phase filters in their digital components. It is my understanding that the purpose of these filters is to eliminate preringing. I know very little about electronics so I may be wrong about that. With that, I’m out of this discussion.

Here is what Charles described as one of his accomplishments in audio design:

"World’s first disc players to provide user-selectable “Minimum Phase” digital filter responses, including both “slow roll-off” algorithm with improved transient response and “apodizing” algorithm for removal of ringing from digital filters used to produce the disc."

Read more at https://www.audiostream.com/content/qa-charles-hansen-ayre-acoustics#8mhJtBVPIS8iPIDQ.99


clearthink
1,421 posts
 
cj1965"cite some examples of "people who design and build audio equipment" who claim that music reproduction involves "pre and post ring" signals. I use impulse tests on a daily basis. I would like to see evidence..."

hey cj1965 here’s you’re very answer to someone else who asked for some proof. Do you recognize this???
"cj1965Oh puhleeeeeze. Spend five minutes of your time Googling...Nobody is going to waste time giving you proof of the obvious. If you’re that lazy...don’t expect any of us to serve it up to you on a silver platter. 

So it is only fair to now ask you to Google for yourself!
cj1965 OP
124 posts
 
" Charles Hansen of Ayre Acoustics and many other audio manufacturers use Minimum Phase filters in their digital components. It is my understanding that the purpose of these filters is to eliminate preringing.   I know very little about electronics so I may be wrong about that. With that, I'm out of this discussion. "  - tomcy6

Ok. Let's be clear about what minimum phase  filters are. By definition, a minimum phase filter is a filter that remains causal and stable when used in a linear, time invariant system. All that really means is that  phase and gain changes cannot result in oscillation (unstable). For this to occur, the poles and zeros of the transfer function plot have to stay inside the unit circle. A common example of violation of this characteristic happens when an amplifier's overall phase shift is 180 degrees at unity gain. Under those circumstances, a typical feedback amplifier will oscillate.

In terms of filters, it really has nothing to do with impulse response. As noted previously, to generate a very rapidly increasing and rapidly decreasing voltage spike in a linear, time invariant system - requires and extremely sharp (steep) filter. That's where the ringing (instability) comes in. A minimum phase filter in Charlie's case is simply a gradual rolloff (low phase shift with frequency) filter that is causal and stable - minimum phase. We are talking apples and oranges here. There is absolutely nothing whatsoever about an impulse signal that is "minimum phase".
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