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Getting to grips with "clipping" - an ongoing research project

EricW

Active member
Modern speakers too bright ...

Modern speakers too bright ...

[/I]
Is that your argument? If so, that explains much.
Yes, that is exactly what I mean to say. It's speculation, of course, but it seems to make sense as a working hypothesis.
 

weaver

New member
Are these a false conclusions?

Are these a false conclusions?

1. A decently powered amplifier is more important for the upper frequencies than the lower ones.

Recent posts illustrate that clipping has a major effect on high energy, high frequency sound.

Does it therefore follow that from the point of view of amplifier clipping, a decently powered amplifier is more important for the upper frequencies than the lower ones?

(I ask the question as my previous understanding would have been that given the choice of using separate amplifiers for woofer and tweeter the more powerful one would go to the woofer and the less powerful to the tweeter.)

2. Reverberation is essential to 'air' and 'life'.

At the weekend I was at a local village fair (outdoors) where a four-piece jazz band was playing. As you might imagine I took the opportunity to have a listen to the snare drum. No clipping going on there, but also (as it was outside) no reverberation. Again, recent posts would suggest that reverberations excited by high frequency, high energy sounds are what give music 'air' and 'life'. On this occasion there was no reverberation but at the same time there was nothing getting in the way of the direct sound either. Was the sound in anyway 'dead' - no, far from it.
 

Pluto

New member
Best equaliser for the job

Best equaliser for the job

... if it is true that the major criticism of graphic equalisers (that they degraded the sound through contact losses in the switches/processing) is a load codswallop, then why not bring them back?
Because the standard graphic equalizer is the wrong tool for the job. It consists of a number of fixed frequency controls and provides the ability to boost or cut at each of those frequencies. Typically, there will be little or no control over the ‘width’ of each of the frequency bands, known technically as the Q of the band. There is the further difficulty with this type of equalizer that you are at the mercy of the maker's chosen frequencies. If the room mode you want to address sits right in between two frequencies on the equalizer, that's tough. Furthermore, graphic EQs tend to be bulky, not only because of all those knobs but also, because the user will be paying for the hardware to control 31 bands (or whatever) when the chances are that he will only be using very few of them at any one time.

A rather better approach is the parametric equalizer, which consists of a number of more-or-less identical modules, each one of which has an infinitely variable frequency control (hit that mode at exactly the right frequency), a Q control to adjust the width of the cut or boost and a gain control to adjust the amount (the equivalent of the slider in the graphic EQ). So, in the parametric EQ you are substituting size and cost for flexibility and control.

But, undoubtedly, the best way of equalizing in the context of domestic playback is to do so in the digital domain. Software costing a few $$, running on the cheapest of laptops, will provide all the EQ that you will ever need, plus the ability to store and memorize various settings. What more could you desire?
 

jair44

New member
Parametric EQ for me ...

Parametric EQ for me ...

Because the standard graphic equalizer is the wrong tool for the job. It consists of a number of fixed frequency controls and provides the ability to boost or cut at each of those frequencies. Typically, there will be little or no control over the ‘width’ of each of the frequency bands, known technically as the Q of the band. There is the further difficulty with this type of equalizer that you are at the mercy of the maker's chosen frequencies. If the room mode you want to address sits right in between two frequencies on the equalizer, that's tough. Furthermore, graphic EQs tend to be bulky, not only because of all those knobs but also, because the user will be paying for the hardware to control 31 bands (or whatever) when the chances are that he will only be using very few of them at any one time.

A rather better approach is the parametric equalizer, which consists of a number of more-or-less identical modules, each one of which has an infinitely variable frequency control (hit that mode at exactly the right frequency), a Q control to adjust the width of the cut or boost and a gain control to adjust the amount (the equivalent of the slider in the graphic EQ). So, in the parametric EQ you are substituting size and cost for flexibility and control.

But, undoubtedly, the best way of equalizing in the context of domestic playback is to do so in the digital domain. Software costing a few $$, running on the cheapest of laptops, will provide all the EQ that you will ever need, plus the ability to store and memorize various settings. What more could you desire?
Thanks for that Pluto! I will check out the suitability of a Parametric Equalizer (until an amp with built-in digital equalizing appears).

To think that I'd never heard any mention of one in any Hi-Fi magazine! Perhaps I should have been reading Sound On Sound all these years.
 

mhennessy

Member
Clipping audibility and data reduction generally: listening to the before/after difference

Clipping audibility and data reduction generally: listening to the before/after difference

There have been some really interesting responses to the Dire Straits track with hard limiting - thank you all.

Of the two clips, the first one - the one that was limited to -12dBFS - is fairly subtle, and I was keen to hear what people thought about it. It certainly doesn't sound "wrong", and in isolation, you'd be forgiven for assuming that all was well, but of course, with the original for comparison, one can hear a slight difference.

No-one identified it as clipped or distorted, and had you not seen the screen-shot of the waveforms, I'd be willing to bet that no-one (myself included) would have ever reached that conclusion. This demonstrates that quite severe clipping can go unidentified: while many of us did hear differences, we would never have guessed that clipping was the cause. In the hi-fi scene, where people make great claims about "tweaks" that do absolutely nothing, you could well imagine how a group of listeners could jump on this real change, and declare that Amplifier A - with more power than Amplifier B - is the best amplifier by quite some margin. Sadly, these same people would not allow a simple oscilloscope to enter the listening room, preferring to trust their hearing more than a scientific instrument.

The next excerpt, hard-limited to -18dBFS was much more obvious to me. What I hear is peak distortion, and that's because the clipping is affecting more than just those very narrow snare-drum peaks. With practice, you should hear some distinct "roughness" to the sound.

"Difference tracks" - a useful analytical tool

This is a really useful tool. It involves loading the excerpt into an audio editor on separate tracks. These sum together when played back, doubling the overall level. But if you invert one of the tracks, they subtract from each other, which produces perfect digital silence. Having done that, we can apply an effect to one of the tracks, and when we play this back, we hear just the difference that effect produced. This is a digital version of the Baxendall/Hafler subtraction test (some details of this in the first post on this thread: http://www.diyaudio.com/forums/everything-else/13415-null-difference-testing.html). All of this might need digesting, but trust me; it's a really powerful tool.

I produced a difference track of the previous Dire Straits excerpt with the clipping. The first 20 seconds of this new audio comparison (below) is the difference between the source excerpt and the version with 12dBFS limiting, then a short pause, and following that is 20 seconds of comparison between the source and the -18dBFS version. What you are hearing is the missing audio - or, everything that is louder than the clipping level set at -12dB or -18dB respectively.

Comparing the source file with the version clipped at -12dB, you basically just hear clicks that have been limited-out. Note how quiet they are - you'll probably have to turn up your monitoring. But, it's really interesting to note that the peaks are reaching very high levels; about -7dBFS. The RMS (average) level is incredibly low - not really registering on my level meters - which is why it sounds so quiet. This correlates well with what Alan posted earlier (post #70).

When you listen to the second excerpt, you are hearing everything that reaches and exceeds -18dBFS in the original recording. Or, to put that another way, you're hearing what was taken out when I applied the hard limiting at -18dBFS. The result is grossly distorted, but you'll easily recognise the track.



With all of that sonic information being so casually thrown away, how come the resulting sound was not absolutely terrible? It's something to think about...

MP3 Losses

Just as an aside to all of this, I did think it was worth pausing to consider the MP3 aspect of this. I'm converting my files to MP3 (at 256 kb/s) for upload here, and obviously, any budding audiophile knows that MP3 - or indeed any lossy data reduction scheme - must be the work of the devil! That's not strictly fair, of course, but it is true to say that an MP3 encoder exploits well-researched weaknesses in our hearing to reduce data. Given that we have just established that we do not notice clipping on narrow pulses, I wonder if the people at the Fraunhofer Institute knew this?

I made a difference track from the original unaltered .WAV, and a 256 kb/s MP3 version of the same. And then repeated the test with a 128 kb/s version. And guess what? The transients feature heavily, especially at the lower bit rate. And that's because once the coder has decided to curtail the peaks, it can allocate more of its resources to dealing with the lower-level detail that makes up the bulk of the signal.

Although we're not discussing MP3 specifically in this thread, you might find it interesting to listen to the difference tracks. A lot of nonsense it talked about data-rate reduction, but very few people take the time to create difference tracks to separate out what is really happening from what they imagine might be. Have a listen to the "ghostly", scratchy vocals, and the "phasey" treble. But, most importantly, note how quiet the difference track is - especially the first one (256 kb/s). MP3 is not as bad as some people believe!

The first 20 seconds of the following audio comparison (between the source file, not played, same as above) and the outcome of MP3 encoding/data reduction is 20 seconds at 256 kbps, a pause then 20 seconds at half the data rate (more audio discarded) at 128 kbps.


Because the MP3 conversion - even at 256 kb/s - has a small impact on the transients, you might wish to repeat the earlier tests "locally", if you have a copy of the original Brothers In Arms CD. I'm using Exact Audio Copy to extract the track, and Audacity to apply the effects. Both of these are free downloads. The "Hard limiter" option is under the "Effects" menu.

Does this make sense?

Mark
 

A.S.

Administrator
Staff member
A designer's sonic frame of reference defines his sound

A designer's sonic frame of reference defines his sound

Of the two clips, the first one - the one that was limited to -12dBFS - is fairly subtle, and I was keen to hear what people thought about it. It certainly doesn't sound "wrong", and in isolation, you'd be forgiven for assuming that all was well, but of course, with the original for comparison, one can hear a slight difference.

No-one identified it as clipped or distorted, and had you not seen the screen-shot of the waveforms, I'd be willing to bet that no-one (myself included) would have ever reached that conclusion. This demonstrates that quite severe clipping can go unidentified: while many of us did hear differences, we would never have guessed that clipping was the cause. In the hi-fi scene, where people make great claims about "tweaks" that do absolutely nothing, you could well imagine how a group of listeners could jump on this real change, and declare that Amplifier A - with more power than Amplifier B - is the best amplifier by quite some margin. Sadly, these same people would not allow a simple oscilloscope to enter the listening room, preferring to trust their hearing more than a scientific instrument....
One thing that again comes across when we can hear the consequence of clipping (or in the case of MP3, data reduction) is that it is the impulsive, fast accelerating, high frequency information that suffers. It seems to be a truism that music that has been technically amplitude limited (in the mastering process) sounds warmer and certainly less bright.

As Eric has noted, an equipment designer using amplitude limited music as his quality reference, would hear a sonic balance subjectively tipped away from the high frequencies and towards the middle and lower tones: a 'warming' of sound. It is well known that an absence of energy at one end of the spectrum subjectively skews the listener's perception to the other: excessive treble sound like too little bass: excessive bass sound like too little treble. If, due to signal compression, a recording sound like it is bass heavy, should an equipment designer use this as his reference, he could be motivated - understandably - to add some HF energy boost in his equipment to rebalance the sound.

f a speaker - or microphone - designer lived and worked on a sonic diet of modern, always compressed recordings, his sonic frame of reference would be entirely different to another who did not own a single recording made in the past 20 years or so, who listened primarily to naturally-recorded classical music, and who as need be, made speech recordings using the very best, flattest microphones into recording equipment at a level well below clipping or limiting.

My situation is that I am not attracted to music made since the mid 80s, not long after the CD was introduced, and a decade before the Loudness Wars started. I do not have one pop CD made after about 1990, and unlike some speaker brands who proudly promote their embedded association with the pop music and film industry, I now appreciate that their designer's frame of reference for natural sound is entirely different to mine.

The consequence for you, the home listener is, dramatic and explains why 'modern' microphones and 'modern' speakers that are developed using junk, compressed, synthetic recordings as a reference may well sound extremely bright and fatiguing or 'real' music recorded naturally. In other words, we have a generation of mics and speakers (and even amps and cables?) that have been tweaked to brighten-up modern recordings to give an illusion of a correct LF/MF/HF balance.

It's very worrying indeed.
 

Pharos

Member
CD archive

CD archive

It had never occured to me that the recording quality of CDs since the beginning of the nineties could be a major reason why I found later music unappealing; I had attributed this response to the lack of artistic merit.

So when Alan says "I am not attracted to music made since the mid 80s" and, "I do not have one pop CD made after about 1990", I really have to rethink my responses.
 

Pluto

New member
The designers feedback loop?

The designers feedback loop?

The consequence for you, the home listener is, dramatic and explains why 'modern' microphones and 'modern' speakers that are developed using junk, compressed, synthetic recordings as a reference may well sound extremely bright and fatiguing or 'real' music recorded naturally. In other words, we have a generation of mics and speakers (and even amps and cables?) that have been tweaked to brighten-up modern recordings to give an illusion of a correct LF/MF/HF balance.

It's very worrying indeed.
This would indeed explain the prevalence of a consistently over-bright sound when one does a quick round of various audio shows.

However, assuming the speaker designer is working though a cycle of listen --> formal test --> adjust --> listen, would the testing not reveal that the prototype under development is becoming rather HF heavy and thereby encourage the designer to take a moment to stop and think?
 

A.S.

Administrator
Staff member
Equipment design for the masses

Equipment design for the masses

This would indeed explain the prevalence of a consistently over-bright sound when one does a quick round of various audio shows.

However, assuming the speaker designer is working though a cycle of listen --> formal test --> adjust --> listen, would the testing not reveal that the prototype under development is becoming rather HF heavy and thereby encourage the designer to take a moment to stop and think?
I'm not sure how 'formalised' the speakers designer's approach is. I would imagine that, in many examples, the designer is given by his marketing masters a pile of CDs (or equivalent) which represent the current consumer's tastes (from a study of sales statistics) and told to make the speakers sound good on those. That would seem an entirely sensible approach in tailoring the sound of the speaker to the lifestyle of the would-be consumer and to creating a winning sound under demo conditions with that same compressed music. Why would any other strategy be applied? If market research revealed that a 35 year old prospective buyer never listened to classical music, never listened to folk music, never listened to opera and that any music published before, say, 1990, was invisible and certainly not part of the listener's library, why on earth would the speaker designer waste time and effort playing his favourites? They simply wouldn't be relevant to the voicing and hence marketability of the speaker under design.

One issue for me is that, aside from some contemporary digital recordings of 'classical' music, a handful of analogue recordings (such as the Opus 3 recordings which are astonishingly natural) and some old favourites from the 1970s (poor technically, but played to see if any more detail can be extracted) the music industry might as well have ceased twenty years ago. Nothing whatever in current circulation is of the slightest interest technically or musically. It's all far too intense, too narrow a dynamic and too highly polished. The ultimate - as in the peak of the art form technically and musically - for me was the original 1985 Michael Jackson Bad which has been the backbone of my listening/adjusting/listening approach ever since.

So one of the many questions that the media do not ask of equipment designers is 'exactly what and whom do you listen to when designing your audio equipment?'. We know what modern reviewers use, because the often tell us. Not only do I not recognise the artists' names, I know I wouldn't like the sound and the music would leave me cold.
 

joetjie200

New member
Original rock classics - where to buy?

Original rock classics - where to buy?

First and foremost - thank you Alan and Mark for your ever-enlightening posts on this thread. Though I must admit I don't follow every technical detail - I have once again learnt a lot from this forum.

As a 27 year old Harbeth 30.1 owner and avid reader of this forum - I find myself in quite a pickle when considering the music I play on my Harbeths.

Michael Jackson's Bad album was released one year before I was even born! Though I would love to listen to these acclaimed records on my new system - I can't seem to find any of these recordings that aren't digitally remastered... locally (South Africa) or on popular online stores that deliver to South Africa.

Apart from the tiny collection of over priced albums on HDTRACKS is there anywhere where I could buy some of these original albums?

I absolutely love my Harbeths and would love to get my hands on some older, original albums to listen to.

Regards

Johan
 

Regalins

New member
Better Harbeth design, better balance, better sound?

Better Harbeth design, better balance, better sound?

This is a most interesting discussion. There is no doubt that heavy limiting and compression is a defining aspect of many (most?) modern recordings. But the musical world didn't come to an end in 1985! There are still great records being made every day, all over the globe. They might not be on the radio, but they are out there.

Alan writes:
I would imagine that, in many examples, the designer is given by his marketing masters a pile of CDs (or equivalent) which represent the current consumer's tastes (from a study of sales statistics) and told to make the speakers sound good on those.
This might well be true. But here is a paradox: Why do my Harbeth speakers make all these modern, heavily limited records sound so much better than on any other speakers? The Harbeths don't sound dull on limited/compressed material, and they certainly don't sound as piercing in the high mid/treble as my previous speakers (from one of the top UK manufacturers of professional speakers).

If the "voiced for modern recordings" approach holds any merit, wouldn't these speakers make modern records sound better/more balanced than the Harbeths? Or is it just my brain playing tricks on me?
 
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EricW

Active member
Your frame of reference

Your frame of reference

This might well be true. But here is a paradox: Why do my Harbeth speakers make all these modern, heavily limited records sound so much better than on any other speakers? The Harbeths don't sound dull on limited/compressed material, and they certainly don't sound as piercing in the high mid/treble as my previous speakers (from one of the top UK manufacturers of professional speakers).

If the "voiced for modern recordings" approach holds any merit, wouldn't these speakers make modern records sound better/more balanced than the Harbeths? Or is it just my brain playing tricks on me?
I don't think it's necessarily a paradox. If we accept the premise that many (modern) speaker designs are balanced to be toppy - and there's certainly loads of anecdotal evidence (at least) for that premise - and we hypothesize that one reason, perhaps the major reason, is to optimize those speakers for the reproduction of "modern" overcompressed recordings, it doesn't necessary follow that those speakers will be in every case "better" sounding with such a recording. It depends on what one considers to be better. Harbeths will still have advantages in terms of resolving detail - whatever detail is available - that many other speakers will not have. You may consider that "Harbeths don't sound dull on limited/compressed material" - but what if that material is dull? The Harbeths will certainly reveal that very clearly. It's not the sound of the Harbeth that matters, ultimately, it's the sound of the recording.

The hypothesis is only that the frequency balance of most speakers today is designed using compressed recordings as a reference, and so will tend to be subjectively "bright" with less (or not) compressed recordings. What is "worse" or "better" is subjective in the ear of the beholder, and will depend on large part on what your reference point is.
 

tedwin

New member
Great modern music (?)

Great modern music (?)

Hi Johan & Regalins, thank you for piping up :)
Thought for a minute I'd accidentally wandered into an old people's home!

Completely agree, there is plenty of great, recent, music out there, which sounds great on 'naturally voiced' loudspeakers.

Hopefully the old duffers will get back to the amplifier chat shortly... ;)

Ted.

P.S. I'm not a Nine Inch Nails fan, but it's relevant to recent posts, http://nineinchnails.tumblr.com/post/59587808317/hesitation-marks-was-mastered-in-two-different

(For those not bothered to follow the link its about a popular band who have released their latest album with two different masters. One for volume and the masses, one so "it sounds like it did on the studio monitors")
 

EricW

Active member
The correlation between modern music and speaker design

The correlation between modern music and speaker design

Completely agree, there is plenty of great, recent, music out there, which sounds great on 'naturally voiced' loudspeakers.

Hopefully the old duffers will get back to the amplifier chat shortly... ;)
Ahem. I still listen to and purchase a fair amount of current rock, jazz and other music, and indeed some is very good (although I think the quality of "Top 40" type music has decreased by any standard, whether technical or musical). Some of it is even well-recorded.

Let's leave aside the issue of musical taste - too subjective. I think that what one can say is that up until about 15-odd years ago, quality production and sound was a mainstream value. Big commercial recordings - which is by definition what most people buy or bought - were, for the most part, engineered to sound as good as possible, not as loud as possible. Personally, I'm not a Michael Jackson fan, but I would agree that on a record like Bad there's no shortage of either engineering or musical talent. Steely Dan, Dire Straits, Pink Floyd - all those records were beautifully produced, at least in their original incarnations. There may be a few modern rock artists who understand the destructive effects of over compression and try to do something about it, but this is no longer automatic. Most are unlikely to even be aware of the issue, and of those few who are, how many will have the inclination or ability to do something about it?

The push to increasing loudness probably does coincide with the advances in digital distribution of music, the ease of copying and piracy, the decline of the importance of radio, the rise of the internet, etc. etc. One can posit a variety of factors that may have led record execs and producers to say "let's grab people's attention, first and foremost." It may ultimately have been a failure as a strategy. But for whatever reason, it happened. The increasing compression of commercially-released recordings is a fact that is easily demonstrated, and therefore beyond argument.

So the idea that follows is that the nature of the change in recorded sound may have led to a general change in how loudspeakers are voiced and balanced. I don't know if this is really true, or not true, but it seems logical and in agreement with such evidence as is available. (There is also the "showroom-attention-getting" hypothesis, but if you think about it, the two ideas do not contradict each other, and may even be complementary.)
 

Regalins

New member
Dull is dull!

Dull is dull!

You may consider that "Harbeths don't sound dull on limited/compressed material" - but what if that material is dull?
Dull is dull! I probably should have stuck with "more balanced" as a description of the sound of the Harbeths vs. the competition. But in my mind, a speaker which has a more balanced frequency response is better than a speaker with a "toppy" treble. It can't all be in the ear of the beholder - that's pure relativism.

I believe that I do understand the way in which limiting and compression tips the frequency balance away from the treble, but to which extent? Every record is different in this aspect, having been produced in its own particular manner, and I think it has less of an impact on the treble response one hears through one's own speakers than the ways in which the loudspeaker manufacturer has tailored their speakers to sound more "sparkling", "clear", "revealing" in the showroom. The treble response varies wildly, even in the most "modern" recordings, but if you have a speaker with an exaggerated treble, everything sounds too bright, even if the record itself is "dull".

There is one more thing: There is nothing keeping the mastering engineer from boosting the treble after limiting, compression etc. have been applied, which is something I believe happens often, both during production and mastering, making many "modern" records more top-heavy than the 80s records discussed earlier in this thread.
 

weaver

New member
"Micro" vs "macro" clipping?

"Micro" vs "macro" clipping?

Would it be sensible to make a distinction between what clipping does to an individual note (which we might call the micro level) as opposed to what it does to music as a whole (macro)?

The thrust of this thread has gone in the direction of "clipping affects high energy/high frequency sound which has the result that the sound we here is tipped toward the lower frequencies and is therefore subjectively warmer".

According to the source of this image (http://www.ccp14.ac.uk/ccp/web-mirrors/isotropy/~stokesh/violin.html) what it shows is the frequency spectrum of a single violin note (in this case 497 Hz which is the B above concert A - concert A being the note we hear orchestras tune to before a performance). The fundamental frequency (497 Hz) is labelled 1 with successive harmonics numbered thereafter. It is clear that the even harmonics (2,4,6...) are of greater amplitude than the odd harmonics. If this tone were to be clipped, then the fundamental would be the first to suffer with the even harmonics then being affected sooner than the odd harmonics.

Given that these harmonics are what give a violin its characteristic sound, the 'timbre', then it must follow that by reducing some harmonics relative to others then the quality, the timbre, of that note will also change. Further, it is not a case of the higher harmonics being lowered and thereby subjectively 'warming' the sound, but of some harmonics being lowered relative to others - an effect different to that which we have been shown in relation to the Dire Straits track.

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mhennessy

Member
Audibility of violin harmonics

Audibility of violin harmonics

Hi weaver,

You make a good point; it is one of the things I'm hoping to get towards - it's hopefully just a case of finding the right way to explain it, and the best source material to use for demonstrating it.

To go partway towards answering your question, the largest component (497Hz) will be clipped first, and the clipping will add energy to the odd harmonics (~1.5kHz, ~2.5kHz, etc), which should make it sound slightly more "shrill" compared to the undistorted version. Which is, as you say, quite different to the percussive effects in the Dire Straits track. But most violin players tend to vary their volume as they play, so the effect is very intermittent and can be easy to miss unless the clipping is gross. So much depends - so many variables...

I've got a few ideas of how I can demonstrate further clipping scenarios, and will have a go at putting something together soon. In the meantime, do please keep your comments coming!

All the best,

Mark
 

A.S.

Administrator
Staff member
Multiple MP3 encode, decode, re-encode

Multiple MP3 encode, decode, re-encode

...Although we're not discussing MP3 specifically in this thread, you might find it interesting to listen to the difference tracks. A lot of nonsense it talked about data-rate reduction, but very few people take the time to create difference tracks to separate out what is really happening...
I've finally found the example I made of passing a file multiple times through an MP3 encoder/decoder.

The thread is here. What we can hear is that the high frequency detail is successively reduced to a sandpaper on sandpaper mush. In other words, the precision of detail that we expect from a high fidelity recording is irretrievably lost.
 

npoguy

New member
Resurrection?

Resurrection?

I fear that this thread is suddenly on life support at a critical time! Mark/AS, what can we do to motivate you to keep the "class" going?
 
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