A Second Order Elliptic Equalizer for Vinyl Mastering

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mediatechnology
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Thu Dec 29, 2016 4:13 pm

If its hard clipped there is no improvement.
If it's brick-walled without clipping it may or may not be a little more open - depends a lot on the cut.
If there's audible grunge in the original its garbage in garbage out.

I think the Allpass' merit - if there is one - is a check to see how brick walled material was brick-walled to begin with.
If there's a lot of phase rotation in the original the crest factor increases significantly.
If it was brickwalled by limiting, soft clipping or multiband it doesn't seem to unlimit as much.
So it allows you to identify how it got squashed.

I just built a 200 Hz 4 pole allpass and was able to reproduce the results so its not an EE thing.
If processing through the allpass increases crest factor it will also do it in a broadcast air chain.
It might be worthwhile for an ME to examine material that comes in this way and correct it because it may decrease loudness once aired.

The EE with allpass seems to sound better coming out than going in only by virtue of bass-to-mono IMHO.
For cleaner material there may be some opening up but what I mostly hear is the improved low end focus.

For stuff that is clipped in Sides in the low frequencies, the EE does provide some cleanup because it steers side "mud" into mid where it gets masked.

Unfortunately the allpass phase unwinder does not make a silk purse out of a sows ear.
But it may be useful to spot problems similar to soloing mono, side etc.
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Thu Dec 29, 2016 5:07 pm

The effects of broadcast phase rotation on brickwall-limited source material.

Original is the first waveform, the second after 4 poles of 200 Hz allpass filtering.

Image
Daft Punk again. First waveform is brickwalled original without allpass filtering. Second waveform with 200 Hz 4 pole allpass.

Despite the hard limiting of the original, a broadcast processor's internal AGC, multiband compression and limiting would be processing the second envelope owing to its internal allpass phase rotators.
My hunch is that the brickwall-limiting in the original was also allpass-based due to its symmetry about baseline and the large change in crest factor when double-processed.
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Fri Jan 06, 2017 3:59 pm

After a lot of poking around and experimenting I think I've gotten a better handle on what goes on with brick-walled material.

Phase rotation in the original source is not the cause. Hyper-compression is.

It appears that hyper-compression induces large amounts of odd-order distortion that, when allpassed (or simply undergoes phase rotation) causes peaks to grow in level.
I confirmed this with a track I hyper-compressed myself and then analyzed using a tool called Mastering Visualizer, "MasVis." http://www.lts.a.se/lts/masvis

MasVis has a display that they call "allpassed crest factor" which they claim is an indirect measure of "crush."
The dashed line is the crest factor of the original file, the second curve the crest factor vs. frequency after allpass.

The following MasVis animation shows THAT's Test Track 37, "Funky Bass" unprocessed, limited 20:1 @-9dB and normalized, and then with two degrees of hyper-compression:

Image
Animation showing THAT's Test Track 37, "Funky Bass" unprocessed, limited 20:1 @-9dB and normalized, and then with two degrees of hyper-compression.

As clipping increases note the spread between the file's crest factor and the allpassed crest factor.

MasVis write:

Allpassed crest factor

This graph probably is the most revealing graph in MasVis. It gives an estimate of how much the crest factor has been lowered by processing of the 2-channel master. In the graph, there are four curves, two red for the right channel, and two blue for the left channel. The dashed line shows the crest factor, ie the same information as above the waveform graphs. The solid line shows the crest factor of the signal after it has passed an allpass filter. This second crest factor is a rough, and typically underestimated value of how large the crest factor was prior to level maximation of the 2-channel master.

At first, it may seem impossible to make such an estimate, but experience shows that the method works. Of course it does not give an exact value of the original crest factor, but it is good enough to tell when the track has been subjected to destructive loudness maximation.

The difference between maximum in the solid line and the dashed line correlates quite well with the amount of crest factor loss that is a result of level maximation of the 2-channel mix.

Here is the method that MasVis uses to make the graph:

1. Draw a dashed line at the crest factor of the track.
2. Run the track through seven different first order allpass filters at 20, 60, 200, 600, 2000, 6000 and 20000 Hz
3. For each of the seven filtered signals, calculate a new crest factor value
4. Plot a solid line representing the crest factors of the seven signals as a function of filter frequency


Apparently, when the odd harmonic artifacts of hyper-compression are phase rotated (or undergo any significant phase shift) the peaks are no longer folded inward but instead fold outward causing an increase in peak level but not an increase in RMS.

I also ran some tests to see what the effects of RIAA pre-emphasis are on brickwalled material.

The peak level increases with brickwalled material are huge when inverse RIAA is applied.

And, as it turns out, MasVis' "allpassed crest factor" measurement correlates well with peak level growth with varying degrees of hyper-compression after inverse RIAA.

I ran the THAT 37 test suite and, sure enough, the original and lightly-limited didn't have the peak growth. The ones with hyper-compression do.
The amount of gain that had to be reduced prior to inverse RIAA correlated closely to MasVis allpassed crest factor.

The allpassed crest factor portion of the graph is small and requires some interpretation due to lack of a scale.
MasVis doesn't have inverse RIAA weighting so some visual fudging is required: The higher the center frequency of the allpassed crest factor the more it underestimated due to HF boost.
But its close.

First I normalized the four tracks downward by 0.25 dB to remove the 0 dBFS clips.
I applied inv RIAA EQ to each track and normalized it downward until I got just under 0 dBFS peaks.

(The unprocessed and limited file did not require normalization.)

The file with "+6 dB slam" required input normalization to -2 to prevent post-EQ clipping.
The +12 dB slam file require normalization to -8 to prevent post-EQ clipping.
For an equal post RIAA EQ peak level the lightly-limited file could be recorded 8 dB hotter than the highly-slammed one.

I can establish that inverse-RIAA'd brickwalled material will overshoot more than identical un-brickwalled material.
Put another way hyper-compressed files require large decreases in record level to maintain the same post-EQ peak level.
This is probably not news to any ME but I needed proof.

What are the consequences of increased peak level from the second-order EE?
Apparently very little after RIAA preemphasis.


Material which has not been brickwalled does not show significant peak level increases.
Files which have been brickwalled do.
.

The first example are the files with no EE, a second order 300 Hz EE with time delay correction, and a first-order EE.
The expansion in peaks in the second-order example are significant.

Image
Daft Punk Harder Faster Stronger No EE, 300 Hz Second Order, 300 Hz First Order Comparison

When the same three segments are inverse RIAA equalized the differences almost disappear:

Image
Daft Punk Harder Faster Stronger No EE, 300 Hz Second Order, 300 Hz First Order Comparison with Inverse RIAA applied.

The peak levels are:

-1.58 dB in the original
-0.79 dB in the 300 Hz Second-Order EE
-1.62 dB in the 300 Hz First-Order EE

About 0.8 dB more in the second-order EE.
The final difference may be due to the effect of EE which will steer LF Side to Mid.

Even though the second segment in the original looks hotter - and in terms of peak level it is - it is not significantly hotter after Inverse RIAA EQ.

The second-order EE "unwraps" the embedded peak overshoots but the RIAA pre-emphasized output does not have significantly increased peak level.

When inverse RIAA is applied to the output of the second-order EE, whose phase shift is at low frequencies, the peak increase isn't pronounced because LF are highly attenuated on the preemphasis curve.

What I think I might do is provide a means to bypass the "time delay correction" in Mid.
The user could ultimately have it three ways:

1) Single-order
2) Second-order with Mid delay correction (symmetric response)
3) Second-order without correction (asymmetric response)
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by ricardo » Fri Jan 06, 2017 8:55 pm

I think ANY non trivial EQ will 'unwrap' the 'hidden peaks' in a severely compressed/clipped signal.

The limiting case is MLS as used to test speakers. A long pseudo random sequence .. which is a 2 level digital signal is LP filtered and the result is near Gaussian Random White Noise. Loadsa funky maths about the amount of filtering and the resulting randomness .. but you don't have to do much to get very high Peak/Mean ratios as you would with 'real' GRWN.

I've had to process stuff that was recorded on a smart phone. The original looks like 21st Century 'pop' with a dynamic range of 3dB or less .. being both clipped & compressed by the phones AGC

Just correcting some timbre makes the signal look more like 'real' music with peaks.

Gotta admit this is purely cosmetic as I object to issueing anything that even 'looks' like 21st century 'pop'. Any audible improvement is probably due to my response tweaking. Of course, your all-pass doesn't introduce any frequency response tweaking! :mrgreen:

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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Sat Jan 07, 2017 7:46 am

I can't really say that there's any audible improvement from "unwrapping" them.
The clips are just hidden in the waveform.
They look better but that's it.
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Effects of Inverse RIAA on Hyper-Compressed Mate

Post by mediatechnology » Sat Jan 07, 2017 8:52 pm

I wanted to provide another set of visuals to illustrate the self-defeating nature of hyper-compression when the crushed version of the master is used for vinyl.

This test involves applying inverse RIAA EQ and comparing the output RMS and peak levels with varying degrees of processing.

This is a screen capture of THAT Track 37 "Funky Bass" with no processing, limiting at 20:1, 6dB of overdrive (slam) and finally 12 dB of overdrive.
Each of the original files was separately normalized to 0 dB to make them have equal peak level.
The assembled file is normalized to -10 to prevent clipping when Inverse RIAA EQ'd.

Without Inverse RIAA EQ

Image

After Inverse RIAA EQ:

Image

Statistics for the Inverse RIAA Eq'd Files:

Track 37 No Processing:

Peak Amplitude: -12 dB -12.01 dB
Total RMS Power: -32.78 dB -32.78 dB

Track 37 Limiting (20:1 @-9dBFS in original):

Peak Amplitude: -7.55 dB -7.54 dB
Total RMS Power: -28.93 dB -28.93 dB

Track 37 with +6 dB Squash:

Peak Amplitude: -6.01 dB -6.01 dB
Total RMS Power: -26.94 dB -26.94 dB

Track 37 with +12 dB Squash:

Peak Amplitude: 0 dB 0 dB
Total RMS Power: -21.92 dB -21.92 dB

Conclusions:

The first set of files have identical peak level but the RMS values are progressively increased due to limiting.
The second set of files show the RMS progressively increase as a result of compression from -33 dB to -22 dB and the peak levels also increase from -12dB to 0 dB.

The compressed peaks "come back" after Inverse RIAA EQ.

The low crest factor/high RMS waveform in the fourth cut would have to have its overall level - and its RMS values - reduced by 12 dB to produce the same peak level as the first cut.
Putting it another way the first unprocessed cut could be cut 12 dB hotter than the last one - and have 12 dB higher RMS - to produce the same peak modulation.

Compression and limiting is a good thing.
In a preemphasized environment hyper-compression is simply self-defeating.
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by ricardo » Sun Jan 08, 2017 10:18 pm

mediatechnology wrote:I wanted to provide another set of visuals to illustrate the self-defeating nature of hyper-compression when the crushed version of the master is used for vinyl.

Compression and limiting is a good thing.
In a preemphasized environment hyper-compression is simply self-defeating.
But but BUT ... 3dB dynamic range always sounds better (ie louder) on my iThing :o

Not sure about it being a good thing .. but it's hard to record stuff like rim shots or clap sticks without compression/limiting without an average level your customer will object to. :mrgreen:

Who listens to vinyl anyway? Sorry Paul. I'm just jealous cos my vinyl collection is on the other side of the continent and probably will never see the light of day :cry:

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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by emrr » Thu Jan 12, 2017 9:48 am

Eye opening results.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Thu Jan 12, 2017 12:13 pm

It was surprising.

When you normalize them back the squashed one loses about 1.1 dB RMS.
Not only does it require lower level it's distorted as heck.
I tried some stuff that had higher frequency content and there was some with 2-3 dB RMS gain from being squashed but it wasn't enough to justify squashing it to death.

The vinyl always seems to have higher DR: http://dr.loudness-war.info/

Comparing HDTracks to some of my CD rips of the same release it appears that they are able to get better masters.
http://www.hdtracks.com/
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by Gold » Thu Jan 12, 2017 1:31 pm

ricardo wrote:
Who listens to vinyl anyway? Sorry Paul. I'm just jealous cos my vinyl collection is on the other side of the continent and probably will never see the light of day :cry:
I enjoy making the donuts and fixing the donut maker. I love music but I rarely listen to it for pleasure anymore. One great thing about being in the woods for 10 days without music was that I actually started to miss it. Luckily I've culled the client list so I rarely work on stuff I don't like.

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