A Second Order Elliptic Equalizer for Vinyl Mastering

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Gold
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by Gold » Thu Jan 12, 2017 1:45 pm

mediatechnology wrote: 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/
For HD tracks it's doubtful that the CD is the same. I know I generally use the same files for HD tracks as I do for MFiT. They are 24/44.1. I don't have to mash stuff too bad for most things but if I did I'd have to lower the level about 1dB to meet the MFiT spec.

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mediatechnology
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A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Fri Jan 13, 2017 2:37 pm

I've given a lot of thought about how to build the second-order elliptic equalizer so that it is as flexible as possible.

My current line of thinking is to have the EE inserted into the existing MS board in the decode path at insert point "F."

By making it part of the MS board, as an option, the balanced MS inserts can still be used ahead of the downstream elliptic EQ.
It's also possible to use insert "C" and perform EE upstream of the MS insert sends.

Image
MS Matrix Block Diagram

The Elliptic Equalizer has three modes:

1) Symmetrical 12dB per octave vertical and crosstalk slopes with Allpass time-correction in mid.
2) Symmetrical 6 dB per octave vertical and crosstalk slopes.
3) Asymmetric with 12 dB per octave vertical and 6 dB per octave crosstalk slope.

Frequency Selection:

On-board Relay-selected frequencies of 75, 150 or 300 Hz.
External stepped switch at customizeable frequencies.

Image
Elliptic Equalizer Block Diagram

I'll post a scan of the Side and Mid filters soon.

Signal Flow:

The Side path consists of two simple cascaded first-order filter selections with relay-selected resistive tuning.

The default frequency is 75 Hz with relays used to select 150 and 300 Hz.
An external four pole stepped switch can also be used.

K1 bypasses Side.
Relay K3 selects the filter slope of 6 dB or 12 dB per octave.
K3B switches the filter outputs.
Because the individual first order sections are tuned to approximately 0.64*Fc, K3A raises the first section's Fc to 75, 150 or 300 Hz when the Side filter is 6 dB per octave.

The Mid path has two cascaded first-order allpass filters to align Mid with Side.
The allpass filter provides symmetrical vertical and crosstalk slopes.


K2 byapsses the allpass under three conditions:
(1) When the EE is bypassed,
(2) When the EE is first-order, or
(3) When the EE is being used to provide an uncorrected, asymmetric, response.

The coil of K2 will be wired so that the allpass filter cannot be engaged unless the EE is in the 12dB/octave mode and not bypassed.

By configuring the elliptic EQ this way its easy to compare first-order to second-order responses on-the-fly.

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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by Gold » Fri Jan 13, 2017 3:10 pm

Is there a reason you are putting the EE section in the M/S board and not the Widener board? I think it might be because the crosstalk slope is baked in with the widener board. The widener board should be more transparent all things being equal, right?

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mediatechnology
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Fri Jan 13, 2017 3:22 pm

Is there a reason you are putting the EE section in the M/S board and not the Widener board? I think it might be because the crosstalk slope is baked in with the widener board. The widener board should be more transparent all things being equal, right?
Thanks Paul.

There are two advantages to using the MS instead of the widener for high-order EE.

The Stereo Width Controller does not derive Mid.
To correct the time alignment of the Side low pass filter, two separate allpass networks would be required one each for left and right.
The filter switching requires six switch poles; MS requires "only" four.

The Stereo Width Controller when it is second order and not time corrected peaks in Vertical - the worst possible place.
The vertical response is also only 6 dB/octave.

The MS, when it is second order and not time corrected has a vertical response that is 12 dB/octave.
Its mono-compatible peaking (widening) is in Left and Right.

For single-order both MS and the Stereo Width Controller produce identical results.
For second-order with correction the MS and Stereo Width Controller produce identical results but the Width requires 6 switch poles versus the 4 of the MS to tune it.
All things being equal they are the same in terms of transparency.
So it's an issue of complexity.

Using the MS also preserves its inserts for additional processing.

Here's a plot I did yesterday using the MS with a 150 Hz EE.

Image

Red and burnt red are the 12 dB and 6 dB crosstalk slopes.
I didn't have the relay installed that switched the 6 dB Fc so there's a slight error.
Purple is the bypassed crosstalk.

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mediatechnology
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Fri Jan 13, 2017 5:02 pm

BTW - I got the filter Fc for single order to switch with the 6dB/12dB switch.

Now that I can switch on-the-fly the audible midrange footprint of a single order 150 Hz EE is huge compared to the 150 Hz corrected second-order.

EDIT: A very quick way to compare the difference between first and second-order responses is to drive only one input and listen to the opposing channel's crosstalk. It doesn't require any critical listening to hear the difference or require time to find a wide low end source. And it's huge...The 6 dB per octave with a 150 Hz EE point contains a lot of low-midrange. The 12 dB just bass. With Left muted a Right-only image should be out to the side at 3 o'clock. With the 6 dB 150 Hz EE it's at about 1:30 to 2 o'clock in the low midrange. With the 12 dB it's still out to the side.

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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Sun Jan 15, 2017 2:05 pm

Here is a test I did to illustrate the differences in crosstalk between a second-order and first order elliptic equalizer.

I used Depeche Mode feeding only a single channel.
The opposite channel's crosstalk was recorded.
The EE frequency is 150 Hz.
The recording began with 12 dB/octave EE and was switched at about 0:12 to 6 dB per octave and was switched back to 12 dB per octave at 0:29.

This is a mono mp3 of the crosstalk: http://www.proaudiodesignforum.com/cont ... _Audio.mp3

The waveform and spectrum:

Image
Comparison of a 12 dB per octave Elliptic Equalizer to a 6 dB per octave EE.


When monitoring using headphones in stereo with a single channel driven you can easily hear the lower midrange pulled inward when it's 6 dB/octave.
At 12 dB per octave its still completely out to the side.

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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Thu Jul 20, 2017 4:00 pm

I've finally gotten back to the Second Order Elliptic Equalizer and documented the prototype.
I haven't done the layout yet.

The design will have optional on-board Mid Side encoding/decoding which can be jumper-bypassed if the signal is already MS encoded.

Image
A Second-Order 12 dB/Octave Elliptic Equalizer with Allpass Delay Correction in Mid.

Larger image: http://www.proaudiodesignforum.com/imag ... heet_1.png

Circuit Description

IC1 and IC2 provide balanced inputs for the MS encoder formed by IC3 and IC4.
J1 and J2 bypass the MS encoder to accept external single-ended inputs which are already MS encoded.
The external MS configuration is useful when the EE is incorporated into an existing MS PC board at insert point F: viewtopic.php?f=7&t=262#p2918
Using an existing MS board provides the advantage of having the existing MS inserts ahead of the EE and one less MS encode/decode.
The EE is bypassed "hardwire" by RY1 and RY2.

Side High Pass Filter

Op amps A, B and C provide a single-order or second order high pass filter for the Side channel.
RY3 switches the filter from 12 dB/octave to 6 dB/octave.

Op amp A is a unity-gain voltage follower to buffer the filter input.
Maintaining precise unity gain within the filter passband is important to maintaining separation.
Very small gain errors cause very large changes in crosstalk above the EE frequency.

R1 provides bias for op amp A.
R2, and similar resistors R4, R6 and R14 limit inverting input current for NPN op amps such as the NE5532 or LME49720 that have back-to-back input protection diodes.
With some op amp types these may be reduced to 0Ω and with others having them in place can also provide some distortion cancellation.
With the NE5532, NJM2114 or LME40720 they are essential to prevent latch-up.

Op amp B is the first stage high pass filter for the Side channel.
For second-order EE filtering the HP network is provided by C3 and R3 plus R10 and R11.
When the EE is switched to single-order, the parallel combination of C1||C2 in series with C3 shift the cutoff frequency.

Op amp C and associated components provide a second, cascaded first order stage.

Side Filter Tuning

The filter frequencies can be tuned with either on-board relays or an external 4 pole switch.
In most cases three EE frequencies selected by relays is sufficient.
Four pole switches can be very costly: A 24 position 4 pole Goldpoint/Elma V24C, unwired, is about $191 US. http://goldpt.com/prices.html
Twelve position switches are similarly-priced. Six positions drops the cost to about $122 US.

Rather than use simpler single op amp second-order filters that would have unequal resistor values on each switch deck, a decision was made to use simpler cascaded single-order filters with all four poles of the switch having identical resistor values.

Two simple 6 dB/octave filters are cascaded to form a 12 dB/octave filter.

The EE frequency of the second-order filter is 1.56 times the cutoff of each single-order section.
Each stage of the 75 Hz filter will have an Fc that is 48 Hz. The 150 Hz Fc is 96 Hz; 300 Hz Fc is 192 Hz.

R3 and R5 limit the maximum frequency.
RY4 and RY5 switch the filter frequency by adding R10-R13 to step the frequency from 75 Hz to 150 and 300 Hz.

When stepped switches are used R10-13and RY4-7 are not installed.
Headers S1A-S1D provide a switch tie point.
DC bias current must be maintained through the off-board resistor string.
The switch common should ground the unused resistors to change tuning.

When the Side filter is set to 6 dB/octave the parallel combination of C1 and C2 are switched in by RY3.
When C1 and C2 in parallel are in series with C3, the effective capacitance is 0.14 uF.
The shifts the cutoff frequency to the actual 75, 150 or 300 Hz.

Mid Allpass Correction Filter

An allpass filter is used to match the phase response of the Side filter to achieve true second-order response of the crosstalk curve and eliminate uncorrected response peaking. (See earlier posts for various response curves.)

Op amp E is a unity gain voltage follower similar to op amp A that is used to buffer the allpass filter input.

Op amps F and G are allpass filter sections tuned identically to the second-order Side filter.
To match the phase response of the Side filter C8+C9 (or C10+C11) must be twice the value of C3 and C4.

RY6 and RY7 switch the allpass filter frequency using an identical resistive and relay network to the Side HP filter.

MS Decoder

The filtered outputs, which are in MS format, are decoded by IC8 and IC9.
MS decoded produces 2L and 2R outputs.
Op amps D and H attenuate the MS decoder to provide unity gain operation.

THAT1646 line drivers buffer the decoded L and R outputs.
Note that the 1646 output polarities are redefined to correct the inversion of op amps D and H.

When the MS decoder is not needed, J3 and J4 can be jumpered to bypass it.
When J3/J4 are used to link the filter outputs to an external MS decoder the THAT1646s must not be installed.

Bypass

RY1 bypasses the Side filter.
RY2 bypasses the Mid allpass filter.
RY3 switches the Side filter output from the second-order output to the first filter stage.

Relay Coil Wiring

Image
A Second-Order 12 dB/Octave Elliptic Equalizer with Allpass Delay Correction in Mid. Sheet 2. Relay Coil Wiring.

Larger image: http://www.proaudiodesignforum.com/imag ... heet_2.png

I've tried to design a lot of flexibility into the EE's relay switching to permit a lot of front panel options and the ability to optionally switch it to 6 dB/octave.

If the Elliptic EQ is only going to be used at 12 dB/octave RY3 does not need to be installed and can be jumpered in the normally-closed position.
C1 and C2 can also be eliminated.

J5 provides a bypass option when the frequency relays are used.

When in the "A" position, connecting any Frequency control line to relay common will automatically engage the EE filter.
This configuration permits a single pole 3 or 4 position switch to act as both frequency select and bypass.
When a three position switch is used a bypass switch for A/B comparison can be in series with the common.
A four position switch could also be used to provide Off-75-150-300 Hz positions.

When J5 is in the "B" position a separate bypass pushbutton can be used to activate the EE.
If an external rotary switch is used to set the EE frequency J5 should be in the "B" position.

The second block of switch connections, labeled "Sum" and "Difference" are optional SPDT switch connections to alter filter slopes to 6 dB/octave.
If these are not used J6 and J7 are installed.

If a stepped frequency switch is used RY4-RY7 are not installed.

Relay Power and Indicators

The EE will have LED outputs to show actuation of RY1, RY2 and RY3.
Resistors R30-R33 provide current limiting.

When the EE is configured to operate at only 12 dB/octave only the "EE In" LED is used.

If switches are installed to permit 6 dB/octave operation the Difference and Sum LED outputs may be useful.

Relay power for the EE is galvanically-isolated on the board from audio ground.
External coil power may be unregulated or regulated.
The on-board 24V regulator has a separate input connection.
Jumper J8 connects the 7824 regulator to the Relay Supply connection.
When the on-board regulator is used the relay supply connections become 24V outputs.

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mediatechnology
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Sun Jul 23, 2017 8:46 pm

I updated the previous post to add the power distribution and relay coil wiring.

A conventional EE has identical crossover slopes in both directions.
The second-order Elliptic EQ has optional switchable filter slopes of either 6 or 12 dB/octave.

I labeled the switching to show the switchable filter slopes as "Sum" and "Difference."

Here's why:

"Sum" is the slope of the filter as frequency decreases and the signal folds down from stereo to mono.
Sum is the "bass to mono" "vertical crossover" slope.

"Difference" is the slope of the filter as frequency increases and the signal expands from mono to stereo.
Difference is the "midrange to stereo" or crosstalk slope.

As shown in the drawing, the slope of "Sum" can be made switchable from 12 dB/octave to 6 dB/octave.
If the slope of "Sum" is 12 dB/octave, the Difference slope can be switched from 12 dB/octave to 6 dB/octave.

J6 and J7 bypass the slope switches for people wanting only 12 dB/octave response.

Whats not intuitive is that when the allpass filter is inserted into Mid, it's the "difference" or stereo crosstalk slope that changes.
Conversely, when a switchable 6 or 12 dB/octave highpass filter is inserted into Side, the result is that the " sum" or mono slope changes.

That's why Sum switches control Side via RY1 and RY3 and Difference controls Mid with RY2.
Confusing? I thought so.

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mediatechnology
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Tue Aug 22, 2017 11:59 am

I just ordered the first pre-production run of Second-Order Elliptic EQ boards.

Image
Second Order Elliptic Equalizer Layout

Image
Second Order Elliptic Equalizer Layout

Image
A Second-Order 12 dB/Octave Elliptic Equalizer with Allpass Delay Correction in Mid.

Larger image: http://www.proaudiodesignforum.com/imag ... heet_1.png

Image
A Second-Order 12 dB/Octave Elliptic Equalizer with Allpass Delay Correction in Mid. Sheet 2. Relay Coil Wiring.

Larger image: http://www.proaudiodesignforum.com/imag ... heet_2.png

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mediatechnology
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Re: A Second Order Elliptic Equalizer for Vinyl Mastering

Post by mediatechnology » Thu Aug 24, 2017 11:16 am

Got 4 boards in a day early.
Provided they pass test three are already sold.

Image
Second-Order Elliptic EQ PC Board

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