Re: Using the Precision MS Matrix for Mono Crossover LF Blen
Posted: Mon Mar 10, 2014 4:43 pm
I've had a chance to revisit the Neumann VAB-84's technique to transform LF Side information to mono to provide an improved Elliptical Equalizer.
This is the circuit built using the Precision MS Matrix. The board really is a Swiss Army Knife of Mid/Side Processing.
In this implementation the Mid channel isn't even used.
Emulating Neumann VAB-84 Side to Mono Transformation Using the Precision MS Matrix
Here's a quick breakdown of the signal flow.
Note that the points shown on the board correspond to these drawings:
http://www.ka-electronics.com/Images/jp ... iagram.jpg
http://www.ka-electronics.com/Images/jp ... _Large.JPG
Left and Right enter the Decoder inputs as balanced signals at points EM and ES.
The Right output appears on FS Out and is carried into the FS Input.
The Left output appears on FM Out and is carried into the FM Input.
The FM and FS inputs are differenced by U11 (internal to the board) to provide a Side Output.
Side is at the GR Output.
Both Left and Right feed the summing nodes at GL In and GR In through 10K resistors.
(Internal resistors R4 and R5 on the MS PCB are made 0 Ohm.)
The "Encode" section of the matrix board provides balanced I/O for the external EQ and to provide polarity inversion.
The Side input at CS In is single-ended and converted to balanced to drive the DS Output.
The balanced output from the EQ is received at Input AR.
The filtered Side output is at BR Out.
BR Out feeds a summing node for the right channel through a 20K resistor.
The BR Output also feeds BR In to provide polarity inversion.
The CS Output is polarity inverted with respect to BR In.
The CS Out is summed into the Left channel summing node through a 20K resistor.
The overall Left and Right Outputs appear at HM and HS.
The Mid channels are not processed: IC1, IC3, IC5 and IC10 are not used on the PC board.
I've been playing around with various filter 3 dB points and have found that an aggressive 300Hz (-3dB) 12dB/octave response isn't that audible in terms of mid-band separation. I also played around with a 600 Hz filter and the effect on imaging was minimal. Neither the EE70, EE77 or VAB84 are capable of a LF crossover greater than 6 dB per octave. Having a steeper filter option allows a larger chunk of LF to be mono'd (when required) without as great an effect on imaging.
Steering Math
Let's take a look at the math the VAB-84 uses to convert Side to Mono.
Nuemann VAB84 Block Diagram Showing Low Pass-Filtered "Side" Converted to "Mid" By Side to Mono Conversion.
Lout = L -1/2(L-R)m
Rout = R -1/2(R-L)m
Where "m" is the filter transfer function.
Assume for sake of discussion that the filter is infinitely steep.
If the Side information is within the bandwidth of the filter then m = 1.
If the Side information is outside the filter pass band then m = 0.
Outside the pass band of the filter (where m = 0) L=L and R=R because the difference terms disappear.
Inside the low pass filter pass band (m=1) the following occurs:
L = L -(L/2) + (R/2) = L/2 + R/2
R = R -(R/2) + (L/2) = R/2 + L/2
Thus, below the LP filter cutoff, L and R are summed.
I hope to provide some sound files to provide comparisons.
I'm not having a hard time finding things mastered with a wide low end.
The next step is to build a conventional EE70/EE77 subtractive EE and provide comparisons to no EE and the VAB84 additive type.
What I've found is that aggressive amounts of EE (a high crossover) using the VAB-84 method isn't that audible when the Side info is folded into mono.
Some of this may be due to the fact that no information is actually lost, the other may be due to a higher filter order.
Another interesting attribute is that as tones are swept through the passband and filter skirt of the VAB84 EE the mono sum remains constant.
There is no perceptual reduction in low end as the VAB84 EE is switched in or out or if the result is mono'd.
I should also point out that if one wanted to get really creative, a bandpass filter could be used to reduce separation of a particular frequency range for more difficult repairs and/or cleanup.
This is the circuit built using the Precision MS Matrix. The board really is a Swiss Army Knife of Mid/Side Processing.
In this implementation the Mid channel isn't even used.
Emulating Neumann VAB-84 Side to Mono Transformation Using the Precision MS Matrix
Here's a quick breakdown of the signal flow.
Note that the points shown on the board correspond to these drawings:
http://www.ka-electronics.com/Images/jp ... iagram.jpg
http://www.ka-electronics.com/Images/jp ... _Large.JPG
Left and Right enter the Decoder inputs as balanced signals at points EM and ES.
The Right output appears on FS Out and is carried into the FS Input.
The Left output appears on FM Out and is carried into the FM Input.
The FM and FS inputs are differenced by U11 (internal to the board) to provide a Side Output.
Side is at the GR Output.
Both Left and Right feed the summing nodes at GL In and GR In through 10K resistors.
(Internal resistors R4 and R5 on the MS PCB are made 0 Ohm.)
The "Encode" section of the matrix board provides balanced I/O for the external EQ and to provide polarity inversion.
The Side input at CS In is single-ended and converted to balanced to drive the DS Output.
The balanced output from the EQ is received at Input AR.
The filtered Side output is at BR Out.
BR Out feeds a summing node for the right channel through a 20K resistor.
The BR Output also feeds BR In to provide polarity inversion.
The CS Output is polarity inverted with respect to BR In.
The CS Out is summed into the Left channel summing node through a 20K resistor.
The overall Left and Right Outputs appear at HM and HS.
The Mid channels are not processed: IC1, IC3, IC5 and IC10 are not used on the PC board.
I've been playing around with various filter 3 dB points and have found that an aggressive 300Hz (-3dB) 12dB/octave response isn't that audible in terms of mid-band separation. I also played around with a 600 Hz filter and the effect on imaging was minimal. Neither the EE70, EE77 or VAB84 are capable of a LF crossover greater than 6 dB per octave. Having a steeper filter option allows a larger chunk of LF to be mono'd (when required) without as great an effect on imaging.
Steering Math
Let's take a look at the math the VAB-84 uses to convert Side to Mono.
Nuemann VAB84 Block Diagram Showing Low Pass-Filtered "Side" Converted to "Mid" By Side to Mono Conversion.
Lout = L -1/2(L-R)m
Rout = R -1/2(R-L)m
Where "m" is the filter transfer function.
Assume for sake of discussion that the filter is infinitely steep.
If the Side information is within the bandwidth of the filter then m = 1.
If the Side information is outside the filter pass band then m = 0.
Outside the pass band of the filter (where m = 0) L=L and R=R because the difference terms disappear.
Inside the low pass filter pass band (m=1) the following occurs:
L = L -(L/2) + (R/2) = L/2 + R/2
R = R -(R/2) + (L/2) = R/2 + L/2
Thus, below the LP filter cutoff, L and R are summed.
I hope to provide some sound files to provide comparisons.
I'm not having a hard time finding things mastered with a wide low end.
The next step is to build a conventional EE70/EE77 subtractive EE and provide comparisons to no EE and the VAB84 additive type.
What I've found is that aggressive amounts of EE (a high crossover) using the VAB-84 method isn't that audible when the Side info is folded into mono.
Some of this may be due to the fact that no information is actually lost, the other may be due to a higher filter order.
Another interesting attribute is that as tones are swept through the passband and filter skirt of the VAB84 EE the mono sum remains constant.
There is no perceptual reduction in low end as the VAB84 EE is switched in or out or if the result is mono'd.
I should also point out that if one wanted to get really creative, a bandpass filter could be used to reduce separation of a particular frequency range for more difficult repairs and/or cleanup.