In-ear finalizer/mastering device: anybody interested???

Hi all,


I am working as a live sound engineer and want to build an analog finalizer which will be used on in-ear mixes, but might also serve as a mastering device.

Main features of the design should be:

-A 5-band parametric EQ with band 1 and 5 being switchable bell/shelving
-A multiband compressor, preferably with user-adjustable cross-over frequency (continuous variable or preset)
-A Width control, to alter the width of the stereo image.


Since this design will be stereo, the most important issue is to have the left and right side of the EQ and compressor being as
equal as possible.

To achieve this, it will probably be needed to use rotary switches instead of potmeters.

The range of the EQ can be as low as +/-6 dB (in half dB steps).

The ratio of the comp should be adjustable in between 1.5:1 and 4:1



Questions:

Which type of EQ is best chosen for this purpose? I like a lot the Midas XL4 state variable, which is very similar to the amek M2500
and fairly easy to build. Also the ability to easily disable one of the filter sections is an advantage.

Will clicks be audible when the pots in state variable designs are replaced by rotaries?


Compressor: What cross-over to use? Are there simple designs with variable XO frequency, and more important: after the summing
of the three bands will linear phase be retained? The standard XO is Linkwitz-Riley, but I haven't seen a lot of LR designs with variable
XO frequency.


The comp itself should be transparant. What gain reduction topology recommended, FET, VCA or opto/EL? I guess VCA will allow for best tracking.


The Width control circuit: I have no info at all about this. Any example schematics are very welcome.


If other people are interested in helping out, this might become a nice Lab project.

Since there has been a lot of comment about just copying existing stuff, I would like to customize the EQ and comp rather than just copy and paste something existing.

Comments please...


Greetz,


Rogy

What cross-over to use?

Click to expand...

For a multiband comp you want a very gentle slope to avoid heavy freq changes during processing. So I would use a 1. order high pass filter, say @ 4 kHz, for the top band. To get the mid band, sum the inverted top band with the unsplitted original signal and add a 1. order high pass @ 800 Hz. To get the low band, add the inverted top and mid band. This way, you'll get linear phase as long as all bands have the same gain reduction and very mild phase distortion otherwise (linear phase in this situation is not possible with analog technology).

For best results, you add an auto-release circuit for each band and some type of interaction control for the three bands.

What gain reduction topology recommended, FET, VCA or opto/EL?

Click to expand...

VCA, IMO.

The Width control circuit: I have no info at all about this.

Click to expand...

Use a M/S-matrix, a +-10 dB trim for the S-channel and again a M/S-matrix. You might want to include a high-pass filter for the S-channel to avoid a wobbling low-freq stereo image (this is called an "elliptic eq").

Samuel

Rogy for the XO have a look at the Rane site, there has to be something there. For a transparent compressor, you must be looking at something Aphex - look at my Aphex references here:

http://www.groupdiy.com/index.php?topic=2501&highlight=

Stephen

The trouble with the finalizer ... L1, L2 type of approach is that these units are so powerful at altering the mix that you may find that you don't know what the talent is hearing.

Of course if you have an IE set up yourself to monitor with then all can be very good.
Added to this is those that mix IE with powerful stage monitors may have feed back trouble with such sompression devices.

The basic question needed to be asked is to do with threshold. Does this conpression kick in only for loud unexpected peaks OR is it expected to be IN comp mode most of the time ... increasing RMS level ... as with a Maximiser.


I'm always interested.

Does this conpression kick in only for loud unexpected peaks OR is it expected to be IN comp mode most of the time

Click to expand...

With a multiband design, it's very hard to get absolute peak level control. All broadcast processors (and "finalizers") I know use a full band limiter after the band processing. I think we should add this. A lookahead-design would be nice...

Samuel

[quote author="Samuel Groner"]A lookahead-design would be nice...
[/quote]

Yes, but that would involve signal delay. I don't think you'd want that at live performances!

Jakob E.

Yes, but that would involve signal delay. I don't think you'd want that at live performances!

Click to expand...

True, but all digital processor (including those designed for IE monitoring) will have far more delay than we get with our bessel filters or phase shift networks due to the linear phase filters in the AD-DA converters - so I don't expect this to be a problem. Anyway, we make it switchable!

Samuel

Here is a selection of Crossovers on the Rane Site:
http://www.rane.com/procat.html#cross
Stephen

[quote author="Samuel Groner"].. True, but all digital processor (including .... - so I don't expect this to be a problem. [/quote]

yes and I do think it is a problem
I've tried a few digital units to carry the primary signal and not just as an effect return and there is a proble with latency.

The same musicians during recording have felt uncomfortable BUT have survived recording and over-dubs yet when we take the same equipment LIVE just can't settle for it.
I'm all analog for live performaces.
Remember this is primary signal and not effect return.

The feed back arrestors are unusable for this reason. I should say that I haven't had the oppotunity to use the top flight units.

I've tried a few digital units to carry the primary signal and not just as an effect return and there is a proble with latency.

Click to expand...

I did not wanted to say that the delay of digital gear is a non-issue. I wanted to say that 100 us delay is very likely to be neglegible, but still helpful for a fast limiter.

Samuel

Thanks all and especially Samuel for the linear phase crossover approach.

I just ran some Pspice sims and instead of using twice a HPF as described by Samuel the same "optimal phase result" can be achieved using a HPF for the high band, a LPF for the low band and one summing amp which mixes the outputs from high and low bands with an inverted feed from the full range input signal. Since the summing amp is inverting, the output of the midrange band has the same phase as the high and low bands.

So all what's needed to create the crossover is one C and R and a buffer for the high band, one C and R and a buffer for the low band and one inverter and a summing amp for the mid band.


That's easier than I hoped for when I started out...


I will post spice results later.


Greetz,

Rogy

Hi all,

I've been simulating a possible multiband comp crossover.

Results can be found here: http://users.pandora.be/Rogy/IEMFinalizer/Multiband%20comp/

The crossover points are user adjustable in between 40Hz-800Hz for the low/mid band and 2K7-20K for the mid/high band.

This two files are bode plots of the three bands and a resultant full range signal after summing the three bands.
http://users.pandora.be/Rogy/IEMFinalizer/Multiband%20comp/40Hz20KHzdB.pdf

http://users.pandora.be/Rogy/IEMFinalizer/Multiband%20comp/800Hz2K7HzdB.pdf

This two files are phase plots of the three bands and the output of the sommator. Note that the output of the sommator
is 180° out of phase, so an inverter should be added.
http://users.pandora.be/Rogy/IEMFinalizer/Multiband%20comp/40Hz20KHzPhase.pdf

http://users.pandora.be/Rogy/IEMFinalizer/Multiband%20comp/800Hz2K7HzPhase.pdf

The last two plots are a bode and a phase plot with the mid band attenuated by 2 dB. As can be seen, the deviation of linear
phase after the sommator is only 7 degrees, which is good.

http://users.pandora.be/Rogy/IEMFinalizer/Multiband%20comp/40Hz20KHzMid-2dB.pdf

http://users.pandora.be/Rogy/IEMFinalizer/Multiband%20comp/40Hz20KHzMid-2Phase.pdf

And finally the schematic link:

http://users.pandora.be/Rogy/IEMFinalizer/Multiband%20comp/Multiband%20XO%20schem.pdf

I will redraw the schematic ASAP as it was drawn in a hurry and it's very messy.

All simulations are done using orcad Unison Suite 9


Greetz,


Rogy

Great Rogy!

Fantastic efforts - just a side question..

Do you think this could be moulded into an awesome three-way loudspeaker crossover with great phase performance? With some nice high performance discrete opamps and a similar filter topology.

I'm just thinking ahead to a project many many moons away.....

Anyway back to your discussion.

Cheers Tom

Do you think this could be moulded into an awesome three-way loudspeaker crossover with great phase performance? With some nice high performance discrete opamps and a similar filter topology.

Click to expand...

You could use that as an active xover, but it would be a poor performer. For a speaker xover, we like steep curves (which means higher order filters) to get good separation between the tweeter and the woofer.

As I tried to point out in my first post, a multiband comp has entirely different requirements.

Samuel

...

Thanks again Samuel for the explaination on the auto-release topology.

I was looking up some M/S decode info for the Width control and came up with the following schematic :

http://www.paia.com/msdecwrk.htm

Could you please point out where the +/-10dB Side trim should be implemented?

So I just use two of these decoders in series; should the trim be placed in between the Right out of the first decoder and the Side input of the second decoder? How about the HPF?


Thanks,

Rogy

So I just use two of these decoders in series; should the trim be placed in between the Right out of the first decoder and the Side input of the second decoder?

Click to expand...

Yes.

How about the HPF?

Click to expand...

Goes between Right out and Side in as well. The idea of an elliptic EQ is to make the lower frequencies mono, i.e. removing them from the S-signal.

The schemo you posted has too many features - get rid of the pots. And we could replace some inverting stages with noninverting ones, which would result in lower noise. Give me a few days and I'll think it over...

Samuel

For the EQ:

Check the Barry Porter NetEQ.

http://www.wavefront.mcmail.com/neteq.pdf
http://www.wavefront.mcmail.com/readmeeq.pdf

It could be adaptet to more bands.

Samuel

Thanks for the EQ tip.

However I think I will give Steve Dove's CAPS (Constant Amplitude Phase Shift) design a go. I'm looking at it for years but never came to building it. A description of the topology can be found on Fred Forssel's website www.forsselltech.com in the excellent article "Evolution of an EQ design"
http://www.forsselltech.com/Evolution%20of%20an%20EQ%20Design2.pdf

I already ran some sims of the CAPS EQ; will post results ASAP.


BTW; Barry Porter made a PCB for the NetEQ described above. There have been copies of the PCB artwork and even gerbers on mr. Porters website, which doesn't exist anymore. Anybody interested in the PCB or gerbers, let me know and I'll send you a copy.

Greetz,

Rogy

Here's my suggestion for the width control:
[removed]

Nothing breathtaking, but exactly what we need. The resistors are of equal value. I did change the trim to a cross-ganged one to minimize amplitude changes.

Samuel