New project idea

[k brown]

Finally finished this up.

It uses Martin's 2nd Order ZDFs and 1 pole Shelf Filters; sounds very good.

The Shelf freqs include 30kHz and 60kHz - not sure if they're even doing anything; Martin included a warning about Nyquist limitation in the Peak filters used, but not in the Shelf filters, so -- ?

I selected Q for ea. Peak frequency based on the Cello Audio Palette Mastering EQ:

10kHz: 1.0
4kHz: 0.75
1.5kHz: 0.5

500Hz: 0.5
250Hz: 0.75
125Hz: 1.0

So, the slopes get more gentle at mid freqs.

Those Decca boys chose their limitations well.

This a a very subtle EQ for polishing a final mix; not for torturing your synth!

Thanks tons kortezzzz for the rotating labels - so cool!

You can choose black or blue!

Dekka EQ fsm.zip

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Dekka EQ_blue.jpg (15.62 KiB) Viewed 8719 times

[k brown]

This zip includes a version with three units, two of which can be bypassed.

Dekka EQ.zip

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Dekka EQ III.jpg (37.48 KiB) Viewed 8710 times

[martinvicanek]

Awesome! Perhaps add some scratches to make it look really old.
Under the hood, I would recommend the Peaking EQ and the shelving filters from here. The Peaking EQ you have picked is geared towards modulation (irrelevant here). Moreover, the new filters will handle center frequencies above Nyquist (the ones in your schematic won't).
Edit: I took the liberty to carry out these modifications. Discovered a little bug in my shelving filters along the way, so thanks!

[k brown]

Wow, Martin - thanks!

Unfortunately, the version of FS4 I have (3.0.9 Beta 3) wont open it


I should have posted the 3.0.6 schem to start with (I keep forgetting about that, since the 64 is still a moving target).

The original's been updated with knobs that function more like the hardware.

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Dekka EQ_IIe.fsm

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BTW - there's still a mystery about the Decca EQ that remains - I posted about it on Gearspace, but no has offered a guess about it. There is an additional position on the 'db' switches at full CW (past '8'), that appears to be marked 'R' - what could have been the function of that position?

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The only thing I could think of was that it stood for 'Reference', as in reference tone; but that seems unlikely, as these were passive EQs. I suppose it could have been a routing switch for an external tone generator?

[martinvicanek]

Oh, I see. Never mind, I replaced those filters in Dekka EQ IIe, too. BTW the shelf-peek sequence in the bass seems reversed, don't you think?
I have no idea what what R stands for.

[k brown]

Great - thanks so much!

Yes the control arrangements are, shall we say eccentric. Really feels foreign to turn a knob CW to cut and CCW to boost.


Here is the Triple version, with Martin's updates:

Dekka EQ_MV.zip

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[k brown]

Question for you Martin (or anyone else who might know) would there be any point in having bypass switches in the two sections; would there be any audible change in the sound if no boost or cut is applied - phase shift or anything?
Is that the point of Zero Delay Feedback, that there is no phase shift through the filter?

As awesome as your filters are, could there be any impact on the sound when no boost or cut is being done? Or would having bypass switches just be silly?

Thanks

[martinvicanek]

Bypass switches are not entirely silly. When there is no cut or boost, there is no need for processing, so bypassing the filter will save a tiny bit of CPU. On the other hand, if you pass the signal through the filter in neutral position, it won't introduce any delay or phase shift. I suppose it could add noise or distortion albeit at a very low level, something like -300 dB (I haven't measured it).

Zero delay feedback is perhaps a misleading term for those filters - it has nothing to do with the group delay. It is more a technical term for a topology (delayless loop) which has been thought of as unsolvable or unimplementable originally. Therefore, people would introduce an additional one sample delay in such loops to make them fit. However, that manipulation spoils the filter's properties at high frequencies.

It turns out that zero delay feedback can in fact be solved and implemented. What's more, these topologies result in more accurate and stable codes at little extra CPU cost if any.

[k brown]

OK, thanks so much (x100).

[k brown]

Here's another Decca-inspired EQ I made with the same filters.

This one's much more flexible and based on the currently-made "Broadhurst Gardens #3", by D.A.V. Electronics. Designed by Michael Hinton who used to be a technical engineer at Decca's Broadhurst Gardens studio, the BG3 is an updated version of a mastering EQ he designed for Decca in their later years.

I emulated a few mods that are commonly done to the BG3 -

- The number of switch positions doubled for the Gain controls for finer adjustments.
- The HPF freq can be lowered from the stock 50Hz to 25Hz (by right-clicking on the toggle sw).
- The Lo Peak freq range can be raised to 60Hz - 2kHz (by right-clicking on the knob's "Hz" label).

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BeeGee 3.fsm

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