SSL Duende

DSP Plug-in Processor

Published in SOS August 2006
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Reviews : Processor

This sleek silver box promises to bring all the mixing power of a 32-channel SSL console into your computer recording setup.

Paul White

ssl 1 Header.s
Photos: Mark Ewing

Few musicians need an introduction to SSL mixing consoles, as they've been an integral part of the high-end studio scene for as long as most of us can remember. As the project studio world has expanded, SSL have moved into that market too, with products derived from their flagship consoles. We've already seen their XLogic analogue outboard gear, based on elements of their console channel strips, but the new Duende sees SSL entering the DSP-powered DAW plug-in arena alongside TC Electronic's Powercore, Universal Audio's UAD1, Waves' APA units and the recently launched Focusrite Liquid Mix.

Duende went into development in 2005, with the aim of recreating the EQ and dynamics processing sections of an SSL console as plug-ins that could be hosted on a rackmounting processor unit, hooked up to your computer via Firewire. Audio Units and VST plug-in formats are supported, and the plug-ins can also be used in Pro Tools thanks to an included version of FXpansion's VST To RTAS Adapter wrapper software. The Mac OS version of Duende is already shipping, and a Windows version is expected in the autumn. Duende makes 32 channels of EQ and dynamics available simultaneously at sample rates up to 48kHz, with half that at 88.2 and 96 kHz, and you also get an SSL Bus Compressor plug-in recreating the hardware unit that is a must-have tool for many professional mix engineers. Four SHARC-based DSPs provide the power for Duende, utilising 40-bit floating-point maths.

But wait a minute, surely any digital plug-ins purporting to be SSL channel strips must simply be emulations, because the big SSL consoles are analogue, right? The answer isn't as straightforward as it seems, because SSL also make the large-format C-series digital consoles, which run digital emulations of their analogue channel strips. However, these consoles are recognised as having a sound in their own right, and to complicate things further, Duende doesn't run exactly the same algorithms as the C-series console either, because SSL's engineers revised and tweaked the C-series algorithms to get them even closer to the sound of SSL's analogue consoles in the course of developing Duende — which presumably leaves them wondering whether they need to update the C-series console software to keep up!

And why the weird name? Apparently 'duende' is a Spanish word meaning 'spirit' and the SSL guys liked it.

Way To Go, Duende

The Duende hardware is an elegant 1U box with little more than a power switch and a pair of rear-panel Firewire (400) connectors. In most cases it can be buss-powered, though a PSU that adapts to the local voltage is included for situations where buss power is not available. However, the real face of Duende is in its plug-in windows. Rather than providing a suite of different plug-ins, Duende ships with just two — Channel Strip and Bus Compressor — though more may be added in the future.

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Within the plug-in window, parameters are adjusted by dragging and clicking the controls in the usual way and a small diagram at the bottom of the page shows the way the channel-strip elements are routed. A black box at the top of the channel strip displays the current parameter value, which will also show up above individual controls during adjustment in software version 1.1 onwards (due for release as I'm finishing this review). As you'd expect, plug-in parameters can be automated to the extent that plug-in automation is supported by the host application, and all the plug-in settings are saved in your host DAW's song file in the normal way.

A Preferences panel shows information about the Duende hardware and also displays DSP loading information so you can see how much free DSP you have available at any time. There are 32 slots available (16 at high sample rates) and each slot may be filled with either a Channel Strip or a Bus Compressor. Note, however, that a stereo version of a plug-in takes up two slots, and each half of a stereo plug-in must run on the same one of the four DSP chips, rather than being spread over two chips. If you end up with just two slots left on different DSP chips and you need to load in another stereo instance, closing and then reloading the song should remedy the problem, as the DSP resources are reallocated when a song opens.

Latency & System Load
One major concern about DSP-assisted Firewire devices is the inevitable increase in latency caused by having to send the audio out of the computer, process it and then send it back. Duende is no exception to the general rule that your host software's audio driver latency will be roughly doubled. This isn't an issue when mixing but may mean it is best to record your session, including any overdubs, before opening up any Duende (or other DSP-powered) plug-ins. As Duende's plug-ins are intended for mixing, unlike soft synths and some other plug-ins, this isn't really a problem, but it is something you need to be aware of, and if SSL could add a control to bypass all of Duende's plug-ins at once for those occasions where you need to come back and do an overdub, that would be a great help.
Most host applications automatically compensate for plug-in latency providing the plug-in correctly publishes its delay characteristics to the host software, so all your tracks retain the correct time relationships with each other. Providing PDC is switched on, the process is transparent to the user. Note, however, that not all host applications offer delay compensation for sends, group busses or outputs, and in the case of programs like Logic, you have the option to switch this on or off — so you should ensure it is switched on if you are using plug-ins in buss insert points. (This is of course true of any plug-in, but as DSP-powered plug-ins tend to double the latency set by your original buffer size selection, the result can be very noticeable if you forget!) Pro Tools LE 7.1 and earlier does not feature PDC so any compensation has to be done manually.
Currently there is no expander option to allow 32-channel operation at high sample rates, and multiple Duende units are not supported, but SSL are actively researching ways around this limitation. I also did some tests to see how much CPU load Duende placed on the host processor; some administrative load is inevitable, but on a Mac G5 the extra load is pretty much insignificant — probably around the same as running one or two native compressor plug-ins.
EQ And Dynamics

The most commonly used of Duende's two plug-ins is likely to be Channel Strip, which is based around the characteristics of the EQ and dynamics sections of SSL's XL9000K console. Channel Strip is set out as a horizontal version of what you'd find on one of their hardware consoles, and its features include input filtering, an expander/gate, a compressor, low and high-pass filters and a four-band EQ.

The filters comprise a 12dB/octave low-pass filter and an 18dB/octave high-pass filter with cutoff frequencies continuously variable from 20Hz to 500Hz and 3kHz to 22kHz respectively. Both can be bypassed and the filters can be switched out of the audio path and into the dynamics side-chain if required. Additionally, the filters can be switched pre- or post- the main EQ. Filters of this kind are very useful for constraining the frequency range of tracks when you're trying to create separation in a mix. A routing block diagram is always shown in the plug-in window, so you can always check that the signal is being routed as you expect.

A four-band equaliser handles the main EQ functions, and like the XL9000K EQ, has two distinct switchable characteristics based on the SSL G and E-series designs. The four EQ bands comprise high and low-frequency shelving equalisers, which can also be switched to peaking curves using a Bell button (which I personally find a very useful option), and two variable-Q, variable-frequency mid-range parametric sections. The lower of the shelving bands has a range of 40 to 600 Hz, with ±16.5dB of gain available, while the higher works from 1.5 to 22 kHz and offers a generous 20dB of cut or boost. The two parametric bands offer ranges from 200Hz to 2kHz and 600Hz to 7kHz, with Q adjustable from 0.5 to 2.5. Again, ±20dB of gain is available.

SSL 3 Channel Strip.s

In the G-series mode, the shelving curves EQ exhibit some overshoot, like their analogue counterparts, while switching to E-series mode gives a gentler slope with no overshoot. In G-series mode, the bandwidth of the parametric bands varies with the amount of cut or boost applied, whereas the E-series version operates in constant-Q mode; this may be perceived as being less subtle, and you have a definite choice of EQ flavours here. It's also possible to bypass the EQ section of the plug-in independently of the rest, or place the EQ in the dynamics side-chain. If you've already assigned the filters to the side-chain, the EQ comes before the filters in the signal path.

The dynamics section of the channel strip is relatively simple but nevertheless very powerful. Compression can sound different depending whether it is applied pre- or post-EQ so there a Pre EQ switch to provide this choice as well as a bypass button. When the dynamics is switched pre-EQ, it still comes after the input high- and low-pass filters.

Featuring a continuously variable ratio from 1:1 to infinity (hard limiting), the compressor/limiter has a threshold control adjustable from -20dB to +10dB, a choice of auto or fixed 1ms attack times, and a release time variable from 0.1 to 4 seconds. There's also a choice of detection algorithms. Choosing Peak means the compressor responds to high signal levels regardless of how short they are in duration, while RMS makes the compressor respond more to the average input signal level as it would be perceived by a human listener. As a rule, peak compression is more assertive and is well suited to percussive material. In this design, the RMS mode uses a soft-knee compression curve whereas the Peak setting reverts to a conventional hard knee, so the RMS mode is clearly the more gentle of the two. A meter with yellow and red 'LEDs' shows the amount of gain reduction being applied.

Expander/gates are always useful for mopping up noise in pauses, and the one here features an attenuation range variable from 0 to 40 dB, a threshold variable from -30dB to +10dB, switchable normal or fast attack times and a hold control variable up to 4 seconds. At the fast attack time, it takes 0.1ms to reach 40dB attenuation, while the normal attack time is 1.5ms per 40dB. Hold is an important control as it prevents the gate 'chattering' when the audio envelope fluctuates, as it can otherwise do if a fast release time is used. The release time can be set anywhere from from 0.1 to 4 seconds.

Because the filters and equaliser can be inserted into the dynamics side-chain, frequency-conscious gating is possible. Green 'LEDs' show the amount of gain reduction applied when the gate or expander is closed. In expander mode, the expansion ratio is fixed at 1:2. At the output of the dynamics section is a ±20dB output gain control (to compensate for any gain loss in the compressor), and a six-segment level meter. A side-chain listen button enables the user to hear the side-chain signal complete with any filtering or EQ that has been applied.

You could buy a TC Powercore plus the Sony Oxford EQ and Oxford Dynamics plug-ins, which offer the same calibre of performance, but they aren't the same thing as SSL algorithms. Similarly, the Focusrite Liquid Mix will give you a greater choice of different EQs and compressors, but if you want genuine SSL, then Duende is the only game in town.
Magic Buss

The Bus Compressor plug-in is definitely Duende's secret weapon and is, again, derived from the buss compressor used in the XL9000 K-series console. It is available in stereo or mono versions and may be deployed anywhere, though it is commonly used for processing subgroups or complete stereo mixes. Unlike the real thing, however, you can open multiple instances up to the maximum of 32 DSP slots. There's a meter to show gain reduction (3 or 4 dB is usually enough!) while the threshold can be varied from -20dB to +20dB. As with the original, the attack time is switchable in six steps from 0.1 to 30 ms, while the release is also switchable, this time from 0.1 to 1.2 seconds in five steps. Make-up gain compensates for compressor gain reduction and is adjustable from -5dB to +15dB. A Compressor In button acts as a bypass. There may be relatively few controls, but the compressor itself is based on a very complex circuit that has a particular character and adds density and punch without sounding unmusical. This buss compressor is often favoured by hip-hop producers, but it is applicable to many types of pop and rock music, and depending on how it is set up, it can add subtle body to a mix or really make it pump.

Duende In The Studio

Duende installed without fuss in my system, and a firmware updater on the CD-ROM was used to bring the hardware up to the current spec. It ended up sharing a Firewire port with my TC Powercore, which was also being used in the test song, and I didn't encounter any problems there, successfully running all 32 channels at the same time as a fair number of Powercore plug-ins while also streaming audio from an external Firewire drive, though this was connected to a different port on my G5. If you mix 'in the box' rather than streaming multiple audio channels out to an analogue mixer, therefore, you may well get away with using your computer's built-in Firewire ports. However, Firewire bandwidth isn't infinite, so it might be a good plan to add an additional multi-port Firewire card to your computer if you have a free PCI slot, especially if you also need to communicate with external hard drives or other DSP processing boxes.

When I spoke to the SSL engineers, they also pointed out that unlike some DSP boxes, version 1.0 of Duende isn't able to handle off-line bounces, so mixes need to be rendered in real time. I checked this out and they're absolutely right — if you try to work off-line you get glitches. Apparently software version v1.1 will allow off-line bouncing, although this will still be no faster than real-time operation. While offering no speed advantage, it will at least avoid inadvertently creating files with glitches in them!

SSL 4 Bus Compressor.s

I tried out the Bus Compressor plug-in first as part of a Mix Rescue project I was doing and it worked a treat, fattening the mix, adding lots of loudness and somehow blending the various elements without losing clarity or focus. If you push the ratio and amount of gain reduction up you can definitely hear it working, but in a way that is still musically useful for many styles, so you get plenty of creative leeway with this particular plug-in. I also used it on a second remix project involving a very nice jazz outfit where more subtle settings were in order, and again, it helped knit the mix together without getting in the way of the music.

When I came to try the channel strip I was impressed by how predictably everything worked, making it fast and easy to get the job done. Both EQ options sound 'right' and you only need to apply small amounts of cut or boost to make the required changes to the sound, just like with a good analogue equaliser. The high and low-cut filters are also extremely useful, though the designers may have missed a trick by only allowing you to put these in the side-chain for the whole dynamics section, rather than allowing you to filter just the compressor or just the gate side-chain. This may be a feature of the original but there's nothing to say that software shouldn't be more flexible than the original in areas where it doesn't detract from the authenticity of the sound. Other than that, the EQ sections behaved beautifully, and you don't seem to have to fight to get the right results. The only slight disappointment for me was that the low-mid parametric band only goes down to 200Hz, and I often find there are low-mid problems that need addressing just below this frequency, so a bit more range would have been appreciated. In an ideal world, I'd have liked this band to go down another octave and the upper mid to go up by a further octave.

Though it has relatively few controls, the channel-strip compressor works really well in both its 'stealth' RMS mode and more overt peak mode. Whether you need subtle gain control or the effect of something being audibly knocked into shape, you've got it. Indeed, there are few if any applications where I'd feel the need to resort to a different compressor other than for ducking (as there's no way to source the side-chain from a different audio track). Whether you are levelling vocals, adding punch to drums or adding foundation to a bass guitar, this compressor makes a good job of it without any fuss whatsoever and without making the sound dull or 'spongey'.

Bargain Or What?

At just short of £1000, Duende might look like a lot to pay for a couple of plug-ins, even ones that can be run with virtually no CPU hit. Looked at more logically, though, it gives you the essential functionality and tonal character of 32 channels of SSL console to use inside your own DAW, for less than the cost of the VAT on a hardware SSL power supply — or so I'd imagine! The audio quality of these plug-ins is first-rate, and because they are based on the algorithms already used in SSL's own digital consoles, they're more than just emulations that don't quite hit the mark. I've pointed out a few limitations inherent in the originals that could have been worked around in the software versions, but there's plenty of time for SSL to issue updates if they feel the need. As it stands, these plug-ins do pretty much what you can do on a C-series SSL console, and in my book, the ease with which they get the job done is a major bonus. You can spend all day twiddling the knobs on an indifferent EQ without getting the results you're after, but here it all seems effortless.

If you like the SSL sound but are never likely to be in a position to own one of their consoles, you can now have all the sonic benefits from within your own Mac-based DAW at a very affordable price. I've only been playing with it for a few days and already I'm hooked. 

SSL Duende £999
Sensibly priced.
Both the Channel Strip and Bus Compressor plug-ins are excellent.
Thirty-two plug-ins always available, except at high sample rates when 16 are available (stereo counts as two plug-ins).
Easy to use.
Mac-only at present.
Currently, only one Duende can be used at a time.
Some limitations of the original have been retained when they could have been avoided.
When it comes down to it, 32 channels of real SSL EQ and dynamics is seriously attractive. It's just a pity that PC users will have to wait a little longer for it.
£999 including VAT.
Sound Technology +44 (0)1462 480000.
+44 (0)1462 480800.

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