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SPC2000 Serial/Parallel Compressor

The McDSP SPC2000 Serial / Parallel compressor provides a collection of multi-stage compressors (two, three, and four-stage configurations) with plenty of routing combinations. The SPC2000 also uses the award-winning CompressorBank plug-in algorithms to emulate a variety of classic compression responses or create a new and unique compressor sound. In short, the SPC2000 is a serious audio plug-in, in our completely biased opinion. So read on, fellow seriously biased audio engineers! This edition of Colin’s Corner is an overview of the SPC2000 and the finer details therein.

Serial and Parallel Compression

Serial and parallel compression are not new studio techniques. The capability to compress a signal in multiple stages has existed as long as audio engineers have been able to own at least two outboard compressors. When you consider the gear collection nature of the lot of us, the implication is serial and parallel compression have been around for a long time!! But to be sure we’re all on the same page, some explanations.

Serial compression is the process of taking the output of one compressor and feeding it into the input of another compressor. This technique allows for some seriously over compressed sounds if both compressors have a low threshold (-40 dB), high makeup gain (+24 dB), and a high compression ratio (8:1, 10:1). Alternatively the same technique can yield very natural sounds when each compressor is applied gently to the signal, using a high threshold (-12 dB, -6 dB), small amounts of makeup gain (+3 dB, +6 dB), and a low compression ratio (2:1, 1.5:1). These combinations are just the tip of the iceberg. A combination of heavy, medium, and light compression can yield a variety of results, one of which is bound the make the client happy and willing to pay you for all that extra experimentation.

Parallel compression is the process of splitting a signal into two (or three, four, …) paths and feeding each into its own compressor, and then blending the output of those compressors together. The mixing of a heavily compressed track and barely compressed track is a common use of parallel compression. The heavily compressed sound can have lots of room tone, is fairly level (un-dynamic if you will!), and has a transient/attack character very different from the original sound. When blended with the barely compressed track, the results can be great. Gently compressed natural sounds mixed with ambient crushed sounds have made many a rock drummer famous. Or at least louder in the mix.

Bring in the SPC2000

As much fun as experimenting in the studio can be, often there is not enough time, gear, and/or patch cables to do all the serial and parallel compression experimentation one wants to do. Here is where the SPC2000 is a great tool, in my completely biased opinion. Starting with the now legendary CompressorBank plug-in, the SPC2000 takes two, three, or four of these fine dynamic range processors and makes them available at the click of a mouse in serial, parallel, and many other combinations. The user can even link the compression bands together to make quick changes and find the ultimate sound in a jiffy (people still say ‘in a jiffy’, yes?).


The SPC2000 is in fact three plug-ins – the SPC202, SPC303, and SPC404 – which as their name implies, are two, three, and four stages of compression, configurable in a variety of routings. Looking at the SPC404 (the four band version – yeah let’s start with the big guy), the compressor routing is displayed graphically at the top of the interface, followed by the four stages of compression.

The compression controls in each stage are mostly standard – makeup gain, threshold, compression ratio, attack, and release. Screen Shot 2014-01-21 at 8.54.21 AMThe additional controls – knee and BITE are unique to CompressorBank (since 1999!!!). The CompressorBank knee control actually morphs the compression curve between different models of classic units (*) like the dbx 165, UREI 1176, Neve 33609C, and LA2A. Not a bad feature to have in a single compressor! The BITE control is an acronym meaning Bi-directional Intelligent Transient Enhancement – or for the non-nerd, BITE allows the compressor to operate more like an older compressor, where high frequency signals (aka transients) are passed through the compressor nearly unaffected. The BITE control could be considered an ‘older’ control – the higher the BITE value, the more the compressor operates like a vintage dynamic processor, whose circuitry has not been designed to handle modern high fidelity signals. There are also multiple time constant (TC) circuits in each compression stage (and the original CompressorBank plug-in) – auto, type 1, and type 2. Auto is automatic attack and release values (attack and release controls have no affect), type 1 is a linear release, and type 2 is a program dependent release.

To facilitate fast edits and quick setup, the compression stages can be linked and soloed. On the right hand side of each compression stage is a set of meters for input, gain reduction, and output metering. Screen Shot 2014-01-21 at 8.55.29 AMThere are also buttons marked “M” for master, and “S” for solo. When “M” is pressed in a compression stage, that compression stage will slave all the other stages’ controls to it, keeping their relative offsets, so you can operate all the compression stages at one time from the ‘master’ stage. The slaved stages can still be adjusted independently as needed. When “S” is pressed, only that compressor stage is audible. If there are other compression stages in the audio path, they are bypassed so that only the ‘soloed’ compressor stage affect is heard.

Screen Shot 2014-01-21 at 8.54.55 AMSelecting the actual compression routing is easy. The popup control located in the upper display of the SPC404 (and SPC202, and SPC303) selects the compression routing. A small graphic immediately to the right of the popup, and a larger graphic above, show how the audio signal is routed through the available compression stages. The large display also shows the compression curve for each compression stage.
Master input and output controls are useful for adjusting levels into and out of the SPC2000 without having to adjust any of the compression stages.

Application Tips

Many compression routing possibilities exist in the SPC2000. Here are a few…

Gentle Multi-Stage Compression

Insert a SPC404 on a track. Select the basic ‘serial’ routing selection to place all four compression stages in series with each other. Press “M” in compressor 1 to make it the master. Now using just the controls in compressor 1, set the threshold at – 6 dB, compression ratio at 2:1, keep the makeup gain, attack, and release at their default values. Use the master input and output to control how much compression is actually occurring and how much makeup gain is needed. Upon hitting play, you want to see no more than 3 dB of gain reduction in the gain reduction meters in each compression stage. This is the classic “1 dB at a time” serial compression style useful for vocals and acoustic instruments that need to retain a natural sound.

This technique is great, but not if the compressor clips the signal at 0 dB. Such behavior was mostly only common to the older TDM hardware, however many host-based compressors have the same limitation. McDSP compressors handle signal level well above the 0 dB maximum, so when using the multi-stage compression technique described above, signal peaks jumping over 0 dB are ok, and the final output can be trimmed with the master output control of the SPC2000.

Parallel Compression for Drums

Insert a SPC202 on a drum buss, and reset the parameters to their factory default if they are not already at the factory default settings. Now select the basic parallel routing mode. Use the stage 1 compressor to crush the drums – lower the threshold to about -40 dB, set the compression ratio to 8:1 or great, use an attack between 1 and 3 msec, and a release of < 100 msec. Turn up the makeup gain so you can hear the heavily compressed sound. Using the factory default settings for the second band, set up the threshold control so be just above the peaks of the drums so that only 2-4 dB of gain reduction is occurring. Blend the heavily compressed sound from stage 1 and the relatively uncompressed sound from stage 2. Taa daa!! Parallel buss compression for rocks bands everywhere.

Parallel Compression Plus

Now observe as you blend the two compressors in SPC202, some signal peaks are still troublesome – there is some amount of overshoot, perhaps even into the red (!!) when the compressed and uncompressed signals are combined. If only someone would make a 3-stage compressor that can take the combined output of the heavy and not-so-heavy compressor to prevent any amount of signal peaks to exceed a certain maximum. Fortunately, McDSP has! Replace the SPC202 in your session with the SPC303 – each configuration will attempt to read the others presets values. Select the XXX [need to look up] routing mode so stage 3 compressor is placed after the combined compressor 1 and 2 signals. Using almost the same ‘barely compressing’ settings as stage 2, the third stage should prevent digital overshoots above 0 dB.

Wait There Is More

A few of the routing combinations – serial, parallel, and parallel “plus”, have been detailed. But the SPC2000 has a whole lot more to offer. Here are some other ideas to get you started using the SPC2000 on your next project…

– using SPC303 – two gentle compressors in parallel with a heavy compression stage
– using SPC303 – a gentle, medium, and heavy compression stages using basic serial mode
– using SPC404 – two heavy compression stages in series (!), then in parallel with two gentle compression stages
– using SPC404 – three gentle compression stages, followed by a ‘touch up’ stage to control any remaining errant peaks

Compression Conclusion

There are lots of ways to process your tracks. Serial and parallel compression are some good processing techniques. Hopefully the SPC2000 will your be your goto for serial and parallel compression on your next project and beyond!

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