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November 1, 2001

StereoPack and MixPack are two VST plug-in bundles, from Professional Sound Projects (PSP) in Poland, that offer a broad range of mixing and tracking tools at a modest price. StereoPack contains four plug-ins to create, enhance, and correct stereo imaging and comes with a graphic stereo analyzer. MixPack comprises four plug-ins for bass and treble enhancement, saturation, and compression. With a VST host program, the PSP plug-ins can process solo tracks, enhance submixes, and polish final masters. Each plug-in includes numerous presets.


The StereoPack bundle contains StereoController, PseudoStereo, StereoEnhancer, and StereoAnalyser. With those plug-ins, you can view, simulate, manipulate, intensify, and repair an audio file's stereo image.

StereoController is designed to reduce problems in a stereo mix. Four sliders let you balance the left and right channels, the mono and side components, and the left and right side components. You can also swap channels, phase-invert either channel, and pan the signal's mono component. Side-mono and left-right meters provide visual feedback.

In addition to making stereo repairs, two StereoControllers in series can remove vocals for karaoke recordings. That technique relies on the assumption that the solo material is panned near the mix's center. The idea is to invert one stereo channel's phase and then mix it with the other channel to reduce or eliminate a solo vocal performance. You can move the solo to the center with the first StereoController's rebalancing controls and then invert the phase and sum the channels with the second StereoController. To nudge the solo into position, StereoController has separate control of the side balance, channel balance, and center panning.

PseudoStereo and StereoEnhancer use delay lines to simulate stereo from a mono track and to manipulate a stereo track's imaging, respectively. Both plug-ins operate on the principle that when you mix a delayed version of a signal with itself, some frequencies are attenuated as others are reinforced, producing a comb-filtering effect in which the comb “teeth” are harmonically spaced.

PseudoStereo mixes normal and phase-inverted versions of the delayed signal with the original. The result is two channels with complementary attenuation and reinforcement frequencies. If you apply PseudoStereo to stereo tracks, it first mixes them to mono and then back to stereo — not desired typically, but sometimes effective.

StereoEnhancer performs the same trick on both channels of a stereo input signal. Except for providing three modes to remix the enhanced stereo channels, StereoEnhancer's controls are identical to PseudoStereo's (see Fig. 1). The Depth control determines the amount of delayed signal, thereby controlling the stereo image's depth. A depth of 0 percent leaves only the original mono signal in the mix. The Freq control actually sets the delay time within 1 to 50 ms, but because that control specifies the fundamental frequency of the comb filter, its display is in hertz. The Emph control specifies the cutoff frequency of a highpass filter applied before the delay line. Higher emphasis settings leave low frequencies sounding more monaural than stereo, which adds realism because lower frequencies tend to have less stereo imaging.

StereoAnalyser is a software oscilloscope that displays stereo imaging by separating the common (mono) and side-signal (difference) parts of an audio file's stereo signal. The oscilloscope displays the levels of those two parts on its vertical and horizontal axes, respectively. The real-time signal is displayed in light green, and a Hold option displays the signal over time in dark green. You can switch the meters to show the levels of either the left and right stereo channels or the mono and side signals. For example, in two analyses of the same stereo mix, the right channel is phase-inverted in the analysis on the right (see Fig. 2). In a proper stereo mix, the mono component dominates, and most of the trace is in the vertical section between the L and R diagonal lines. If the differential component dominates and the trace is mostly horizontal, you may have problems with phase, mono compatibility, and excessive spread. In the example, inverting the phase of one channel wipes out the signal's mono component.

StereoAnalyser is invaluable for setting up and analyzing the results of the additional StereoPack plug-ins. If you insert StereoAnalyser immediately after one of the others, you can toggle the extra plug-in in and out of the signal path to analyze its effect. StereoAnalyser is also useful at the end of the signal path for providing visual feedback when setting up a mix.


MixPack contains four plug-ins — MixBass, MixTreble, MixSaturator, and MixPressor — in mono and stereo versions. The first two process a signal's low- and high-frequency components, and the latter two provide saturation and compression.

MixBass splits the signal into low- and high-frequency components. The lows are routed through a downward compressor, followed by a coloration stage that simultaneously applies low-shelf filtering and adds harmonics to enhance the bass signal. The highs are then mixed back in, and the signal passes through a saturation stage. That emulates analog saturation and lets you boost the signal about 3 dB without audible distortion.

As its 21 factory presets show, MixBass has applications that go well beyond bass-track processing. MixBass is effective for subtly warming a final mix, punching up drum loops, and enhancing the low end of pads and keyboards. Particularly useful is the ability to compress and color the bass and then to apply low-cut shelving.

MixTreble consists of five processing stages: a hiss remover, a high-frequency expander, a high-frequency stereo enhancer, a coloration stage for adding high-frequency harmonics, and a soft saturator. To conserve resources, you can independently toggle each stage on and off for a variety of effects combinations.

MixTreble's hiss remover does a nice job of de-essing vocals, but MixTreble is best used for adding color and sizzle to percussion tracks. On most other material, the effects were a bit hard to tame, and the results seemed somewhat harsh.

MixPressor is a soft-knee limiter followed by an optional hard limiter. To control the limiting effect, MixPressor passes the signal through a variable Q, bandpass-filtered sidechain. You can audition the sidechain signal for setup purposes. The main limiter has controls for threshold (labeled Compress and calibrated in percentage), Attack (with optional delay), Hold, and Release. The last three controls and the sidechain give the limiter compressor-like characteristics; for example, a delayed, slow attack allows transients above the peak limit. An optional automatic makeup feature compensates for the level reduction caused by limiting. The hard limiter absolutely limits signal peaks, letting MixPressor function as a maximizer. The second limiter's look-ahead option adds a 2.2 ms delay to soften the effect.

With careful tweaking, MixPressor offers useful limiting functions. Because introducing unwanted audio artifacts is so easy, however, it is better used on individual tracks than on mixes. On percussion tracks, in which coloration is often the goal, MixPressor is a handy tool.

MixSaturator's saturation algorithm applies one of seven nonlinear distortion curves to the signal (see Fig. 3). Using the Shape slider, choose from three tube emulations, three analog-tape emulations, and one digital-clipping emulation. The main saturator is preceded by optional bass saturation and treble compression sections. For postbass and -treble processing, a flexible metering scheme lets you monitor the peak and root mean square levels of the input, the output, or the input to the main saturator.

Subtly applied during mixdown and mastering, MixSaturator adds warmth, especially to low-quality, hard-edged digital recordings. It can also add presence to almost any track. If pushed over the top, MixSaturator can seriously mess with the harmonic content of a track or a mix; such an effect can be useful with hot guitar and synth leads or with any material you want out front that can stand some distortion. The 24 factory presets include generic settings for most contexts.


StereoPack and MixPack are available stateside only as downloads from the Professional Sound Projects Web site. You can download and buy MixPack plug-ins for $30 each. The site has demo versions of the plug-ins and MP3 audio examples.

HTML documentation and plain text files are provided for the bundles, and MixPack includes a PDF manual. The documentation is more than adequate, though some explanations suffer in translation. It's well illustrated, featuring block diagrams of the signal flow and control structures of some plug-ins.


StereoPack and MixPack's audio quality is very good, and the MixPack plug-ins are surprisingly versatile. StereoPack's plug-ins are CPU-efficient and add only a small percentage to overall CPU usage. The MixPack plug-ins do more intense processing and use more resources. Depending on settings, they use 10 to 30 percent CPU power on my Mac G3/300 MHz.

The PSP bundles are a great value with useful applications. Some controls are touchy or oddly labeled, but the control panels are graphically well designed; the metering is excellent too. StereoPack and MixPack won't satisfy all tracking and mixing needs, but they offer unique digital signal processing solutions to fill a few holes in your audio-editing kit.

Len Sasso is a writer and composer living on California's central coast. Contact him through his Web site at


Professional Sound Projects

StereoPack (Mac/Win)
audio plug-in bundle



PROS: Great tool for stereo enhancement and repairs. Graphic stereo analysis.

CONS: Controls sometimes touchy. Stereo simulation not always convincing.


Professional Sound Projects
tel. 48-22-601-963-173


Professional Sound Projects

MixPack (Mac/Win)
audio plug-in bundle



PROS: Great value. Wide variety of useful effects.

CONS: Controls sometimes touchy. Care needed to avoid unwanted artifacts.


Professional Sound Projects
tel. 48-22-601-963-173

Minimum System Requirements

StereoPack and MixPack

MAC: G3/300; 64 MB RAM; OS 8.5; SVGA display; VST host software

PC: Pentium/200; 32 MB RAM;
Windows 95/98; SVGA display;
VST host software

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