TC electronic unwrap. Professional stereo to surround conversion.

November 14, in Hi-Fi Systems Reviews

There are bound to be few products in the surround world that will be more controversial than UnWrap, a new plug-in for the TC System 6000. Simply put, this is an algorithm designed to derive a 5.1 or LCRS mix from a stereo (or even a mono) signal.

Surround mixing engineers everywhere are bound to be in an uproar about this, but TC is careful to explain that UnWrap is not designed to replace the true 5.1 remix; instead, it is meant to present a viable alternative when either the original multitrack masters are unavailable (or, in the case of older recordings, may have never existed at all), or where it is preferable to retain the original effects of the stereo mix (as in, for example, a remix of Beatles classics like “I Am The Walrus” or “Tomorrow Never Knows,” where many of the key effects were flown in live during mixdown).

Because patents are pending, the specifics of the digital processing performed by UnWrap is, of course, proprietary, but TC program manager Thomas Lund explains, “The original left and right signals are analyzed with regards to different angles of phase relationship as well as spectral content. The derived signals can be processed through multi-band envelope generators for enhanced channel separation, and, finally, the surround signals can be further decorrelated in different ways. All of this is done using 48-bit processing, so the output resolution always can be kept at full 24-bit.”

Unlike the cheap pseudo-conversion processors found in consumer products (you know, the “press this button and a slightly delayed, slightly reverbed copy of the front L/R signal magically appears in the surround speakers”), UnWrap provides a wide and flexible range of user controls (some of which may be expanded or added to in future software releases), allowing the skilled engineer to precisely tailor the process to the source material. The most important of these is called L/R Processing – at 0 percent, the original front L/R outputs are preserved although signal is added to the other four speakers; increasing percentages spread the width and add additional center channel signal. Other controls include Input Trims and Output Levels that allow exact level matching; independent Solo and Mute buttons for each channel; and adjustable delays per channel (in both milliseconds and samples – minimum throughput delay is 3.6 ms at both 44.1 and 48 kHz; like all other System 6000 algorithms, UnWrap works with all sample rates from 32 k to 96 k). UnWrap also allows an LFE signal to be derived, with an adjustable low-pass filter (from 10 to 200 Hz, with both 2nd and 4th order filters available).

There are also a series of extensive controls dedicated to the center and surround channels. Adjustable center channel parameters include a Reference Level (adjustable according to the dynamics of the center signal, from 0 to -25 dB) and a choice of two different Contour Styles (frankly, I had difficulty hearing the difference between them). More importantly, there’s a full-featured 4-band parametric equalizer dedicated to the center channel alone, to enable you to selectively suppress frequency ranges that might compromise L/R width, or to boost selected frequencies in order to strengthen the “anchoring.”

Adjustable surround channel parameters include the aforementioned Reference Level and Contour Styles (their effects are slightly more noticeable here; Contour Style 1 provides more clarity, while Style 2 seems to blend the spectral range a little more smoothly), as well as an all-important Decorrelation control, with six options: Dry (no reverb added to the surround channels), Close (short and subtle reverb), Dorsal (a little wetter), Lateral (increased width), Diffuse (significant delays), and Wet (a fairly over-the-top effect somewhat reminiscent of consumer-type artificial “enhancement” controls). Closely associated are a Focus parameter (which appears to adjust phase) and a Decorrelation Tone parameter (essentially 2-band equalization: “0” preserves the original spectral content; increasingly positive values increase the high-frequency content; increasingly negative values reduce the cutoff frequency of what appears to be a low-pass filter). Again, there is a full-featured 4-band parametric equalizer, which in this case affects both surround channels identically.

TC recommends that the output of UnWrap be monitored through the Toolbox 5.1 algorithm (assigned to the succeeding engine) in order to allow you to instantly determine how the derived surround mix collapses to stereo or mono. This is an extremely useful function, as you might imagine – and, of course, a Bypass control also allows you to make instant before-and-after comparisons.

I used UnWrap on a wide variety of source material, and was consistently impressed with the results I was able to achieve with a minimum of tweaking (it generally took me less than an hour to arrive at what I subjectively felt were the optimum settings for a particular track). Using it to process tracks from the superbly engineered Steely Dan Gaucho CD, I was able to create satisfactory results – not nearly as good as Elliot Scheiner’s actual surround remixes, of course – but, when directly A/B comparing the two, it was amazing how close many of UnWrap’s processing “choices” were to Scheiner’s aesthetic decisions. Another eye-opener was using it to derive a 5.1 mix from extremely discrete stereo mixes – specifically, tracks from the original release of Revolver (where, often, a vocal is left channel only, or drums right channel only). The resulting surround mix was astonishingly good, with, of course, the benefit of preserving all of Geoff Emerick’s original equalization and effects.

Needless to say, UnWrap is powerful enough to be abused in the wrong hands, but, under the control of a skilled engineer, it does a great job. More importantly, it solves an important repurposing problem: what to do when the original multitracks are unavailable or non-existent. Kudos to the Danish design engineers who developed this extraordinary product!

PRICE: $ 995 (license fee)
CONTACT: TC Electronic, Tel: 805-373-1828. Web:

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