+ AN INTRODUCTION TO LINE-LEVEL MID/SIDE PROCESSING

Dangerous Music’s chief circuit designer, Chris Muth, was the first person to design a device specifically for implementing Mid/Side processing for line-level audio signals, as well as the first to include insert loops for applying external signal processors like compressors and EQs. Those innovations were a vastly important step forward for mixing and especially mastering. The Muth legend continues in the Dangerous MASTER, a powerful and crystal clear mastering console complete with an all-analog Mid/Side Matrix, width control and insert loops.

In this tutorial, we’ll learn about Mid/Side processing techniques, using the MASTER’s Mid/Side configuration as our example.

Mid/Side processing, as the name suggests, allows the Middle and the Sides (L-R) of a stereo recording to be separated out from each other and manipulated independently. Sometimes Mid/Side processing is called “sum and difference” or “sum and minus,” but we’ll stick with the standard Mid/Side name.

We typically think of stereo as being made up of the left and the right – which is technically accurate – but it is equally valid to think of stereo audio as being made up of the Mid and the Sides that we perceive. It’s also worth keeping in mind that there is no exact line of demarcation between the Mid and the Sides – anyone who’s used a “pan” pot knows intuitively that the stereo field is a continuum between far left and far right. So, when we separate out the Mid and the Sides in Mid/Side processing, we end up with signals mostly in the middle and mostly on the sides. This fact doesn’t present any real-world issues when working in Mid/Side, but it may help you grasp better what’s happening as you experiment with it.

The creative possibilities that arise with Mid/Side processing are vast, and it’s considered one of the most powerful – and sometimes problematic – tools among audio engineers. The possibility of changing levels, tones and dynamics within a finished mix puts formidable power into the hands of the engineer, especially during mastering. We’re going to explore some of those possibilities, especially as it pertains to using the Dangerous MASTER, but before we do, let’s get some basics down.

+ UNDERSTANDING THE MID/SIDE MATRIX

Like most audio devices, you actually don’t need to understand how Mid/Side processing works in order to use it, but it’s helpful and interesting to have a basic sense of it, and of how the Dangerous MASTER’s circuits achieve the definitive Mid/Side processing in the analog domain. Many digital Mid/Side plugins are available, but they have yet to measure up sonically to the sound of Muth’s brilliant analog circuits and often introduce undesirable artifacts.

Mid/Side processing involves a relatively complicated manipulation of audio signals, and it can be used during tracking, mixing and mastering stereo recordings. We’ll leave tracking aside for this tutorial, as Mid/Side recording requires a specific microphone technique. (Note that many MASTER owners use its Matrix for Mid/Side recording). Instead, we will focus on how Mid/Side processing works in the manipulation of line-level audio during mixing and mastering.

+ A FEW QUICK DEFINITIONS

Mid – The middle of the stereo image. We perceive the Mid as a mono audio image in the middle of the stereo field, but is actually a “phantom image” created by the left and right speakers delivering the exact same source at the same volume and at the same time.

Sides – The Left and Right sides of the stereo image.

Matrix – Using a relatively simple formula, the Matrix is the processor that encodes the L/R signal into separate Mid and Side signals and then decodes the M/S signals back into L/R. (The MASTER uses an audiophile-grade, all-analog Matrix.)

Insert Loop – On the Dangerous MASTER, there are three insert loops, and one of them can utilize the Mid/Side matrix, meaning that you can use this insert to process the Mid and Sides of a stereo signal separately.

Width Control – Not found on all Mid/Side processors, this control is an essential part of the MASTER that allows the user to quickly dial in the relative level of the mid and sides, effectively changing the perceived width of the stereo image. Even when not using the Insert Loops, pressing the MASTER’s Mid/Side button engages the Matrix, allowing use of the Width Control.

This diagram shows the basic topology of the MASTER’s analog Mid/Side Matrix.

As you can see in the diagram above, engaging the Mid/Side Matrix gives you separate control of the middle image and the sides of a stereo signal. The Left mono channel of the outboard processor becomes the mono Mid channel and the Right mono channel becomes the Sides. Though a bit counterintuitive, it’s important to keep in mind that the Sides channel is a mono channel within the matrix. By using the external processing loops, you can enter into the powerful world of Mid/Side processing. Below we’ll share a number of examples. As with any audio processing, Mid/Side is only as limited as your imagination.

+ POSSIBILITIES FOR MASTERING WITH MID/SIDE

De-Ess the Vocal – By inserting a dedicated de-esser or other dynamic EQ unit on the Mid, one can control an ess-y vocal while leaving the sides alone. This is often desirable when there are problematic esses, but the sides require an open top-end that the de-esser might interfere with. Especially given the resurgence of vinyl, ess-y vocals can be a particularly nasty problem when cutting vinyl, while maintaining an open stereo image is essential.

Open Up The Wings – It’s not uncommon during mastering to use shelving EQs in the top end to “open up the mix.” (The Dangerous Music BAX EQ is a particularly musical tool for this.) By applying a high-shelf EQ to only the sides of the mix, elements that are commonly panned wide, such as cymbals and guitars, can pick pick up a great deal of high-end energy without causing excessive vocal sibilance, snare splashiness, or other undesirable results that the EQ might cause in the middle of the mix.

De-Ess and Open The Wings – It is not uncommon to use these two techniques in conjunction.

Rebuild The Middle – Another useful technique during mastering is simply turning the sides down on a mix where the engineer has “overpanned” and sucked the center out. This is why the Width Control on the MASTER goes in both directions, allowing you to narrow the mix and emphasize the Mid as needed.

Pump the Middle – Got a rhythm section down the center that’s begging for some hard compression, but you don’t want the sides to start breathing, or pumping, with the beat? Send the Mid into your favorite compressor and dial in edge and sustain up the middle without affecting the sonic elements on the sides. A powerful strategy for music with a heavy beat that needs to hit hard.

Match the Image – Sometimes there’s a track on an album that just doesn’t fit in due to its stereo image (compilation albums are notorious in this way). By using the width control on a unit like the Dangerous Music MASTER, you can dial in a stereo image that sits convincingly among the other tracks on the record.

Targeted EQ’ing – If you’ve got a lead vocal that needs some drastic EQ’ing, a kick drum that could use shaping, or a “snotty” snare that’s dominating the mix, simply use a parametric EQ to process those crucial elements in the middle while leaving the sides alone. Or, what if the middle sounds great, but there are some side elements that need individual attention – just patch an EQ channel to the Sides and go to work. Even more powerful, use individual channels of a stereo EQ to separately carve the Mid and Sides while maintaining consistent tonality from the EQ’s circuits across the stereo field.

Surreptitious Reverb Processing – Whether they openly admit it or not, many mastering engineers will use reverb to help get a record over the line. With Mid/Side processing, you can target the areas of the stereo image that you’d like to add reverb to, allowing you to use more in that specific area rather than bleeding it into the entire stereo image. Add a touch of extra space around a vocal or a solo horn in the center, for example, and let the sides remain unaffected. If you’re the type not to tell your clients you’ve added reverb, you’ll get away with even more!

Orchestral Sound Stage Manipulations – Many an audiophile will obsess over the realism of the soundstage in an orchestral recording, but what many of them don’t know is that those images are often derived quite artificially through the use of Mid/Side processing. Orchestral recording techniques often boil down to just a few simple microphones, and Mid/Side processing – even just control of the width – can add or subtract many yards/meters of perceived soundstage width and depth. Add in some EQ and a touch of additional reverb and you’ll feel like you’re controlling the architecture of the concert hall.

+ POSSIBILITIES FOR MIXING WITH MID/SIDE

Any of the above mastering techniques can be used during mixing. By running any stereo source through the Matrix, you’ve got control over the Mids and the Sides separately. Whether it’s a single stereo recording like a string quartet or a drum overhead pair or something more elaborate like a stereo subgroup of dense background a drum mix, Mid/Side opens up a whole world of possibilities for the creative mixer. Below are just a few possibilities to help get the creative ideas flowing.

Simple Width Control – With a device like the Dangerous Music MASTER’s straightforward width control on hand, adjusting the width of a stereo signal within a mix couldn’t be easier. It’s an effective way to help tracks either fit together, or distinguish themselves, within the bigger picture. Especially helpful when managing dense, layered mixes, where a bit of extra space can help.

Fake Stereo: Delay and Modulation – By “multing” (duplicating) a mono signal and routing it into both the left and right channels and sending it to the Mid/Side Matrix, one can do all kinds of interesting things. Put some short delay (start around 5 to 15ms) on one channel feeding the Matrix and you’ve suddenly got a whole world of slight differences between the Mid and Side content to play with. Modulate that delay with a chorus effect on the Sides, and you’ve got subtle movement in pitch and time with the center staying clear as a bell. This can create a “fake stereo” effect, adding depth and width to your mix. Very useful on pads and guitars.

Fake Stereo: Comb Filtering – Comb filtering occurs in real acoustic spaces when a signal and its reflection are arriving at the same point after having travelled different distances. The interaction of the two signals causes sharp peaks and dips across the frequency spectrum. This is called “comb filtering” because the peaks and valleys resemble the teeth on a comb. The human auditory processing systems use comb filtering to help “localize” a sound source in space. We can recreate comb filtering by choosing a predictable boost and cut pattern on the Mids and the exact opposite on the Sides, thus creating some very convincing artificial stereo effects. Add in a slight delay on one channel (start around 5ms and experiment from there), and the possibilities get quite interesting.

Rhythmic Delay Processing – In electronic music, many of the drum machines and older samples that have become so iconic are typically mono. It’s no secret that adding delay to your drum tracks has become a common production technique that wakes up those mono drums and converts them into wholly new beats. What’s less known is how to use Mid/Side processing to create these effects. For example, using the Dangerous Music MASTER’s Mid/Side send the sides of your drums out to a funky analog delay set to a subdivision of the song’s beat and the center will stay rock solid while the sides rock to an all new rhythm. Incredibly effective, and unique.

Drum Stereo Pair Work – Many simple stereo-pair drum recordings can leave you without options while mixing, and a common problem is splashy or harsh cymbals. EQ’ing the stereo-pair often leaves the center of the kit lackluster – the snare loses its cut, or the kick feels too muffled. By EQ’ing and compressing the Sides of the stereo pair, one can gain control over those cymbals while allowing the center to remain snappy and alive. Or, if you’ve got the opposite problem, EQ and/or compress the Mid while letting the Sides sizzle and crack. Mid/Side processing can make a stereo-pair function like a multi-mic recording.

Pitch-Shifting For Depth – Perfectly pitched recordings often lack character and depth (and are more common in today’s machine driven musical landscape). The magic of music is often found in the slight discrepancies of pitch and timbre. It’s been a long-known trick to shift the pitch between Left and Right order to create a sense of width – just tiny changes, a few cents to start and adjust to taste, sharp or flat, often some of each blended into each side. With Mid/Side you can apply this same technique and, rather than obtaining width, you can get front-to-back depth to emerge. Try this on “gang vocals” or a choir of background vocals, a horn section, or any string pads, and see if you can get a more 3D sonic image.

Reverb Sends – Experiment using the Sides as the stereo send to a reverb while leaving the center of the source dry. Subtle differences in how reverb is triggered by the Sides will often create a greater sense of space and interest around a vocal. Experiment with long pre-delay times in the reverb, and even try slightly modulating or pitch-shifting the send to the reverb for more miasmatic effects.

+ GO BE CREATIVE

These examples above are merely starting points for anyone to get a sense of what’s possible with Mid/Side processing. It’s an area of audio work that’s ripe for experimentation, and more often than not, Mid/Side processing leads to new discoveries. It’s an incredibly powerful tool, and when working in the analog domain, it unleashes all kinds of new possibilities for your investment pieces. Because of its audiophile-grade design and components, the Dangerous Music MASTER’s Matrix is one of the most sought-after Mid/Side processors available, and will deliver rock solid results, whether you’re simply trying to de-ess a vocal or creating a whole new sound.

+ MID/SIDE HISTORY

When it comes to mastering, one of the hottest current trends is mid/side processing. It seems like every new plug-in or hardware device geared towards mastering offers some type of mid/side functionality. Most engineers who have used these functions agree that they are able to come up with masters that are exciting, musical, dynamic, but also nice and loud. That said, there is certainly a veil of confusion over what exactly mid/side or sum/difference processing is designed to accomplish, and equally, there is a good amount of uncertainty regarding how it is supposed to be done correctly.

For starters, it is important to distinguish between the mid-side stereo microphone technique and line-level mid-side processing. In the 1930’s, long before stereophonic records would find their way to the public, engineers in the research and development departments at several different companies had already begun to explore the possibilities of multichannel sound capture and reproduction. One such engineer, Alan Blumlein developed a two-microphone technique for recording stereo sounds, which placed a pair of bi-directional (figure-8) microphone elements in close proximity and perpendicular to one another. The vertex of the right angle between them is pointed towards the sound source, and as direct on-axis stereo sounds were captured along with the room sounds arriving at the back of each microphone a decent image of the players and the room was recorded.

In the 1950‘s Danish engineer Holger Lauridsen, building on Blumlein’s idea, created the mid-side mic technique. In his method, a cardioid mic is pointed directly at the sound source and a bi-directional microphone’s element is placed perpendicular to the cardioid element. The cardioid microphone captures the “mid” signal. The bi-directional mic captures off-axis information arriving from either side. During monitoring and playback, this “side” signal is duplicated and hard-panned left and right with one of the two instances being inverted in polarity. Because the sound being recorded had been hitting the bi-directional mic on opposite sides, sound hitting the left side of the mic capsule was directly out of polarity from sound hitting the right side. The electrical polarity flip of right signal essentially puts it back into a positive polarity relative to the left signal. This two-channel sound is mixed together with the direct “mid” signal and as they interact certain frequencies add and others cancel due to their phase relationship, and an impressive recreation of the original stereo space is achieved. On top of that, a balance of focus to ambience can be manipulated as desired, even after the recording is captured.

One point of contention as to the usefulness of this strategy is that when summed to mono, the “side” information completely cancels, leaving only the dry, direct signal. Some say that is a blessing, while others curse it. However, this mic’ing technique remains very popular today, particularly for sound effects recording. Two small pencil condensers can be packed into a blimp and record stereophonic sounds in the field with far less bulk than could be achieved with any type of X/Y strategy. Because of this, many popular production sound mixers and recorders have built-in mid-side decoders which allow the recordist to monitor the end result even during capture in the field.

The confusion arises because this type of mid-side decoder, which does, in fact, process line-level signals, produces a different result than modern mastering equipment offering a homonymic function. As a result, if a typical left/right stereo mix is processed through the type of decoder found in a field recorder or through a plug-in designed for this same purpose, the left signal will be duplicated and played through the left and right channel, and the right channel will likewise be duplicated to each channel, except that in one channel it will be 180˚ out of phase. To that end, the left channel will feature the left signal added to the right, while the right channel will feature the left signal minus anything that is identical in the right, which would be out of phase, and thus cancelled. This could be referred to as a sum/difference signal.

This type of encoding was used in the original vinyl mastering equipment in order to produce records which could have one groove, play in stereo, and still be backwards compatible with mono equipment. Mono players relied on the back-to-forth movement of the needle to produce sound. When storing sound on new stereo records, all of the sound from the two-channel stereo mix was summed and stored in the horizontal or “lateral” movement within the groove. This way mono records could still play that information. The difference signal was stored in newly introduced up and down movement, or “vertical” movement of the needle.

To that end, sum/difference signals could synonymously be referred to as lateral/vertical mastering in those days. This is why the Fairchild 670, the holy grail of compressors, offers a Lat/Vert mode, which was initially designed to process the middle signals and the side signals separately. That said, the amazing thing about matrix encoding the signal in this way is that after running the sum/difference signal through the same type of matrix that produced it in the first place, if no processing has been applied in between, the original left/right signal is recreated identically.

As newer stereo release mediums were developed, especially CD, the realization that sum/difference processing was no longer necessary ultimately led to its fall from popularity in mastering. Instead mastering engineers relied on true stereo compressors and limiters, and simply EQ’d the entire stereo mix. Despite that, equipment designer Chris Muth, who has a background in vinyl mastering, was not willing to content himself with the fact that sum/difference mastering was now irrelevant. To that end, he built a nondescript, unlabeled black box which contained a sum/difference matrix encoder and decoder. It also was highly innovative in that it featured an insert loop where either signal could be fed to any piece of outboard signal processing gear so that the sides could be affected independently from the middle. The only control on the box itself was a blend control which could adjust the balance of the sides to the middle.

Anyone who heard his famous black magic box wanted one, and eventually Sterling Sound purchased Muth’s company in order to obtain exclusive rights to this circuit, as well as his transfer console and monitor controller designs. They also signed Muth to a 5-year contract to serve as Technical Director to oversee the building of and transition into their then-new facility in Chelsea. Sterling’s world-class mastering engineers using Muth’s transfer consoles were consistently turning out records that sounded loud enough to win in the “loudness wars” while simultaneously maintaining width and dynamics. That trend continues today.

In the 1990’s when Chris Muth paid a visit to Bob Muller’s Dangerous Music recording studio, the two hit it off. Chris became a partner and the two began collaborating on designs for custom recording and mixing equipment. Due to popular demand, they launched their analog summing amp, the Dangerous 2-BUS, commercially in 2001. Once Muth’s contract with Sterling ended in 2003, Dangerous Music was free to design and sell mastering hardware. The Dangerous MASTER, based on Muth’s transfer console, also features the “Sides and Middle” technology from his black magic box allowing the option to split signals out to an insert chain which will process summed signals separately from difference signals and then blend them back together with variable width. This technique allows an engineer to correct issues and enhance a mix in ways that are simply impossible to achieve working in simple left-right stereo.

The technology seems simple enough, however, the power of mid-side processing also bears some responsibility. Maintaining the original stereo image of a mix relies heavily on the way that the two signals interact with one another. The math producing the result is more complicated than what is found is in simple left/right stereo. When decoding the mid-side signal, the sides rely on cues from the mid signal and their relative phase in order to pan to the correct left or right channel. Because of this, if the mid signal is attenuated completely, for example, the sides will collapse into mono. On one hand, playing with this balance can be a good thing, and lead to a manual control over the width of the mix. However, if the components in your mid-side encoder or decoder are not impeccably clean, they will incorrectly alter the phase relationship of the two channels. This can produce undesired results ranging from a flattened stereo image, loss of vocals in the center, or loss in transient response.

From the polarity inverters to the switches, to the summing amplifier blending the mid back together with the sides, to the blend control circuit itself, everything has to be perfect. Many other products on the market claim to be able to offer mid-side processing but only the Dangerous MASTER is going to give you the wide, detailed, accurate sound of the original Chris Muth magic black box that made mid-side processing popular today.