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Line 6: Inside the Tone Modelling Factory

Marcus Ryle • Michel Doidic • Jeff Slingluff By Paul White
Published March 2006

Marcus Ryle (left) and Michel Doidic of Line 6.Marcus Ryle (left) and Michel Doidic of Line 6.

We go behind the scenes at the home of the Pod and Variax to get the latest on developments in the world of guitar modelling technology.

Most musicians know of Line 6 through their digital modelling guitar amps and Pod preamps, but few appreciate the lengths they go to in order to recreate the sounds of classic amps and, more recently, guitars. However, to really understand Line 6, you need to know a little about its two founders, Marcus Ryle and Michel Doidic, who previously worked together under the banner of Fast Forward Designs, where their developments helped to bring such milestone products as the Alesis ADAT into the world.

"That Call's For... Line 6"

Michel, who also enjoys playing guitar, gained a Bachelor of Science degree in electrical engineering from the University Institute of Technology in Angers. Marcus Ryle operates as the company's Senior Vice-President of Product Development, but in a company like Line 6, engineering skills are only part of the story, as musical ears are also important. Marcus has a background as a classically trained pianist, but he was soon seduced by the dark side of electronic keyboards and has been involved with a number of TV and film soundtrack projects, as well as working with professional recording artists as diverse as Barbara Streisand and Chicago.

Both Marcus and Michel worked as design engineers at Oberheim during the '80s, leaving to set up Fast Forward Designs in 1985. Their new company designed technology behind some of the most famous audio products of that era, including the Alesis ADAT range, Quadraverbs and Quadrasynths. They were also involved in Digidesign's Sample Cell, the Fostex RD8 recorder and numerous other pieces of hardware from companies such as Studer, Dynacord, and Steinberg. Eventually, they decided to move away from designing hit products for other people to doing it themselves, and as Marcus Ryle explained to me, DSP chips were just starting to get cheap enough and powerful enough to allow guitarists a taste of the sound design freedom that synthesizer players already enjoyed. In 1996, Line 6 started out on the road to guitar-amp and effects modelling with their first product, the Axsys 212 guitar combo. Apparently the name Line 6 came about because Marcus and Michel were cooking up some guitar-related ideas back in their days at Fast Forward, and to keep their endeavours under wraps, they had all guitar-related phone calls diverted to telephone line 6.

Line 6: Inside the Tone Modelling Factory Line 6: Inside the Tone Modelling Factory Line 6's first product was the Axsys 212 guitar combo (top), but the one that really made their name was the Pod (middle). Their most recent are the Toneport audio interfaces, reviewed in last month's SOS.Line 6's first product was the Axsys 212 guitar combo (top), but the one that really made their name was the Pod (middle). Their most recent are the Toneport audio interfaces, reviewed in last month's SOS.The company is now presided over by Mike Muench, who holds a Business Administration degree from Harvard University and a Bachelor of Science degree in Business Administration from the University of Southern California. Mike joined the company in 1998, prior to which he was vice president of the US Consumer Division at Apple. Could this be why almost every computer I saw at Line 6 is an Apple Mac? Like everyone else I met at Line 6, Mike also has musical connections, and he's worked professionally performing and arranging.

Today Line 6 employs approaching 250 people worldwide, which is a big step up from their original 10-strong team. Research and development is done in their Agoura Hills facility, half an hour's drive north-east of Malibu in California. Warehousing is taken care of in another California facility, but to keep costs down, all their manufacturing is done in China, where Line 6 also maintain an engineering office to liaise with their manufacturing companies. Designs for tooling and test equipment originate in the US, but the toolmaking itself is done in China using CAD blueprints created at Line 6 Agoura Hills. On my tour of their facility, the design department showed me their 'Easy Bake Oven' 3D plastic fabricator which 'grows' 3D plastic parts direct from CAD data. Using this they are able to create and test prototype parts and housings very quickly, including the housing for their new Toneport audio interface hardware.

What struck me during my tour of Line 6's various departments was just how nuts about music, especially guitars, everyone was. All the corridors bear plaques named after famous guitar players (many British I'm pleased to see — see overleaf), and after work, a number of the staff often stay behind to rehearse as a band on a small stage at the end of the cafeteria. Everyone lives and breathes the fine details of guitars, amps and effects pedals to the point that the term anorak could easily be applied (in the nicest possible way) to most of them. Over dinner, I had a long conversation with James Santiago, a very fine guitarist who is often seen demonstrating Line 6 products to their best advantage, and the conversation didn't stray far from the insides of amplifiers and effects pedals until the dessert trolley arrived.

In the design department, I was given an insight into the origins of the Tonecore pedal range, which started life as a simple cardboard mock-up to help the designers position the controls and connectors. Next, I was shown a book full of artist's sketches of what the pedals might look like. Apparently these are passed around key people to get some impression of which is likely to be best received by the target market. There are also some engineering niceties built in at the design stage, such as access to the battery compartment without the need for tools and an external power socket that can accept AC or DC power adaptors, thereby increasing the chance that the user will already have a PSU that will work with the pedal. Once a design had been settled on, CAD drawings were produced and a prototype shell grown from plastic so that all the dimensions could be checked against the circuit board and parts.

Fun With Hum

Lead sound designer Jeff Slingluff told me he once set up a blind listening test for some industry guitar players to see if they could tell the modelled Line 6 amp from the original. To start with, the listeners always picked out the Line 6 model even before a note was played — because the hum went away whenever the signal was switched to the Line 6 amplifier! To make things fairer, he set up a third tube amp between the two, and left it ticking over with no input so that there was background hum all the time, and after he'd done that, the results were split almost 50/50, which is exactly what you should get if the sounds are too close to tell apart.

Guitars & Modelling

Before taking a look at the various R&D departments around the factory, I had the chance to talk to Marcus and Michel, and was curious to find out why they had chosen the road of guitar modelling rather than, for example, synthesizers. Marcus explained that while synthesists were already well catered for, guitar players still found it difficult to get exactly the sound they wanted and in the mid-'90s, DSP power was getting cheap enough to apply to finding a solution. While many players can set up one sound that works for them, life becomes more difficult when they need to switch between sounds, and where external effects are part of the sound, repeatability also becomes a problem. The Line 6 approach was to model not only the sound of the amplifier but also the speaker cabinet and the way it would be miked in a studio session. On top of this, the company developed numerous effects algorithms including a digital emulation of the spring reverbs used in typical guitar combos, and all this technology was put into the Axsys 212 combo. Though this was very easy to use when compared to keyboards, some guitar players still felt it was too daunting, so when the next range of amplifiers was developed, the Line 6 engineers moved much closer to the look and feel of a conventional guitar combo.

The Amp Room, where Line 6 conduct their modelling tests, features a bewildering array of old amps and guitars.The Amp Room, where Line 6 conduct their modelling tests, features a bewildering array of old amps and guitars.It's probably fair to say that the first Line 6 product to really worm its way into the public consciousness was the original Pod, with its distinctive kidney shape and user-friendly controls. This was particularly relevant to SOS readers, as it simplified the process of recording electric guitar sounds and solved the problem of noise, especially when working late at night. As DSP chips became more powerful, the improved Pod XT was developed along with the later Flextone, Spider, and Vetta amplifier ranges.

Part of the appeal of products like the Pod XT, and the later Pod XT Live, is that their software can be updated and expanded. System software updates are generally available as free downloads and can be transferred into the device via USB using the Line 6 Monkey software, but Line 6 also saw the market potential of selling additional amplifier and effects packages known as Model Packs, which add to the repertoire of the device in much the same way as the add-on cards that synth players use to get new sounds and new presets. Currently the Vetta amplifier range is the most powerful and most sophisticated Line 6 amplifier, capable of running two amplifier models at the same time, but owners of the Pod XT or the new Toneport audio interfaces can use the optional Model Packs to upgrade their systems to take advantage of all the Vetta amplifier and effects models.

I asked Marcus and Michel whether they thought DSP was finally fast enough and cheap enough that they no longer had to compromise their algorithm designs to get the product to market at an affordable price. Marcus replied that he felt DSP power was now far less of an issue than it once was, but power consumption was still a concern in the case of battery-powered devices, such as effects pedals and portable amplification. One factor I hadn't appreciated was that although the Tonecore pedals all use exactly the same DSP engine, battery life is heavily dependent on how busy the DSP chips are: some algorithms use power at a greater rate than others. They also let me into a secret regarding these pedals — the control section, which also contains the effect algorithms, is in fact a removable module, so by fitting a different module to any pedal in the range, its function can be changed. The DSP itself is part of the main body of the pedal, but as every pedal has the same DSP engine, any module can be used in any pedal. This modularity is clearly part of a future marketing strategy.

One of the corridor signs at Line 6 headquarters.One of the corridor signs at Line 6 headquarters.As I understand it, the original Pods modelled the behaviour of discrete sections within a guitar amplifier to replicate the behavour of the original EQ, preamp distortion, power-amp distortion and so on. I asked Marcus whether convolution techniques were applicable to guitar amp modelling now that we have enough DSP power to handle it, and he surprised me by revealing that convolution was one of their 'secret weapons' and had been used since the very first Pod as a means of replicating the speaker cabinet and its miking arrangement. Convolution is essentially a brute force number-crunching process most often associated with 'sampled' reverb, but the longer the reverb, the more number-crunching is needed, and the more DSP power that takes up. In the case of miked cabinet modelling, the events being measured are relatively short, which is why Line 6 were able to use convolution right from the start. It also explains why they didn't offer the choice of more distant microphones, as the greater the distance, the longer the convolution period required.

To model the cabinets, they were first set up in a typical recording studio, on the floor, with the correct mics in the designated place, as listed in the Pod or amp settings. If the manual says 'SM57 On-axis', then that's what was used to make the measurements. There's always some interaction with the room, and the engineers wanted the result to be as authentic as possible in reproducing a 'studio' sound, which is why they didn't measure the cabinets in an anechoic chamber. This set me thinking that if you did want the sound of an amplifier miked with both a close and a distant mic, it should be possible using a Line 6 amplifier (or a Pod connected to a suitable amplifier and guitar speaker) if you take the DI feed as your close mic signal and then place a conventional mic at a distance from the speaker to add in as required.

Marcus and Michel seem to have an unwritten code, so if you ask them a question and they start their reply with the word 'interesting', it means that what you have asked is probably already under active consideration, and that further probing won't get you too many answers! I discovered this when I turned my attention to Variax, which takes the modelling concept and applies it to guitars. Variax has already shown that it can effectively replicate the sounds of various classic electric and acoustic guitars and other stringed instruments. Variax uses a divided pickup arrangement, which is necessary for the accurate emulation of slanting pickups, and also allows a pitch-tracking algorithm to be used to optimise the operation of the pitch shifter used for 12-string sounds or for open tunings, which can be set up using the Variax Workbench software (of which more shortly).

Paul White tries to sneak a look at a guitar pedal prototype. "No peeking, now..."Paul White tries to sneak a look at a guitar pedal prototype. "No peeking, now..."This led me to wonder out loud whether Variax would be capable of producing some novel synthetic sounds based on the waveforms generated by the strings — after some heavy processing of course. After all, the ancient Roland GR300 guitar synth worked entirely from the string waveforms, which avoided all the tracking problems associated with most other guitar synths, and although its capabilities were extremely limited, it was still a very musical instrument. Using modern technology, wouldn't it be possible to use Variax to bring this concept up to date, especially given Marcus' interest in synthesizers? Indeed, if the six Variax string signals could be piped into a computer via USB or Firewire, it might even be possible to create software synthesizers driven from Variax. The one-word response to my ponderings was 'interesting!'.

Continuing on the guitar synthesis theme, I asked Marcus why they hadn't included a Roland-compatible output on the Variax, in order that anyone wanting to use a Roland-compatible guitar synthesizer could do so without adding a special pickup to their guitar. After all, the type of person interested in guitar synthesis would overlap with those interested in a technology instrument like Variax, and given that Variax already derives a separate output from each string, this would probably be relatively easy to do.

Their reply was that they didn't want to do this originally, because they wanted to distance Variax from sample-based technologies to avoid confusion. They needed guitarists to understand that Variax really was a guitar and that what you heard from it was the guitar strings themselves, not samples of guitar strings. However, now that Variax is established, my observation is apparently 'Interesting'...!

During my tour of the facility, I asked lead sound designer Jeff Slingluff whether the piezo pickups used in the Variax had a different dynamic response to traditional magnetic pickups. They're totally immune to electromagnetic pickup from monitors, transformers and mains power wiring, which eliminates the usual guitar hum problem, but don't they produce a different sound or playing feel?

Jeff explained that their modelling algorithms apply different decay times to different frequencies, so that what comes off the strings after processing matches the real thing very closely. In the case of the model that's clearly based on the behavour of the Fender Stratocaster, they've even modelled the way the tremolo springs resonate in the rear body cavity, which adds a kind of low-level, coloured reverb to the sound. They start by modelling the pickups and their magnetic aperture, which combined with their position, affects how they pick up the sound from the strings, both singly and in combination. The pickup characteristics combined with the modelled body resonances produce the final emulation. Because the decay of different frequencies can be controlled, non-guitar instruments such as the banjo and sitar can also be recreated using only the output from the strings as their source. I asked Jeff if they deliberately modelled the effect of the magnetic string pull exerted by the pickups, as this also affects the way the note decays, and in some cases introduces atonal harmonics. He told me they hadn't done this specifically, but the way the pickups affect the decay rate of the different harmonics is automatically recorded in their measurements and so this aspect of string pull is modelled.

Modelling Vintage Effects

One of my interests is how to capture the sound of 'classic' effects such as tape flanging and tape echo. Line 6 have already done a lot of work on tape-echo simulation, and Marcus explained that they feature vintage modelling in their new Echo Park pedal, which has switchable analogue, digital, and tape delay characteristics and is based on their popular DL4 delay processor. In the case of the analogue delay simulation, they've even captured the way the clock noise interacts with the audio, which is particularly important in recreating the sound of those old 'bucket-brigade' flanger boxes. The same approach was used in their Liqua Flange pedal, which has digital, analogue and liquid modes, the latter using two delay lines to allow the flange point to pass through zero time, as it does with tape flanging whenever one of the two tape machines overtakes the other. To try to recreate some of the randomness of tape flanging, they've also used a smoothed, random modulation generator, which can be used as an alternative to the more common modulation waveforms (there are 11 different modulation waveforms in all, including random steps). This attention to detail gives these pedals a much more organic sound than we've come to expect from conventional pedal effects and to my ears, the Liqua Flange's analogue mode gets closer to the elusive sound of the original Electric Mistress than anything I've heard before — but without the noise.

Build Your Own

At a later product demonstration, I saw the Variax Workbench software being used, which allows the user to create guitar configurations that aren't available out of the box. Despite being very complex under the surface, Variax Workbench is intuitive to use and allows you to choose any pickup or pickup pair combinations and put them in any position on any of the available guitar bodies, or, indeed, position them part-way up the neck! The pickup angle can also be adjusted, and the sound updates almost instantly, so you can play as you tweak to see what works. It struck me at the time that if the graphics also provided dimensional data for the pickup placement and angle, Variax Workbench would make a very valuable design tool for custom guitar builders. To be truly effective in this role, it would need to include the ability to experiment with different coil taps and series/parallel wiring options, but wouldn't it be great to see how a Telecaster might sound with, for example, a Gibson P90 pickup in one position and a Rickenbacker pickup in another? In fact, my only regret was the limit of two pickups in the current version, because I wanted to try filling the entire space between the neck and bridge with pickups to see if it recreated that original Spinal Tap sound! Variax Workbench is free to owners of the Variax and either a Pod XT Live or a Vetta II.

Antiques Roadshow

A highlight of the facility tour was Jeff showing me around Line 6's vintage amplifier collection — a whole room full of them. These are not just any old examples either — they're all chosen for their sound, which leads onto an interesting aspect of modelling. Jeff explained that although you could model every single resistor and capacitor in the original circuit, it would take an enormous amount of DSP power and still might not produce the optimal result, purely because of the effect of component tolerances in the original. Most older amplifiers used resistors that had a 10-percent tolerance range or even greater, so depending on the cumulative effects of all these variations, you could get either great-sounding amplifiers or not so good ones. The valves are also very important. After Line 6 had purchased a great-sounding early Marshall amplifier that used KT66 output tubes, disaster struck when it was plugged in to do the modelling measurements. The whole thing erupted in a shower of sparks, and the entire set of output valves needed to be replaced. The original tubes were made by Mullard, and when the team tried to find a replacement set, they found them very hard to come by. According to Jeff, buying the new tubes was almost like conducting a drug deal, involving late-night meetings and bags of cash — and the four tubes they needed, though used, set them back a staggering $450 each!

Sound designer Jeff Slingluff denies everything!Sound designer Jeff Slingluff denies everything!While some modelling does go down to component level — and the exact details of the process are kept secret for obvious reasons — Jeff did tell me that they used fairly standard test equipment, such as Audio Precision test sets, to gain some of their data. They also need to look at the dynamic way in which amplifiers distort in response to transient sounds and also how things like power-supply sag affects the sound. Using this information, they can recreate the behaviour of the various key building blocks that comprise the signal path, and I was surprised at the lengths they go to. For example, when a power supply sags after hitting a loud power chord with the amp turned up, power supply ripple breaks through and modulates the audio to a small but significant degree. Line 6 have even replicated this so that the UK amps impose a 50Hz ripple under heavy drive conditions, whereas the US amps have 60Hz ripple!

Another thing that doesn't always come across with modelling is the way the speakers interact with a tube and transformer output stage, as a tube amp is effectively a current device whereas a traditional solid-state amplifier is a voltage device. To get around this in the current Flextone and Vetta amplifier ranges, they use solid-state power amplifier circuits that operate in current mode which, according to Jeff, gets much closer to the trouser-flapping experience of a real tube amplifier. The Pods don't have this, but then their aim is to replicate the sound you would hear over the control room monitors in a typical recording session involving a miked amplifier, not the sound you'd hear standing in front of a stack.

Currently the amps and Pods include user-adjustable noise gates (after the amp but before effects such as reverb and delay) to help clean up the noise, which is inevitable when a guitarist uses a lot of high-gain overdrive. I asked Jeff why they didn't use dynamic noise filters (sliding filters that progressively filter out the high end as the sound decays below a threshold) as I've always found these to be more benign-sounding on guitar tracks than gates. He told me they do use dynamic noise filters in the Variax guitars, and though he didn't say so, I got the impression they were considering them for future amp models or updates. I could swear he used the word 'Interesting'!

From Hardware To Software

Though Line 6 is primarily a hardware company, they have a strong software side, first aired in their Guitarport product (which strove to introduce guitarists to basic loop-composition and multitracking techniques), and now seen in the likes of Variax Workbench and Gearbox, the software that accompanies the new Toneport interfaces (shown below, and reviewed in last month's SOS). For me, these are the most exciting new Line 6 products. Given their attractive pricing, they'd stand up on their own as practical USB audio interfaces, but what sets them apart from others is that the hardware is only a small part of the story — the Gearbox software is really the heart of the product. It's based on selected Pod XT amp, cabinet and effects models, and runs them on your Mac or PC computer. The interface hardware acts as the copy-protection dongle for the software, which can't be run without it. As you'd expect from a Line 6 product, Gearbox functions as an amp modelling plug-in, but Line 6 have cleverly developed what they call Tone Direct monitoring, so that Gearbox operates on the audio input before it ever gets to your audio application. The reason I say cleverly is that Gearbox itself has extremely low latency, and because it operates outside your host application, it isn't affected by the buffer size set for your audio-recording software. This means you can always hear your guitar in real time as you play, even if your audio software is set to a very high buffer size. Post-amp effects such as reverb and delay can be set to be recorded along with the guitar sound, or you can use them just for monitoring if you wish to add other plug-in effects after recording. Gearbox can also be upgraded to a full Pod XT spec or beyond by adding optional Model Packs from Line 6's web site, including all the Vetta amp models.

The Gearbox software that accompanies the new Toneport interfaces.The Gearbox software that accompanies the new Toneport interfaces.

The package includes selected bass amp models taken from the Bass Pod XT and some brand-new vocal preamp models based on real-life hardware, some of them top-shelf classics (old and new), some lo-fi and some just plain quirky. These are augmented by new vocal compressor and de-esser modules. I asked Jeff Slingluff whether there was really that much difference between vocal preamps, and he replied that although they may measure similarly with steady test signals, their dynamic responses are often very different due to the use of audio transformers or time-dependent signal feedback paths in the circuitry. Gearbox lets you use a different signal path for your mic input via the vintage preamp models, compressors and reverbs, while recording your guitar at the same time via its own amp and effects chain. What's more, all the main effects can be used on guitar, bass or vocals, so if you need (say) a wah-wah vocal, you can have it.

Interesting And Friendly

Whatever they do, Line 6 want to make it musician friendly. At the time of my visit to Line 6, the Winter NAMM show was just around the corner, but nobody would confirm whether they would have anything new there to show us. Whatever may come next, I think it is safe to say that the future of Line 6 is going to be 'Interesting!'.