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Real World Computing

Model behaviour

Posted on 22 Nov 2005 at 15:26

The technology your PC uses to make sound is usually based on replaying an audio sample. Brian Heywood looks at alternatives

Back in the early days of PC sound, the main player in computer sound synthesis was usually some variant of the Yamaha OPL chipset, which made a noise reminiscent of a wasp trapped in a jam jar. This chip was based on the FM synthesis algorithms originally used by Yamaha for its DX/TX digital music synthesizers. While the quality of the sound on these early cards was pretty poor compared to today's standards, it was good enough to propel Creative Labs from a small Singapore-based electronics company formed by Sim Wong Hoo in 1981 into a major corporate world player, which incidentally now owns a large chunk of the world's music synthesis development research expertise.

Without delving too deeply into synthesizer technology or history, FM synthesis was Yamaha's take on the leap from analog to digital synthesis. This leap was made possible by the application of DSP (Digital Signal Processing) technology, originally developed for military RADAR systems, to the audio domain. Yamaha's application of John M Chowning's algorithms, which he'd developed at Stanford University in the early 1970s, led to a family of affordable digital music synthesizers that more or less provided the soundtrack to the 1980s. When Yamaha packaged this technology into an OEM chipset, it probably had no idea that it was providing the raw material for an explosion in PC sound card manufacture. First Adlib, then Creative Labs, based their sound cards on this compact chipset and, along with the games authors, created a mass market for high-quality sound hardware.

The demand created in this new marketplace drove up standards as the hardware vendors tried to get a quality edge over their competitors. One way they did this was by either developing new technology or buying in expertise from the musical instrument manufacturers: for instance, Creative bought the sampler pioneer E-MU Systems in the early 1990s. Electronic musicians benefitted both by adopting the increasingly sophisticated PC-based sound generators for music production and by taking advantage of the decrease in price caused by the intense competition. Several of the traditional electronic musical instrument manufacturers also piled into the market, including Roland with its RAP-10 and Sound Canvas PC cards, and Yamaha with the SW1000XG. The bottom line was that by the mid-1990s, musicians and producers had access to audio tools that just a decade before were available only to the very rich or those who had the time and expertise to develop their own hardware and software.

So sound card technology moved onwards to sample-based replay, initially employing wavetable methods but with higher-quality systems using either ROM- or RAM-based sample players. The former contains short samples of different instrument sounds stored in the player's ROM, which are then looped and used like simple oscillators for the sustained portion of the instrument sound. The latter approach is essentially the same technology as found in professional music samplers in a recording studio: in fact, Creative Labs' subsidiary E-MU developed a standard, called SoundFont, which allows the core instrument sounds to be portable between different systems.

Getting Physical

Given the advances in processing power of DSP chips and the increasing expectations of both musicians and listeners, it isn't surprising that the technology would move on from simply replaying an existing audio sample to something more sophisticated. Again, Stanford University was at the forefront when a way was developed there to simplify the mathematical model of a stringed instrument to the point where current DSPs could perform the calculations required in real-time. Snappily entitled Digital Waveguide Synthesis, this concept was developed by Professor (and rock musician) Julius O Smith III from an idea he had on a bus in 1985 and resulted in the first Virtual Modelling synthesizer, the VL1, being launched by Yamaha in 1995. The same approach can also be used to model wind instruments, drums and even the human voice.

Other applications of this computer modelling idea range from simulating the characteristics of different guitar amplifiers and acoustic characteristics of instruments, or even simply modelling the attributes of an acoustic space, like the reverberant field of a concert hall. In sound synthesis, the big advantage of using modelling is that you can add performance nuances, such as vibrato and pitch bending, to the generated sound simply by tweaking the parameters of the mathematical model that describes the instrument. This approach means the result will approximate the way a real instrument behaves. The downside is that your synthesizer takes up a lot more processing power and it takes a lot more effort to create an instrument.

In fact, these downsides seem to be dominant at the moment, because the only PC Card I know that employs physical modelling is the Yamaha SW1000XG when used with the PLG150-VL or PLG150-AN daughterboards. Strangely, the AN variant models analog synthesizers rather than acoustic instruments, which means you're using a powerful DSP to simulate some relatively simple analog circuits. I guess the law of diminishing returns is partly to blame - there was a vast improvement in the quality of audio when sound cards moved from OPL FM synthesis to sample-based techniques. However, while the improvement in sound quality from sampling to modelling systems is significant, it doesn't have that 'hit them straight between the eyes' quality of the previous quantum leap.

The only area where acoustic modelling seems to be making a real impact is in the amplifier and instrument modelling area. Both Roland and Line 6 have products that give guitarists a wide range of guitar effects and amplification options without requiring them to carry round a truckload of actual equipment. These devices represent a sort of halfway house in the world of modelling, as they use the signals from the actual vibrating guitar string as the excitation input for a software model that's running in the DSP. It makes a great deal of sense to do it this way, since the vibrating strings also act as the performance interface, 'encoding' performance aspects like string bends and vibrato. A completely modelled synthesis alternative would need to find some other way to detect such performance nuances, which would be a formidable task in its own right.

Careful with that Variax, Eugene

Line 6's take on this technology is to build the instrument-modelling electronics into an actual instrument, and so it produced a series of electric, acoustic and bass guitars that go under the name of Variax. All these instruments employ individual piezo-electric pickups placed under each string to provide the excitation for the modelling circuitry. The processor, which is built into the guitar body, modifies this signal to give it the attributes of a wide range of different types of guitar, from various classic makes of solid body electric, semi-acoustic and acoustic guitars, to some tenuously related string instruments like sitar, banjo and various types of resonator guitars.

I've been trying out a Variax 700 recently, and after changing the original strings it turns out to be a pretty decent instrument. The built-in DSP must be pretty powerful, because it doesn't exhibit any switching delay, which is my personal bugbear with digital audio guitar signal processing systems. As you'd expect, the sounds themselves are pretty accurate (at least as far as I can tell), although it's important to realise that the playing style and context in which you use the sounds can affect the realism. For me, the least convincing model was the 12-string simulation, and I'd guess that's because the pitch for both strings in each course is directly generated from a single guitar string vibration, so the note frequencies are exactly phase-locked, which wouldn't happen on a real instrument with paired strings.

You can use the Variax as a standalone instrument, but you'll get more out of it if you use it with either the Line 6 Vetta II FVB or the PODxt Live foot controllers. In this case, you connect the guitar to the floor unit via a Variax Digital Input Cable, which is essentially a ruggedised RJ-45 network cable. This connection does a number of things: it supplies external power to the instrument electronics, allows the guitar's built-in systems to be controlled remotely, and provides for a digital audio signal path. This last bit means you can keep the audio signal in the digital domain right through to the computer's hard disk, which is something of a recording engineer's Holy Grail. You can do this because the PODxt has a USB port and lets you use the unit as an audio interface for the PC. Keeping the initial (and only) analog-to-digital conversion as close to the point of sound generation as possible means you avoid all the sources of signal degradation that intervene once you run an audio signal through analog electronics and wiring.

The PODxt Live implements the simulation of stomp box effects, amplifiers, speaker cabinets and the room acoustics within which the virtual amplification setup is used. It has 36 classic and modern amp models, with 24 speaker cabinets, and to replicate the effect of the acoustic space in which you're operating the virtual amplifier setup, you can select between three types of microphone (one with two positions) along with control over how much of the virtual room's 'ambient' reverberant field is added to the overall sound. All these parameters can be controlled from the unit's front, or rather top, panel and the resulting configurations can be saved into 128 internal non-volatile memory locations.

To be honest, programming the beast directly is something of a chore, as it's designed to be a foot-operated floor-based unit. A much better approach is to download the free Java-based editor from the Line 6 website and program the PODxt using your PC, connected either via MIDI or its USB port. The software gives you two windows: one to manage the POD's memory banks and the other to edit the internal configuration. The whole thing is intuitive, which is just as well as the help system is pretty sparse and documentation non-existent. A cute feature is the facility to download POD configurations from the Web, either created by Line 6 or submitted by users. You can also submit your own configurations if you want.

Variax Workbench

While the Variax is an interesting instrument in its own right, integrating it with your PC using the Variax Workbench application means it's pretty much a whole new breed of guitar. Basically, it allows you to totally reconfigure the Variax in real-time from your PC, as well as letting you save the guitar's internal configurations onto your hard disk. You can alter the number, type, position even the angle of the pickups, as well as selecting the body type, tone control parameters and so on. You can even alter the basic tuning of the instrument to give you any open tuning you might require. It does this by pitch-shifting each individual string frequency across the neck, sort of like having a separate capo for each string. And since you can save these settings back to the guitar, you can do cool tricks like retuning your guitar for slide playing or folksy acoustic picking at the flick of a switch.

While I doubt the sounds would completely satisfy a serious guitar connoisseur, the combined Variax Workbench system gives you access to a wide range of 'standard' tonalities without having to spend a lot of dosh buying the real instruments. The flexibility of the Variax/PODxt system means you can access or create just about any possible guitar sound on the planet, along with some impossible ones. The Variax Workbench software costs about £70 (software plus USB interface) or is available free to anyone one who has a Vetta II or PODxt Live.

There are some little niggles: for instance, you can't run the POD and the Variax editors at the same time, since they won't share the USB port and the Variax Editor doesn't have the Internet download facilities of the POD software. However, the editing applications are still being developed and the firmware on the Variax can be upgraded from the Line 6 website using a useful utility called the Line 6 Monkey. Perhaps someday all instruments will be like this.

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