Keith McMillen Instruments StringPort (Mac) Review

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FIG. 1: Despite its small footprint, StringPort''s audio interface sports stereo analog I/O; MIDI In and Out; USB 2 audio and MIDI; and 13-pin I/O from your guitar.

Guitar may not be the most natural portal into the world of computer-driven music, but you''ve seen plenty of ways to bring it into the digital world. State-of-the-art MIDI guitar pickups can now track like a bloodhound, but they have no direct way to translate the natural variations in tone and articulation inherent in electric and acoustic guitars.

In addition to MIDI, Keith McMillen Instruments'' (KMI) StringPort offers audio recording, musical interpretation of audio data for notation, and, most significantly, a variety of processing applications for complex and expressive sound design for guitar. StringPort''s polyphonic treatment of the guitar signal distinguishes it from other software guitar-processing applications. It is a hardware and software workstation for guitar that harnesses your computer''s horsepower to provide audio I/O, process guitar sounds, parse and send MIDI data, and host a rather extensive modulation matrix for MIDI and non-MIDI sounds. A Windows version is in development.

StringPort''s hardware interface is slightly smaller than a VHS videocassette. It connects to your computer''s USB 2 port, but requires a wall-wart power supply. Despite its relatively small size, StringPort sports stereo analog inputs and outputs, MIDI I/O, and D-13 input and output ports (see Fig. 1). The D-13 connector lets you pass your guitar''s polyphonic output to another device, such as a V-guitar unit or MIDI guitar controller that accepts 13-pin input (most do). You can choose 44.1- or 48kHz audio at 16- or 24-bit resolution.

StringPort supplants MIDI with the Acoustic Instrument Message (AIM) protocol, which provides a broad spectral analysis of string behavior and directly relays its parameters to the aforementioned modules. The analysis is similar to that of some MIDI guitar converters, albeit with more detailed information drawn from the vibrating string. Consequently, AIM is backward-compatible with MIDI and Open Sound Control (OSC).

The installation package includes several main software applications and a USB driver for the hardware. Once you register, you are provided with a license to run the software on a single computer. You can request additional licenses if you plan to use the device on more than one machine.

Each software application harbors what KMI calls Application Modules: suites of mini-applications with related features. For example, from the Processing application you can load four different process types: Waveguide, PolyFuzz, Phase Vocoder, and Smack.

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FIG. 2: StringPort''s Mainframe application accesses all of the analysis, setup, modulation, processing, and synthesis modules.

StringPort connects to your guitar with a D-13 cable, a standard connector for MIDI guitar. I used my Brian Moore iGuitar8.13 with built-in piezoelectric pickups, but guitars mounted with divided pickups such as Roland''s GK Series are fair game. As you might expect, there is a bit of setup work involved before StringPort will adapt to your playing style. The Mainframe module serves as a sort of control panel for StringPort''s functionality and a gateway to all of the other StringPort modules (see Fig. 2). From there, you start with the Analysis module. Clicking on the Audio Input button opens a separate panel for adjusting the trim of each pole of your instrument''s divided pickups. You can pick each string separately and have the application automatically trim the pickup based on the force of the attack.

Each string has its own visual tuner in the Input panel, as well as its own set of sensitivity and other parameters so you can tweak the tuning input and output of each string. The former is ideal for nonstandard instruments while the latter is useful when you want to output a tuning other than that of your instrument. Selecting the All button applies the same value to every string, a great time-saver when you want to have all strings at the same sensitivity setting or MIDI channel.

Beyond string gain, there''s more to tweak than I have room to describe, so I''ll move on to some highlights of the sound-shaping applications.

At the top of the Processing module is a window to store and recall any preset combination of processing modules. Just below, you can access four discrete DSP modules. From the top down, the first is the Waveguide module, which uses string impulses to model percussive sounds from short bursts to sustained atonal (and often harsh) timbres. Apart from a couple of blippy sounds with a subtle wah-wah effect, I didn''t find much nuance in the dozen factory presets. The timbral scope of the instrument seems fairly limited. It would benefit from additional waveguides and a few more modulation presets.

I suspected that PolyFuzz would be an emulation of the old hex-fuzz guitar synthesizers, but, in fact, it is a polyphonic guitar modeler. It offers a barrage of effects such as reverb, delay, chorus, ring modulation, pitch shifting, filters, and more. The result is extensively animated and processed guitar sounds. Pitch shifting sounded very natural. The Poly 12 String patch was outstanding: acoustic-sounding, fat, and warm. When mixed with other synths or processors, it produced stunning results (see Web Clip 1). Unlike a standard 12-string guitar, the G to low-E strings were tuned an octave below standard tuning and managed to sound crisp and clear, with none of the flabbiness associated with extreme downward pitch shifting.

Thanks to StringPort''s sophisticated contextual modulation menus, Phase Vocoder can use the guitar to alter sample playback on the fly. For example, using offsets, you can deploy velocity to alter the starting point of a sample, play back in reverse, or control loop duration or speed. All of the presets are geared toward playing loops with a rhythmic component rather than playing conventional sampled instruments, and there is no support for multisamples in that context. Pulldown menus for each string let you choose from Phase Vocoder''s pool of samples, and you can add your own WAV or AIFF files.

Some of the built-in synth sounds and the MIDI-triggered synths didn''t respond to open-string pull-offs. An Open Sens dial lets you increase the sensitivity to open-string notes. An excessively high setting will cause open strings to misfire, and I needed a setting of at least 90 percent—a rather narrow sweet spot.

SMACK, a processor module, is the show-stopper of the lot. SMACK is not an acronym, but somehow it''s descriptive of a drastically mangled guitar signal, and that''s exactly what it does. Envision a number of synthesizer modules controlled by the string envelope for each string. That''s a rough, albeit virtual, description of how SMACK works. It can produce sounds as diverse as a growling synth bass, a scatting wah-wah pedal, or a set of steel pans being attacked by angry, ring-modulated bees (see Web Clip 2).

Its Notation module is a basic 6-channel audio recorder combined with an Analysis window to parse audio data into MIDI notes. You can set a count-off, tempo, a time signature, metronome gain, quantization, and recording sensitivity. Next to Record, Stop, Play, and Clear buttons is a button to analyze the audio when you have finished recording. At the bottom of the module, you can choose to save the audio data and MIDI rendered from the analysis of your recording.

Of course, you can use StringPort to record audio and MIDI data into a full-blown DAW. StringPort leverages the concept of a divided pickup, allowing you to record audio from each string on its own channel. You can also route synthesizers and processors to independent tracks. Alternatively, and more simply, you can record everything from the main outputs to a stereo track.

As a unique hardware and software package for guitarists, StringPort is hugely successful. StringPort''s shortcomings are trivial compared to the advances it confers and those it promises. The user interface could stand some consolidation and refinement; its modular nature can become cluttered and bewildering when shuttling back and forth between modules. The processing and synthesis sections could use more varied and useful patches, and the synths could stand a bit more complexity. MIDI response is exceptionally clean, but it needs to be more responsive (see the Online Bonus Material at for details).

The tactile, responsive nature of AIM promises new and expressive musical protocols, yet despite the newness of the medium, StringPort has gotten so much right that I had no sense of a bleeding-edge technology. Even with its few rough edges, I unequivocally recommend StringPort to anyone looking to explore new and exciting trails to guitar''s future.

Marty Cutler is co-author of MIDI for Guitarists. He''s also a well-known bluegrass banjo picker—go figure.

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Click on the Product Summary box above to view the StringPort product page.