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Oberheim Strummer

Keyboard to Guitar Chord Converter

Article from Music Technology, November 1991

Latest in Oberheim's Perf/X series is a unit designed to bridge the gap between keyboard and guitar. Simon Trask perfects his Jimi Hendrix impression.

Oberheim's latest Perf/X MIDI processor helps you create realistic guitar parts - but it can also give you interesting "out of instrument" experiences.

FOR CENTURIES, A musical instrument's physical dimensions, the materials used in its construction, and the means of getting a sound out of it (such as plucking or bowing a string or blowing into a tube) have determined not only its physical identity but also its sonic identity (its overall sonic character and the variations in sound a skilled performer can get from it) and its performance identity (its pitch range, the number of notes that can be played simultaneously, the combinations of notes which are possible, and the ways in which notes can be articulated). Collectively, these identities have always defined the essential nature of each type of instrument - the "guitar-ness" of a guitar, for example.

Once we enter the electronic realm, however, things are rather different. A digital sampler allows us to capture the sounds of any acoustic instrument we want, but it doesn't allow us to capture the instrument's complete sonic identity, because a sampled sound is no longer rooted in the physical characteristics and physical constraints of the instrument from which it was sampled. However, physical modelling software currently being developed in academic centres like Stanford University's Centre for Computer Research in Music and Acoustics could allow an instrument's sonic identity to be captured much more accurately. If sampling merely gives us snapshots of an acoustic instrument's sonic identity, software which models the physical characteristics of an instrument could eventually give us the movie.

Modelling the performance identity of an acoustic instrument in the electronic realm is something we can all do for ourselves, however - in fact, you could say the solution is in our hands (or under them). Anything you play on an acoustic instrument is automatically legitimate for that instrument, but if you've called up, say, a multisampled guitar sound on your sampler or sample-based synth in order to add a guitar part to your latest track, chances are you're going to be playing the part not on six strings but on a keyboard - in which case it's all too easy to play parts which aren't legitimate for the real thing.

Not that there's anything wrong with doing that, but if you want to create guitar parts which sound realistic, the best way to do it is to play guitar parts which are legitimate - which means you need to emulate the guitar's performance identity on the keyboard by, for instance, keeping within the note range of the guitar, playing no more than six notes at once, playing guitar-legitimate chord voicings and mimicking performance techniques characteristic of the guitar, such as chord strumming.

All of which is easier said than done, of course. Cue the aptly-named Strummer, a MIDI processor from Oberheim which aims to take some of the strain out of creating realistic guitar parts.


STRUMMER TURNS YOUR keyboard chord voicings into guitar-legitimate chord voicings. It also adds simulated chord strums to the latter by reading the MIDI output of your keyboard as you play, manipulating the MIDI note and velocity data of your performance in real time and transmitting the altered performance to your MIDI sound source. It's this altered performance, not your keyboard performance, that you want to hear. Consequently, if you're using a synth or a sampler as both keyboard and MIDI sound source, it needs to have a Local On/off facility so that you can silence your keyboard performance but not the "Strummer-assisted" performance - DX7 owners take note. Strummer responds to MIDI note and velocity data on a single, user-selectable MIDI In Channel, and transmits the manipulated performances on the same channel; any incoming data on the remaining 15 MIDI channels is passed straight to Strummer's MIDI Out port unprocessed. To bypass Strummer's processing altogether, you can either press the dedicated Bypass button on the front panel or else set the MIDI In Channel parameter to "off".

The results of Strummer's manipulations can be impressive. For instance, if you call up Preset 01 on Strummer and play a root-position E minor triad on the keyboard, Strummer turns your triad into a strummed six-note E minor chord which, if you've got a multisampled acoustic guitar selected on your MIDI sound source, sounds extremely realistic. Strummer uses the guitar voicing EBEGBE - the classic beginner's chord, requiring only the fourth and fifth strings of the guitar to be fretted. However, the sense of realism comes not only from the legitimate chord voicing but also from Strummer's very convincing simulation of guitar strings being strummed. Play a few more root-position triads and some first, second and third inversions and Strummer produces the open chord voicings of a thousand and one folk songs.

However, if you don't want to do Bob Dylan impersonations all day long or find yourself playing the chords to 'Streets of London', select a distorted electric guitar sound, call up Preset 02 and play a few fifths on the keyboard and you can pretend you're a heavy metal guitarist, barred power chords and all. If this doesn't take your fancy, stick with the acoustic guitar sound, call up Preset 06 and you can discover another feature of Strummer: arpeggiation. Play a note on the keyboard and Strummer generates a minor 7th arpeggio which uses your note as its root; play another note and you get another minor 7th arpeggio, this time based on the new note. Strummer provides you with a choice of four arpeggios (minor 7th, octaves, up/down octave and 5ths) and three scales (chromatic, whole-tone up and whole-tone down), but doesn't allow you to create your own note sequences, which is a shame.

Play two notes a fourth apart and you get two simultaneous minor 7th arpeggios a fourth apart. Not exactly something you could play on the guitar, but, what the hell, you could be two guitarists - or, bearing in mind that we're in the latter part of the 20th century, you could be routing your guitar through a harmoniser.

Talking of effects, Strummer provides mono echo/delay processing - of the MIDI rather than the audio kind. So if you run out of spare effects processors you can always press Strummer into service as an echo unit for MIDI instrument parts - and because its echoes are actually MIDI notes you can record them as part of a sequence, and therefore free up Strummer for use on other parts. Another advantage of MIDI-created echo is that you can do odd things with it. As well being able to have up to 99 repeats and an echo repeat rate of anywhere from 1-99 MIDI clocks (allowing the echoes to be synced to tempo), you can set a transposition interval for the echoes anywhere within a ± one octave range, and create echo fade-outs. The repeat rate is referenced to either an internal programmable tempo or an external tempo. The repeat number and repeat rate settings also apply to Strummer's arpeggios and scales when they're enabled, and so by extension the arpeggios and scales are also referenced to internal or external tempo.

Another feature, Chord Capture, is in effect a monophonic performance mode which allows you to trigger chords instead of individual notes. With Chord Capture enabled for a Preset, any chord you play on the keyboard is automatically "captured" in Strummer's memory and can immediately be triggered from single notes. In response to different trigger notes, Strummer can either transpose the captured chord, treating the trigger note as the lowest note of the chord, or else create an inversion of it and transpose the inversion into the octave of the trigger note.

To change the chord, all you have to do is play the relevant chord on the keyboard and then revert to playing single notes again - spontaneous stuff.

"Strum Mute is presumably meant to mimic the effect of damping guitar strings with the right hand - in use it's extremely effective."

And talking of spontaneity, Chord Capture can be switched in and out at any time, providing it's enabled for the active Preset, by clicking on a footswitch connected to footswitch input three on Strummer's rear panel. So one moment you can be triggering a captured chord, the next moment you can be playing a succession of single notes, then the next moment you can be triggering chords again. It's a simple but thoughtful touch which greatly enhances the flexibility of Chord Capture by ensuring you're not locked into it when playing.

To an extent, transposing a chord shape up and down a keyboard by playing it from different keys produces the same result as shifting a chord shape up and down a guitar neck by moving your left hand to different frets. I say "to an extent" because these are different physical activities requiring different degrees of physical movement, co-ordination and exertion, and the differences have a bearing on the end result. The physical act of performing makes its mark on the performance. Consequently, creating realistic guitar parts isn't only a matter of getting the chord voicings right. You also need to be sensitive to the physical circumstances which help shape a guitar performance - and that's not something a MIDI box can help you with.

At the same time, Chord Capture can be used in ways which don't conform to any instrument but the virtual one in your head, so why not make the most of it? There's more to life than confining yourself to a box labelled "authenticity" - there's creativity, for instance. Being original. Being deliberately inauthentic in order to discover your own authentic voice.

Strummer's Riff feature, accessible via the front-panel Record/Stop button, allows you to record a series of monophonic or polyphonic note sequences (the manual suggests around 25 ten-note Riffs with the default 8K of RAM fitted - this can be upgraded to 32K, though) and trigger each one from a MIDI note - shades of Zyklus and, more recently, the Realtime Phrase Sequencer on Roland's MV30. Riffs can be either one-shot or looping, and their tempo is defined by either the internal programmable tempo or an external tempo. A Riff can be erased at any time by pressing Record, playing the Riff's trigger note and then pressing Record again.

Only one Riff can play at a time, and if you trigger a second Riff while a first is playing, Strummer jumps straight to the second one. Also, if one Riff contains the trigger note of a second, Strummer moves straight to the second Riff on encountering it. I suppose you could use this feature to chain Riffs together, with some careful pre-planning, but it strikes me as being more trouble than it's worth. What it means in practice is that you have to keep your trigger notes out of the range of your Riffs. You also need to keep them outside of your playing range, because, once selected, a trigger note is always active and, well, accidents can happen.

Some accidents can be fortuitous, though: for instance, a Riff recorded with one Preset selected can yield altogether different results when you select other Presets, which each process the Riff's notes according to their own parameter settings.

Riffs are extremely easy to set up and extremely easy to delete, so you can use Riff recording and triggering as a kind of scratchpad feature, programming in a short looping riff and playing over it one moment, and doing the same with a chord sequence the next moment. However, it's a shame you can't give the Riffs their own MIDI transmit channel, as then they could use a different sound from the one assigned to the main channel.


STRUMMER HAS THE same casing as Drummer (reviewed MT, September '91) and all the other Perf/X units, which means it has the same practical shortcomings as well: the front-panel ventilation slots which expose its circuit board to the elements and stray pints of beer, the worryingly delicate power on/off switch, the limited information display capabilities of a two-digit LED, a restricting user interface and a general lack of robustness.

In one important way it comes off worse than the other Perf/X units, however. Unlike them, it doesn't provide a parameter listing on its front panel. In fact, its 33 parameters are referred to only by number in the LED. You have to consult a parameter list printed in the accompanying manual to find out not only what the parameters are but also which number selects which parameter. Manuals can be misplaced, however; Oberheim could at least have done what they did on another Perf/X unit, Systemizer, namely include a non-detachable laminated card, listing all the parameters, which could be pulled out from a slot in the casing and slid back in when you didn't need it. Strummer's lack of approachability does it no favours, and will no doubt put some people off getting to grips with it.

Strummer's rear panel contains the power on/off switch and power input socket (the unit comes supplied with an external AC adaptor), a MIDI In socket, a MIDI Out socket and four footswitch jack sockets. The third one I've already mentioned; sockets two and four double the functions of the front-panel Bypass and Record/Stop buttons respectively, while the function of the first is software-switchable between outputting a metronome click or switching off active echoes and arpeggios.

"You could perhaps liken Strummer to a guitarist who's moderately competent but not particularly adventurous or original."

Like Drummer, Strummer can sense the polarity of each footswitch connected to it when you switch it on, and adjusts its response accordingly, so you can mix 'n' match different types of footswitch.


PRESETS, IN CASE you hadn't already guessed, are Strummer's patches, containing settings for its parameters. Strummer has 96 of them, 64 of which are factory presets stored in ROM, leaving you with 32 RAM locations for your own settings. Sixty-four ROM Presets might sound a lot, but in a sense there are only 20, as Presets 21-64 basically take Presets 1-20 and stick them either above or below a keyboard split-point which is fixed at middle C.

Preset data can be transmitted and received via MIDI SysEx, but only one Preset at a time - unfortunately, there's no bulk dump option. Strummer transmits whichever Preset is in the edit buffer, and similarly an incoming Preset is received into the edit buffer, and from there has to be Stored into a RAM location.


BEFORE IT CAN translate your keyboard chords into guitar chords, Strummer has to work out which notes arriving at its MIDI In are part of a chord and which aren't. It does this by referencing them to a "time window": if a succession of notes fall within and are sustained through this window, Strummer considers them to be a chord.

Parameter 31, Chord Detect, allows you to set the window time yourself (from 1-40). According to the manual, the default time is 20 milliseconds - so, logically, it's set by a value of, er, 18 in the LED window. Obviously it's best to get the detect time down to as short a time as possible, because that way you can minimise the delay that Strummer introduces while it waits to see if it has a chord on its hands or not. Too short a time and Strummer won't always register the chords you play, simply because the notes won't all get to it within the timing window. You can do your bit, though, by playing chords with as tight a timing as possible in the first place - which is ironic, given that the notes are subsequently going to be separated out in time by Strummer's strum function. I found that a Chord Detect setting of 10 was comfortable, while around 6 or 7 was possible but pushing it. Also, it makes sense not to play unnecessarily thick chord voicings on the keyboard - just play the minimum number of notes that are needed to describe the chord.


PRESET PARAMETER 8, Chord Transposition Type, provides the heart of Strummer's chord voicing feature. If you select a value of 1 for it, Strummer plays open chords where possible, and keeps within the first five frets of the guitar. This is the setting which is used by Preset 01 to create all those folky strums. If you select a value of 2, on the other hand, Strummer plays barre-chord voicings as they would be played up and down the guitar neck, with the lowest note of your chord effectively placed on the 6th string. Finally, if you select a value of 3, Strummer produces spread chords, transposing notes so that there's a minimum of a perfect fifth interval between adjacent strings.

A number of other parameters also have a say in the chord revoicings which Strummer produces. Most obviously, Chord Low String (12) and Chord High String (13) together allow you to limit the number of strings (and therefore the number of notes) Strummer can use for its chords. Two further parameters, Note Low String (16) and Note High String (17) limit single notes to certain strings. However, a rather unfortunate discrepancy with the real world crops up here: Strummer seems to think that guitar strings are numbered 1-6 from the lowest to the highest pitched, whereas in the real world they are numbered 1-6 from the highest to the lowest.

If Parameter 15, Note Transposition, is set to "on", Strummer transposes all notes into the note range of the guitar. However, it has a strange conception of what that note range is - as you soon discover if you try to play single notes above G# on the first (highest) string. Further investigation and analysis reveals that Strummer is being very logical in what it's doing but it's following spurious logic which says that the note limit of each string is a semitone below the open pitch of the next string up (the G# being a semitone below an "imaginary" string tuned a fourth up from the high E). However, you don't get this problem with barred and spread chord voicings, though with open chord voicings Strummer does treat G# rather than A as its top note.

Another parameter which hasn't quite got its act together is parameter 3, Lead Enhancement. When set to "on", it's supposed to accent the first note of each chord, but this it does not do. In fact, it doesn't seem to do anything at all.

But ultimately these are peripheral shortcomings. I for one can live without Lead Enhancement, while Note Transposition isn't essential - with a little care it's not too difficult to stay within the note range of the guitar. What really matters is the legitimacy of Strummer's chord (re)voicings, and here it acquits itself very well. Firstly, are its voicings playable? Well, there were some chord voicings I came across which you'd need double-jointed second and third fingers to be able to play, and yes, I did come across the occasional chord voicing which no-one would be able to play, but basically Strummer is pretty solid here.

"The recreative guitaristic stuff makes Strummer interesting, but the 'what the hell instrument is that?' stuff makes it exciting."

Secondly, do its chord voicings make musical sense? And are they the sort of voicings a guitarist would actually play? Again, Strummer is pretty solid here without being perfect. You need to give it firm guidance in your keyboard chord voicings, give it the proper raw material to work with. For instance, if you want notes to be in the final chord, they need to be in your keyboard chord. Don't go heavy on the jazzy colourings at the expense of the foundation notes, and don't miss out the root, because Strummer won't know to put it in.


STRUMMER SIMULATES GUITAR strumming by giving all the notes of a revoiced chord the same velocity value (arrived at by averaging the velocities of the input notes), separating them out in time by inserting delays between them, and transmitting them via MIDI in ascending pitch order. Preset parameter 0, Strum Rate, determines the delay time between the notes, and can be set to "off" or a value from 1-99 - from a fast, almost "incidental" strum to a relatively slow, deliberate strum. With parameter 1, Strum Velocity, set to "on" you can vary the strum rate subtly from chord to chord by varying how hard you hit the keys.

Parameter 4, Strum Direction, lets you use downward strums as well. In conjunction with parameter 26, Keyboard Split, and parameter 27, Keyboard Split Direction, you can set this parameter so that chords above the selected splitpoint are strummed down. Alternatively, chords above a selected velocity split-point (parameter 5) can be strummed down - though, despite what the manual says, chords which are below the velocity splitpoint can't be strummed down (a bug, perhaps?). In fact, I came across a number of discrepancies between what the manual's parameter list said should happen with certain parameter values and what actually happened - perhaps because the relevant pages in the manual were headed "version 2.7" while MT's review model was running version 3.1 software.

Parameter 6, Strum Mute, is presumably meant to mimic the effect of damping guitar strings with the right hand, or perhaps choking them with the left, while playing. Whatever, Strum Muting involves sending a matching note off command very shorty after each note on. You need to experiment with different sounds and amplitude envelopes, however, to find the right sort of result. Strummer can Mute notes above or below the programmed velocity threshold, in every other chord, or above the programmed keyboard splitpoint (but not below as the manual says).

Strum Muting is not so much a strum parameter as a parameter which can work well with strumming. The same can be said of Chord Velocity Effect (14). With this set to "on", chord velocity values lower than the programmed velocity threshold cause notes to be removed from the chord - the more softly the chord is played, the more notes are removed (down to a minimum of two). In use, it's extremely effective.


TWO PRESET PARAMETERS, Next Channel Assignment (24) and Number of Next (25), allow you to create some interesting "animated" textures in conjunction with a multitimbral synth or sampler. The Next channel is defined as the next consecutive channel up from the MIDI In Channel. Strummer can be set to transmit either its chords, its notes, its echoes/arpeggios or inverted velocity out on the Next channel. Inverted velocity allows you to create dynamic velocity crossfades between the sounds assigned to the MIDI In and Next channels; Strummer transmits the same notes on both channels, but gives notes on the Next channel velocity values which are the inverse of those on the MIDI In channel.

If you set the Number of Next parameter to a value greater than one, Strummer rotates note assignments round the appropriate number of channels. For instance, you could have the first note of an arpeggio on the MIDI In Channel playing a piano sound, then rotate subsequent notes around strings and atmospheric sounds on two consecutive Next channels, so creating a sort of internally-sequenced multitimbral patch. I got some very effective results by triggering the Combis on Korg's 01/W FD synth in this way.


STRUMMER'S CHORD REVOICING and strumming features represent its foundations, and pretty solid they are, too. But Oberheim have also built a lot on top of those foundations, and produced a pretty interesting building in the process, one with plenty of interesting nooks and crannies to explore. It's a shame that it presents such an unfriendly and uninviting face to the world, and it has to be said that it does look rather tacky, but once you get inside it and learn to find your way around it, the unfriendliness begins to fall away.

You could perhaps liken Strummer to a guitarist who's moderately competent but not particularly adventurous or original, a guitarist who can play the uncomplicated open chords and barre chords, and maybe throw in the odd 9th and 13th chords while being wary of all the clever jazzy stuff with the chord substitutions and the missing roots. Don't demand too much of it, don't expect it to be Joe Pass when it's Joe Doe, don't expect it to be John Scofield when it's John Smith.

For me, the recreative guitaristic stuff makes Strummer interesting, but it's the creative "what the hell instrument is that?" stuff that makes it exciting. I have to say, however, that its general unfriendliness and unapproachability mean that you're not going to get very far with it on only a casual acquaintance. Perseverance and patience are needed, plus a feeling that the effort is going to be worth it - and that's something you have to make up your own mind about.

Price £186.83 Including VAT.

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Publisher: Music Technology - Music Maker Publications (UK), Future Publishing.

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Music Technology - Nov 1991

Gear in this article:

MIDI FX > Oberheim > Strummer

Review by Simon Trask

Previous article in this issue:

> Patchwork

Next article in this issue:

> Second Foundation

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