GR-500 24-pin Connector Change: Thanks to Matteo Mannoni who sent another piece of Roland history to share: documentation of the change from the original, pin-type C24 connector to the much more common C24 positive (locking frame) connector.
2 oscillators per voice
VCOs (voltage controlled oscillator) are directly harmonically locked to each string, but can be tuned separately
VCF (voltage controlled filter, low pass) -24 dB per octave, with envelope modulation (attack and sensitivity)
Low Frequency Oscillator, for Vibrato Effects
Built-in foot switch controls the VCO mode (single/dual), VCO harmonize pitch (detuning of the VCO's), and VCF mode (on, bypass, or inverted)
Pedal control input for the VCF
The GR-300 can output either the guitar, the synth, or a mix of the two
Synchronized, flashing LED status indicators
Dimensions: 15.7" (W) 11.4" (D) 3.9" (H)
Power Requirements: 20 watts Response Time: 29.88 ms
Roland G-808, GR-300 and FS-3
Introduction to the Roland GR-300:
At the end of the 1970s, Roland Corporation introduced the world to the GR-300 analog guitar synthesizer. While there have been numerous advancements in music technology since then, the GR-300 remains the pinnacle of analog guitar synthesis. Although the sound palette of the GR-300 is very limited, the GR-300 has the fastest, most accurate tracking ever developed. Unlike much of guitar-to-MIDI pitch recognition technology, the GR-300 guitar synthesizer does not require the player to adapt technique to get astonishing results.
It is not that the GR-300 ignores fret board misfires, or translates them into wildly inaccurate notes as some MIDI systems will. The GR-300 instead creates a unique analog synth equivalent. For example, the initial atonal pick attack is converted into a sound very reminiscent of the "spit" sound heard at the beginning of a trumpet phrase.
Roland GR-300, G-808, and G-303
The trumpet comparison continues: the GR-300 has a waveform very similar to a sawtooth, with a brassy, aggressive tone. But the GR-300 waveform does something unique: it changes shape as the player moves up the fret board. An "E" played one octave above an open "E" string will not only be sounding at twice the frequency, but the harmonic content will be very different as well. This is the happy consequence of the brute force synthesis used in the GR-300. Inside the GR-300, the amplitude (volume) of each note is related to its pitch. The low "E" will have twice the amplitude of an "E" one octave above. Similarly, the high open "E" string, two octaves above the low "E" string, will have one quarter the amplitude of the low "E". To eliminate this volume difference, Roland used a "chopper-gate" circuit to basically crop the top of the waveforms. The higher notes look more like a classic sawtooth waveform, while lower notes have a more rounded tone. The result is a rich, complex sound where every note played on the guitar across the instrument will have an individual sound. An unfortunate consequence of this design is that higher notes will have less sustain than lower ones.
The GR-300 has two oscillators, both harmonically locked to the string pitch, but one oscillator can be offset by a range of plus or minus one octave. So it is possible to play one oscillator an octave below pitch, or to layer two oscillators together at the interval of a perfect fifth. These pitch intervals can be preset as pitch offset "A" and "B." There is also an adjustable pitch rise and fall time, to create a portamento/glide effect. The -24 db per octave low-pass filter in the GR-300 also tracks pitch, so as pitch preset "A" or "B" are engaged, the filter will also change. Higher pitches open the filter more, making for a brighter sound. For more than 30 years, guitarist Pat Metheny has been thrilling audiences with his GR-300 solos. Pat will often engage the pitch offset to lift the end of his solos one octave up. Not only does this raise the pitch, but it also opens the filter up more, making for a brighter sound. The GR-300 includes a LFO vibrato circuit, and basic filter envelope modulation. There are controls for filter attack time and sensitivity, and the filter modulation can be inverted as well.
The original G-202/303/505/808 guitars have a hexaphonic fuzz circuit, and the GR-300 is the only Roland guitar synthesizer to access this sound. With the GR-300, players can play either the hexaphonic fuzz, the VCO synth sound, or both of these sounds combined.
Pat Metheny with Roland GR-300 and G-303
The GR-300 has maintained a unique level of recognition thanks in large part to musical genius of Pat Metheny. Guitar players who see Pat play for the first time want to know how to get "that sound." While Pat has continued to embrace new guitar technology, he has maintained a close relationship with his G-303/GR-300 combination. Pat has demonstrated with the ease of a virtuoso just how expressive a guitar synthesizer can be. For players used to working with the kludgy, unforgiving MIDI synthesizer systems, the GR-300 is a refreshing, responsive system.
Unfortunately, as mentioned before, the GR-300 does have a very limited sound palette, and it can be difficult to use the GR-300 in a way that does not invoke Pat Metheny. Other guitar players who championed the GR-300 in their careers include Andy Summers of The Police, who tended to use the unit as a source of pads, or dark, swelling tones. This can be heard on the break of "Don't Stand so Close to Me". And Robert Fripp and Adrian Belew of King Crimson used the GR-300 extensive in the early 80s, most notably on their stunning album "Discipline." The lead distinctive synth sound heard at the beginning of "The Sheltering Sky" is a GR-300.
The Ibanez AR300GR and AR305GR, vintage Roland-Ready guitars. Click on either image to enlarge.
With interest in the GR-300 stronger today than ever, Roland reintroduced the GR-300 in 2007 as a part of the breakthrough VG-99 guitar synthesizer. The VG-99 uses a software model of the GR-300, reproducing all the unique qualities of the GR-300 mentioned in this summary. The VG-99 very closely reproduces the GR-300 experience, without the cost and maintenance problems associated with vintage equipment. The VG-99, with its dual COSM engines, also allows players to play two virtual GR-300s at the same time. And the VG-99 includes the hexaphonic fuzz option. The more recent GR-55, introduced at the 2011 NAMM show, also includes the GR-300 emulation. For guitarists interested in playing a GR-300, these modern alternatives recreate the GR-300 experience with about 98% accuracy.
There are three versions of the Roland GR-300. These are distinguished by the letters "A", "B" or "C" appended to the part number printed on the top voice board. The only way to know which version you have is to remove the bottom panel, and look at the printing in the lower right-hand corner of the voice board. You will see either 052-539A, 52-539B, or 52-539C.
Of the three versions, "C" is the most common, and the most desirable. Electronically and functionally, these GR-300s are the same. However, version "C" is the version referred to in the well-documented Roland GR-300 service manual. Version "A" is notable for the fact that many components are found tack soldered onto the circuit board. Apparently when the circuit board was produced, some components were left off the circuit board, and so capacitors, transistors, etc. were soldered to existing components to complete the circuit. Also, there is a small network of resistors added to the power supply, not documented in the service manual, and not included in the more common version "B" and "C".
The final curiosity are the two, 12 pin ribbon connectors. In all other Roland products, the two 12-pin connectors are positioned parallel, side-by-side. This configuration is in line with the way the ribbon connects to the 24-pin cable connectors. However, on the version "A" synth, the two 12-pin connectors are in series, which makes it more difficult to connect the ribbon to them.
There is a companion Roland G-303 guitar, version "A" as well. Like the version "A" GR-300, the version "A" G-303/808 electronics have the ribbon connector position inline. And the circuit board layout for the version "A" guitar is similarly different from the published documentation in the service manual.
One final note: when I first found the Anderton GR-300 modifications, which inspired me to create this website, I was unable to follow Craig's directions on locating the parts on the GR-300 circuit board. It was not until many years later, when I saw my first revision "A" board, that I realized that Craig's plans were referencing the earlier, rare version "A" board, and not the version "C" board in my possession.
Footswitches: there are two versions of footswitches found on the GR-300. The most common footswitch is the larger, chunky rubber footswitch. This is the same part used on the Roland GR-100, GR-33B and US-2. Some GR-300s have a lower profile footswitch made of harder rubber. I have not been able to make a clear correlation between production date, and the use of these different footswitches, though it appears more common to find the lower profile switch in the later, revision "C" GR-300s. I will note that the actual switch mechanism used in these footswitches underneath the rubber cover is the same in both designs, and it is the same switch used in the Roland GR-700.
These three YouTube clips run through the 20-point GR-300 (PDF) checklist. The 20-point checklist tests all the GR-300 functions. You can hear how the Envelope Inverse feature should work, compression, etc. Use these clips as a reference to see how your GR-300 is working.
Three Simple GR-300 Mods
For an encore to my two columns on guitar synthesis (July and August 1982 issues), here are three simple modifications that let you get more out of the Roland GR-300 guitar synthesizer system. You won't have to drill any holes in either the GR-300 electronics or G-series guitar, or even remove any circuit boards. The only cautions are: Solder with a fine-tip, low-wattage soldering iron (no more than about 40 watts), use rosin-core solder, unplug the GR-300 before working on it, and finally, don'tt forget that doing these mods will void your warranty. Now, on to the three mods.
Improving hex fuzz high-frequency response
The hex fuzz section of a Roland G-series guitar (which is built into the guitar) mixes the fuzzed signal from each string into a single output. Note, though, that this hex fuzz mixer starts rolling off high frequencies around 2k Hz. To eliminate this roll-off, remove the metal plate on the back of the G-series guitar'ts body (the one on the other side from the controls and switches). Next, orient the guitar'ts circuit board so that the lettering is right side up, and look for the capacitor labeled C72 (470 pF). On my guitar, this cap is located a little to the right of center of the board, in the upper middle section. Once you've found the cap, snip one of its leads with a diagonal cutter - you will be rewarded by a brighter fuzz sound with more presence.
Interior G-303 guitar. The clip is attached to C72, located just below the ribbon connector.
Interior G-303 guitar. The yellow arrow points to C72. C72 is a flat, ceramic capacitor.
Interior G-505 guitar. The yellow arrow points to C72. The layout is similar to a G-303.
Interior G-505 guitar. Note C72 in the lower left corner. The op amp IC8 is part of the hex fuzz.
Separate hex fuzz out
This mod (which is particularly effective for studio work) taps off the hex fuzz output and routes this signal to its own output jack (the original hex fuzz signal path remains undisturbed). Thus, you can send the hex fuzz signal, VCO signal, and straight guitar all into separate channels and process them individually. The hex fuzz would come from the jack we're about to add, the VCO output from the mix/synth jack (with the guitar's balance control set for all synth and the dist/VCO switch set for VCO only), and the straight guitar sound from the guitar jack. With a little processing, you will get some absolutely incredible stereo effects.
To install the hex fuzz output, take off the bottom plate (12 screws). Orient the unit top-down, so that you are facing the rear panel. This jack connects to two ribbon connectors, which plug into matching sockets (B4 and B5) soldered to the circuit board. Fig. 1 shows a detail of the left side of B5. Note the designation "6 SG" just above the left side of the connector, "3 SG" to the right of that, and a solder connection to the right of "3 SG." A jumper wire goes from this solder connection to another solder connection directly above it. Both of these carry the isolated hex fuzz signal.
Decision time: It you don't want to drill any holes in the GR-300, you will have to give up one of the jack functions on the back panel so that this jack can carry the hex fuzz out. I recommend giving up either the string select or sweep on/off jack. Both of these have three terminals; the ground terminal busses to the other jacks via bare wire, one terminal is unsoldered, and the remaining "hot" terminal (the one closest to the circuit board) has a colored wire going to it. Disconnect this soldered wire and tape up the end, so it won't short to anything. Connect the "-" end of a 10 micro farad electrolytic capacitor and one end of a 100K resistor to the hot terminal. Connect the other end of the resistor to the ground terminal, and the remaining end of the capacitor to a wire whose free end solders to either of the jumper solder pads shown in Fig. 1. You now have an individual hex fuzz output.
This diagram is from the original article, but refers to the version "A" circuit board. The most common version is "B" or "C".
The yellow arrows points to the connection point I used. Photos from "C" version GR-300.
LFO jack and the separate, distortion guitar output jack.
I don't like the GR-300's vibrato touch plates, so I disabled them and added a foot pedal option where pushing down on the pedal increases the vibrato depth. The pedal must be a control voltage pedal,as described in my article in the February 1982 issue of Keyboard magazine (page 24). Otherwise, use a standard volume pedal, connect a 9V battery connector to a 1/4" phone plug (red wire to hot, black wire to ground), and insert the plug into the pedal's instrument input. As you push down, the pedal output will go from 0 to 9 volts.
As with the last mod, you will have to give up another jack's function; I recommend the VCF pedal jack's hot terminal (the closest to the circuit board), and re-solder this wire to the ground terminal. Now, referring to Fig. 2 (which details the right side of B4), use an X-acto knife to break the circuit board trace where indicated. Next, follow along the right-hand side of the circuit board. As you move towards the foot switches, you will see the legend for resistor R179 about two-thirds of the way up the board and about an inch from the side. Move towards the foot switches from this point, and you'll find a jumper wire (see Fig. 3). Solder a wire to the right of this jumper, and connect the other of this wire to the hot terminal of the VCF pedal jack.
Check your wiring over carefully, make sure you didn't short out any adjacent traces (important!), and reassemble the unit. Plug a control voltage pedal output into the VCF pedal's input to verify that the pedal vibrato function works, and also check that there's a signal present at the new hex fuzz output jack. If all is well, play away!
This diagram is from the original article, referring to version "A" circuit board.
This is a photo of the connection near B4.
The yellow arrow points to the cut in the circuit trace.
This is the second jumper, near the power transformer.
Revised Modifications, April 2010:
Revised Anderton Modifications. Click for larger view.
After around 10 years, I have some suggested revisions to the original Craig Anderton modifications. Typically, these came about as the result of a suggestion by a GR-300 user. User Giotto asked me to make these modifications when I was doing the ALG Audio Labs of Georgia output upgrade to his GR-300.
I liked these new revisions quite a bit, they add a lot to Craig's original modifications, they do not require a battery for the LFO pedal, and offer a great deal of flexibility with the hex fuzz sound. In addition, all the changes are made by working on the ribbon connector, so you do not have to completely disassemble the GR-300, or make changes to the main circuit board. You can have a look at my notes for this work. I used a Neutrik NSY234 tip-ring-sleeve for these modifications.
If you watch the YouTube video play list below, you can get all the details on how this modifcations was done, and how it works..
GR-100/GR-300/GR-33B Power Supply Failure - Replacing Blue Tantalum Capacitors:
As a safety precaution, I strongly recommend replacing the orignal blue tantalum capacitors. These capacitors, while very popular in the late seventies, are known to fail. Here is a list of typical failing blue tantalum capacitors symptoms:
GR-100/GR-300/GR-33B suddenly stops working
Unit powers up, but does not make any sound
GR-300/GR-33B string LEDs stay on constantly.
GR-100/GR-300/GR-33B gets hot very quickly.
Before you warm up the soldering iron, it is a good idea to test the GR-300 power supply to determine if you do have a power supply problem. To do this test, you need a basic multi-meter that can read DC voltages, in the 0-20 volt range. Here are the photos of testing the power supply:
Negative test probe attached to grounding bus wire between jacks.
Testing +15 volt supply on voice board.
Multi-meter display for +15 volt supply.
Power supply ribbon block is just in front of the 24-pin connector
Testing -15 volt supply on voice board.
Multi-meter display for -15 volt supply.
If you get a reading of "0" volts, or anything less than +/- 14 volts, you most likely have a blue tantalum capacitor failure. As a quick check of your GR-100 or GR-300, you can simply unsolder one leg of the capacitor to remove it from the circuit. You can power your synth up and confirm if the capacitor is the source of the failure. As I mentioned, I recommend replacing these tantalum capacitors with newer capacitors, since a capacitor failure can destroy your entire power supply. A great replacement part is any modern, 10 uF electrolytic capacitor, with a rating of 15 volts or higher. Feel free to use a 10 uF capacitor rated at 25 volts, or 50 volts.
Be sure to check the polarity of your capacitors! You will notice that one side of the capacitor is marked negative, or "-." When installing the capacitor for the positive side of the power supply, the "-" leg of the capacitor will go to ground. For the negative side of the power supply, the "-" leg of the capacitor will go to the power supply.
If you are unsure how to proceed, get some help! Otherwise you may find yourself with a noisy, smelly explosion!
Blue tantalum capacitors in place next to TA7179 voltage regulator chip
Blue tantalum capacitors in place next to TA7179 voltage regulator chip (GR-100) .
Blue tantalum capacitors in place next to TA7179 voltage regulator chip (GR-33B)
Page six of the 1984 Roland GR series brochure lists several options and accessories for the GR-100, GR-300 and GR-700.
FS-1, FS-2 and FS-3 Footswitches: The footswitches worked with the GR-300 to control compression, pitch sweep, and string select. The GR-33B also has a string select footswitch input.
FV-200 volume pedal: The FV-200 volume pedal could be used to control the filter cutoff on all the vintage Roland synths, GR-100, GR-300, GR-33B and the GR-700. The FV-200 could also be used as a pitch pedal with the Roland GR-700. The both the Roland GR-100 and GR-300 owner's manual list the FV-20 volume pedal for use with the GR-300.
CB-300 case: The rarest of all vintage accessories, Roland made a leatherette case for the Roland GR-100, GR-300 and GR-33B.
And page five lists the Roland US-2 Unit Selector and the C-24D connecting cable.
Most of these accessories are impossible to find now, but there are modern equivalents that you can use to expand the playing experience with the vintage Roland GR-300.
Roland FS-3 Footswitch
GR-300 Rear Panel Switch Inputs
The early Roland GR-300 brochures often picture the GR-300 with the Roland FS-3 footswitch. The FS-3 is just three independent footswitches in a single enclosure. This is a perfect match for the GR-300, since the GR-300 has three connections on the back for foot switches: compression, pitch sweep, and string select. The switches all work a little differently. For example, the compression feature also has a top panel switch to turn compression on or off, with a status LED as well. Plugging a foot switch into the rear panel compression jack defeats the top panel switch.
As for pitch sweep, this feature is always on. As a result, most GR-300s have the "rise time" and "fall time" knobs turned all the way counterclockwise to a setting of zero. The pitch sweep feature also has a status LED. Notice that it is always on! Plugging a foot switch into the rear panel sweep jack lets you turn this feature on or off.
Finally, there is the string select jack on the back. String select only works in mode 3, the "synthesizer only" mode. This is the mode the GR-300 is in when the switch on the guitar is in the "up" position. In mode 3, you can individually turn strings on or off using the six string select switches on the top panel. There is no dedicated status LED for string select, like there is for compression or sweep, but if you have a string switched off with the top panel switches, the associated LED will not light when the string is played. I have to admit that a few times I thought I had a problem with my GR-300, only to notice I had inadvertently switch a string off!
Do-It-Yourself GR-300 Footswitch:
It is quite easy to make your own GR-300 footswitch. Grab a box, drill three holes for foot switches and three holes for input jacks, and you are set! Wire the switching terminals to the sleeve and tip connections on the output jack. I picked up a junked foot switch at a used gear shop, sanded it down and gave it a coat of primer, and used to transfer lettering to label the switches. Several coats of clear coat were added to protect the lettering.
One of the best features of the Roland GR-300 is the excellent -24 dB per octave filter. This filter gives the GR-300 much of its character, and it sounds about as good as any analog filter I have ever heard. In addition to the dramatic sweep of a -24 dB per octave filter, the GR-300 is also capable of high levels of self-resonance. It is also very easy to expand on the range and power of the filter on the GR-300. On the rear of the GR-300 is a filter pedal input. You can plug any volume pedal into this jack, and easily sweep the filter. Some pedals work better than others, depending on your musical requirements.
The filter pedal input works by moderating a control voltage that is summed in the Voltage Controlled Filter circuit. The greater the resistance, the greater the range of the sweep of the filter. A typical volume pedal has a 10K potentiometer. A typical CV pedal uses a 50K potentiometer, and there are some pedals with 100K potentiometers. As you can tell by listening to the samples below, the higher the value of the potentiometer in the pedal, the greater the filter effects.
Also, the filter pedal input works in conjunction with the filter knob on the guitar. The filter knob setting on the guitar sets a "base line" that the filter sweeps from. With the knob on the guitar set to "zero," the filter pedal will sweep from a very dark sound to a brighter tone. With the filter knob on the guitar set to "5," the filter pedal will start with an open sound, and then sweep to an even brighter sound than is possible with the guitar alone. This is an important point: you can actually extend the range of the filter on the GR-300 by adding a filter pedal. If you want really bright, screaming synth tones, or dangerously loud resonance levels, adding a filter pedal gives more range to the GR-300.
If you are using a volume pedal, you will plug a standard guitar cable from the GR-300 filter pedal jack to the output of the volume pedal. The GR-300 will then measure the amount of resistance from the wiper to the ground terminal. Similarly, with a Roland EV-5 Pedal, you will plug a standard guitar cable from the GR-300 filter pedal jack to the EV-5. Again, the GR-300 will then measure the amount of resistance from the wiper to the ground terminal.
Click on knob to hear a filter sweep
I recorded the audio samples at right with a 10K, 50K and 100K potentiometer, and maximum resonance. Also, the sweeps were done with the initial guitar filter setting of 0 (minimum filter). You can hear in all these examples how the external filter pedal expands the range of the filter. Notice how the 10K pedal does not move the filter frequency very much. The 100K really opens the filter up, but it does so too quickly. There does not seem to be any effect once the filter is open, so about half of the travel of the pedal is useless. The 50K pedal seems to be the "just right" value. It does not get quite as bright as the 100K, but overall the range is useful and effective.
My favorite pedal is the flexible, versatile Korg EXP-2. With a Korg EXP-2, you do not have to re-wire the pedal, since it also doubles as a volume pedal. Just plug a standard guitar cable from the GR-300 filter pedal jack to the output of the EXP-2 volume pedal jack, and you will get a wide, musical range of filter sweeps.
The Behringer FCV-100 is a pedal that can function as a dual, stereo volume pedal, or as a control voltage pedal with a 50K pot. However, the FCV-100 cannot be used directly with the GR-300 or GR-700. Since the FCV-100 has an internal PC board, the easiest way to adapt the pedal for the GR-300 or GR-700 is to modify a TRS tip-ring-sleeve cable.
To make the FCV-100 compatible with the GR-300, just reverse the tip and ring wires on one end of a tip-ring-sleeve cable. The other end should be left unchanged. This will reverse the operation of the pedal, resulting in pedal up (dark, filter closed), pedal down (bright, filter opened), as shown in the YouTube clip. I have tested this with both a GR-300, and GR-700. It works with the GR-700 with either the filter or pitch input. It does not matter which end of the cable is plugged into the synth.
Click on image to enlarge
I would note that GR-300 user Joe Bartone, of the band Thelonius Dub, found that the FCV-100 did not hold up well on the gig. By the third gig, the rubber pad came off the pedal. Joe patiently bought some epoxy and took care of that problem...then the bottom of the pedal disconnected from the chassic. Not a big issue, but it meant constantly putting it back together every time he took it out for use. Finally, Joe reported that the pedal went "soft" meaning that it would tilt down when he took his foot off it. It does not stay in position like a quality pedal does. Taking Joe's remarks in hand, I would not give the FCV-100 a strong recommendation at this time.
GR-300 Remote Control Box with switches and filter control
GR-300 filter preset remote control box
The GR-300 has many cool features that few players use, like compression or pitch sweep, mainly because these functions work best with a foot switch. The pitch sweep feature is dramatic, but it is not the kind of thing you want to hear every time you engage the pitch presets. The GR-300 remote puts controls for these features, plus filter presets, in one box.
After I built the original Remote Control Box, I built a newer unit to only select presets for the GR-300. This Filter Preset unit was built to match the same design as the BX-13 V2 and V3. Otherwise, it works exactly the same as the original Remote Control Box. I really liked playing with the filter control, and enjoyed using the pedal for some cool filter sweeps. I also tried some different pots. Why different values? In critical listening tests, the 100K pot opened the filter up slightly more, but the 50K pot yielded the most useable range. If you are building for musicality, I would keep both pots at 50K.
Foot Switch 1, marked "preset" selects between filter preset 1 or preset 2. Foot Switch 2, marked "pedal" selects between the preset knobs and the pedal. There are LEDs associated with each knob so you can know which filter knob is active, even when the pedal is on. Also, keep in mind that the amount of filter control depends on the filter control setting on the GR series controller, or the GK-1. The control knobs, or the pedal, make the sound of the GR-300 brighter. Therefore, if you want maximum range, set the filter control on the guitar to minimum.
YouTube clips of the GR-300 Remote Pedal in Action:
Click on any image for larger view.
Japanese Greco 1981 Catalog:
Click on any image to view original pdf.
Roland 1982 Guitar Synthesizer Brochure:
Click on any image for larger view.
Roland 1984 Guitar Synthesizer Brochure:
Click on any image for larger view.
Guitar Player 1981 - 1982 Advertising:
Guitar Player - October 1981 -Click on any image for larger view.
Guitar Player - April 1982 -Click on any image for larger view.
Vintage Roland Gear in UK Magazines:
Click on any image for larger view.
How the GR-300 Works:
Here is a simple explanation of the genius behind the GR-300. I gleaned this information from talking to Mike Bacich, studying the GR-300 service manual, and reading the original GR-300 Patent Application.
Application drawing of a G-303 without a Mode Switch
Patent application sketch of GR-300.
2 - The World's Smallest Humbucking Pickup
One Roland guitar synthesizer technology innovation is the world’s smallest humbucking pickup.
Each element in the Roland guitar synth pickup is actually a tiny humbucking pickup. As you can see, the patent illustration (on the left) depicts a classic humbucking pickup, with dual coils wrapped around opposite magnetic poles. This configuration cancels out noise while amplifying the essential guitar signal. By using tiny humbuckers, Roland was able to apply the incredible amounts of gain needed to take the tiny microvolt output from the pickups to a 25 volt, peak-to-peak signal used to directly drive the Voltage-Controlled Oscillators in the GR-300.
Interestingly enough, the earlier GS-500 used what appears to be a collection of six tape machine pickup heads to make the divided pickup. With the G-303/808 Roland introduced the divided hex pickup design still in use today, with minor modications.
3 - Adaptive Filter
Before the GR-300 can create a synthesizer waveform, the guitar input signal must be filtered to eliminate any overtones and to emphasize the fundamental tone, (or root pitch of the note). After passing through a simple low-pass filter and compression circuit, the guitar input arrives at a two-stage band-pass filter. This adaptive filter changes its frequency response curve depending on what note is played.
When notes are played from the open string to around the sixth fret, the filter takes the shape of "F1", corresponding to the fundamental of the open string. This filter attenuates 1st overtones or harmonics by 24 dB. As higher notes are played on the guitar, the filter response curve shifts to shape "F2a" and "F2b."
4 - Square Wave to Sawtooth Wave to GR-300 Wave
Other guitar synthesizers used different methods to try to detect the peaks of a waveform. The idea is that the time between waveform peaks will determine the pitch of the note played on the guitar.
Roland took a different approach, and is probably the only company that tried zero crossing techniques to detect the pitch of a guitar signal. While it is possible to have several zero crossings in the guitar cycle (resulting in octave jumping), the adaptive filter is so efficient that this rarely happens.
Here is an explanation of the diagram on the left:
A. Raw input guitar waveform
B. Square wave created by processing the input waveform through filtering and zero crossing circuit.
C. 1 uS pulses created by the leading edge of the square wave
D. Sawtooth waveform created by 1 uS pulses.
E. Distinct GR-300 waveform after "chopper-gate" clips the top of sawtooth waveform.
The raw guitar waveform is filtered and processed through a zero crossing detector to produce a square wave. The edges of the square wave are then used to create 1 microsecond pulses. As a pulse is received by the waveform generating circuit, a steadily rising voltage is generated by a capacitor. When the next pulse is received, the waveform resets to a value of zero, and the cycle starts all over again. Roland calls this a time-to-voltage circuit. Oscillators are tuned by varying the current to the capacitor, which controls the rate the waveform rises. This design leaves one problem: the lower the pitch of the note, the louder the note is. Notice in the example above the widest oscillator pulse (or lowest in pitch) is also the tallest (or loudest). To keep all the GR-300 notes at the same volume, the circuit uses a "chopper-gate" to basically chop off the tops of the sawtooth waveform. The result is the very distinctive GR-300 waveform, unique among analog synthesizers.
4 - Final Signal Processing
The rest of the signal processing in the GR-300 is very much in the classic analog synthesizer design. The sawtooth oscillator outputs feed into a 24 dB per octave voltage controlled lowpass filter. At the same time, signals from the hex pickup are used to drive an envelope generator. The output of the envelope generator provides the control signals for the voltage controlled amplifier. The output from the envelope generator can also be used to modulate the voltage controlled filter, (either in regular or in inverted mode).
The final bit of genius in the GR-300 is a circuit Mark Smart describes as a "squelcher" circuit. This is a circuit designed to suppress false notes created by second harmonics. It is perhaps too complicated to summarize here, but if you would like to know more about the gory details of how the GR-300 works, there is no better resource than the GR-300 Service Manual, available from Roland. The Service Manual goes into great detail with additional diagrams and notes. My friend Mike Bacich first pointed out to me that the GR-300 technology could easily be expanded to incorporate more features, such as multiple waveform outputs, square wave, triangle, etc. Mark Smart has gone so far as to start to develop an expanded GR-300 that could be used as a controller for a more powerful analog synthesizer.
I do not know if Noboru Suenaga, the person credited with inventing the GR-300 is still in the business, but I want to thank him for his invention, and for all the inspiration his guitar synthesizer has given musicians around the world.