Index

Exploring Gilmour's Style
Defining Sound on Sound
Finding A Suitable Delay
White Paper For An SoS Router
Practical Implementations: Transformer Coupling
Delayed Drop-Off
Switching Without Pops
Concealing The Relay Switching Current Surge
Connectivity
Miscellaneous
Programming The DD-500
Sonic Beautifiers
Using The SoS Router
Sound Samples
Reference

Exploring Gilmour's Style

The SoS Router  (click on the picture to load larger image)

As a guitar player, there is no way you can avoid David Gilmour forever. It was time to explore Gilmour’s style. One miniscule tangent at least. In Shine On You Crazy Diamond a pleasant background drone (for the lack of a better word) can be heard for long passages, which I had always thought was made with synthesizers. With the advent of ambient guitar music, it dawned on me that this was made basically with a guitar and an echo device. Further investigation lead to Gilmour’s Sound on Sound technique, a fancy routing of effects and amps. In order to explore this method, a signal routing device has been put together to mimic Gilmour’s Sound on Sound technique. Cost is very small and in no relation to the wealth of sound-scapes that are lurking behind The Wall. Pun intended.

The basic setup is described here: [Gilmourish]

Gilmour has devised a method to steer the guitar signal either into his normal / lead sound effect chain or into to the effects chain that creates the drone sound, controlled by one central manipulation mechanism, linked to a volume pedal. He thus can create drone sounds in real time and immediately switch to solo playing. This is not possible with either common phrase loopers, nor freeze pedals nor plain long delays.

Both sound chains are supposedly electrically independent, since he uses multiple amps in times, which opens up an enormous space for creative pedal use and musical expression.

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As a guitar player, there is no way
you can avoid David Gilmour forever

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A nifty little signal routing device is described in the following, that can do all that and allows us to mimic Gilmour’s Sound on Sound routing method. Nothing comes without price, so the penalty here is that the top notch delay used will be condemned to eke the rest of its lifetime solely with a secondary task.

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Defining Sound On Sound

(hereafter: SoS).
Before we proceed, let us make clear what we speak about here, since there is no mutually agreed upon definition of what Sound on Sound is.

"In this mode [SOS mode], you can record up to 23 seconds of material and repeatedly play back the phrases from the material. (...) you can record new phrases and add them to the previous material (overdub) as many times as you like."    - DD-20 User Manual,  p.18, SOS (Sound On Sound)

"Sound on Sound recording lets you record without erasing the existing part (i.e., the new recording is added to the existing one). "    - MultitrackStudio,  Sound On Sound Recording, https://www.multitrackstudio.com/soundonsound.php

According to that definition they clearly speak of a phrase looper:

"Music looping is recording on the fly into a seamless phrase that plays continuously over and over again. This can be in sections of a song or elements within a song, such as a beat, vocal, bass or synth line. Sound-on-sound looping means recording additional passes over top of the original phrase. "    - LANDR blog, What Is Looping and How to Use It in Your Music Production,     https://blog.landr.com/looping/

but what is a phrase?

"A phrase is a single unit of music that makes complete musical sense when heard on its own. It is most notably heard as a melody and it is made up of smaller units, like motifs, cells, or individual notes."   -  https://hellomusictheory.com/learn/phrase/

So this all is not what Gilmour does with SoS, he does not play a musical phrase, although all the above definitions surely share some common ground. By the looks of it, Gilmour took the liberty to interpret the term his own way. He certainly is not liable to anybody for what he does.

Ambient music players "write" into a very long delay and permanently over-record the content. This is partially what Gilmour's setup does.

The closest approximation to the effect is the EHX Freeze, but it does not (and none of them does) the clever splitting into dedicated branches and amps. Keep that all in mind when you here the term...

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The technique is not new. Robert Fripp did similar experiments, those were called “Frippertronics”. An online music-gear oriented magazine is existent with the same name, but they have an entirely different background. Strymon used the term in conjunction with their Volante, but it clearly refers to a phrase looper according to the above definition. For all sorts of phrase loopers applies: to activate a loop, it takes several footsteps and a pre-recording of phrases before they can be used, which is totally unsuitable for the envisioned task.

So, for our purpose, it is a combination of a long delay dedicated explicitly to the drone generation, combined with some clever signal routing.

Although my approach towards an SoS router very likely differs from Gilmour’s approach as far as the technical realization goes, using the hold function can bring us into the same territory. The signal routing governed by one central control element is comparable. Well enough for exploring that technique anyway.

Using a setup with two independent sound and effect chains allows us to potentially apply the same creative sound manipulation processes as Gilmour does, as different to being restricted to a single dedicated freeze box that squeezes everything through it into a common amp. By taking this route (pun intended) the effort is moderately increased (the biggest challenge being the cabling), but the creative potential is increased by a magnitude.

Before we start to sketch out such a routing device, we must find a suitable delay unit.

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Finding A Suitable Delay

►	A word beforehand. I am by no means affiliated within any of the companies mentioned in the following. I act upon my own responsibility and interest.

Core element of the drone is a delay device. Any plain delay that has a sufficiently long delay time can be coaxed into a drone sound-scape, but it cannot be used like Gilmour's SoS rig without extensively modified  entourage. I suspect he has his volume pedal modified with a touch-switch or proximity switch to make a central control element. We want to avoid this to keep things simple. Fortunately, a delay with a freezing function (whatever the maker calls this...) will do most of the work for us, without fading in with a volume pedal. In the next chapter we will learn what is needed and why.

Most of the available delay units offer a freeze function of sorts, but lack the external connectivity we need for this purpose. Some have functions with a promising name, but turn out to be phrase loopers.

I already had the STRYMON Volante, but this phased out due to its inflexibility. It has a lot under the hood, besides an infinite hold function, but all that is only accessible via MIDI control. Not everybody will be willing to go that far, certainly not me. The Volante is equipped with a phrase looper, which they call Sound-on-Sound according to the above de-facto specification.

I then obtained a second-hand BOSS DD-500. Effects like freezing a delay (what they call “hold”) can within a few minutes be activated with a simple external momentary switch and a few changes in the menu, or even using the tap switch for experimenting. For this matter, any unit with an externally accessible "hold" function will work, likely other members of the BOSS family. By the looks of it, there is not many.

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Its smaller sibling, the DD-200 has both the connectivity and the hold function. It cannot be switched into "simultaneous mode". Most of the other smaller BOSS models do have hold, but no external control connectivity. Many devices are available with a similar function, yet with no external control possibility - except - according to the manual, the Verdict Polyphonic Octave by Klowra (maybe needs a different control wiring).

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White Paper For An SoS Router

Compared to the major hurdle (of finding a useful freeze device with external switching capability) the principle of devising a signal router is utterly trivial: it is a fancy momentary A/B switch.

Block diagram and wiring scheme: (click on the picture to load larger image) Internal details are not depicted, thus this will not work with a plain A/B switch.

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Somebody owning a full-blown signal router like the BOSS ES-5 or ES-8 can implement this routing mechanism directly into the signal router (you likely need the bigger one), since those have all the necessary building blocks available, besides making the delay unit available for entirely different tasks upon the pushing of a button. Restricting the DD-500 (or whatever unit you choose) to a single task that is pretty subordinate and rare too, is a waste. Using a DD-2 with some afterburner (optional, but probably desirable) might do the same job, while freeing the DD-500 for more mundane tasks. An EH Freeze pedal might have worked out of the box – but unfortunately they did not add a jack for external control.

A commercial signal router is neither cheap nor easy to configure, which leads us to the first point:

• building an SoS router must  be simple and low cost.
• The SoS router must be first in the row.
• The intended default sound chain is as follows: Guitar > Router (buffered) > normal / lead sound chain (default) OR Drone sound chain, while the signal into the respective other chain is muted.

►	The front plate of the unit has the routing printed onto it to show the signal flow.

• Key element: all necessary external switching actions for initiating a new drone cycle must converge into the one switch.
• Said switch must be the only switch that needs to be visited. Initiating a drone must be precise, since it has to follow the musical beat, and there is no time stomping several switches again and again.
• A single activation of said switch must do all the necessary action.
  

►	Akin to Gilmour’s SoS setup, both chains eventually go to separate amplifiers. To avoid ground loops, the chains have to be isolated from each other. The default chain (normal / lead) shares common ground with the input, the drone chain is transformer-coupled.

• The activation of the hold function on the DD-500 is accomplished with one part of a 2p2t momentary foot-switch and happens immediately. The control signal must be isolated too.
  
• The DD-500’s hold function samples the signal during the activated time of the switch (i.e. during depressing).
• The chord destined for the drone must be strung immediately after the switch is depressed (in sync with the beat), and it must be held until after the switch is released.
• In order to not spill the remains of the decaying chord into the normal / lead chain, the SoS router must not release the signal switching relay for another short moment, which gives the player time to mute the signal.
• The drone part replays endlessly until

1.  either the effect is bypassed on the DD-500 (in which case tone fades out or stops abruptly dependent on the programming) or
2.  the button is pressed again for a new chord.

• Switching pops are to be avoided at all cost. (see below)
  
• The effect must have compact dimensions
  

To download the schematic, click on the icon By doing so, you agree to the Terms of Use

PCB of SoS router / mounted in the case: (click on the picture to load larger image) View of the PCB (3D rendering), and mounted in the case (Hammond 1590N1). Jacks are Amphenol ACJS-MH.

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Practical Implementations: Transformer Coupling

The transformer used for decoupling is a small and cheap one, which certainly fulfills no Hi-Fi expectations, but was deemed more than sufficient for guitar purposes. A suitable design was proposed by R.G. Keen [KEEN1], [KEEN2].
Certainly not a hindrance for creating drone sound-scapes.

Hum does not necessarily appear, but it might. Only one chains needs to be isolated (from the other), so since the default setting (= non-activated) is on normal / lead chain, and since the drone chain is considered a secondary function, the drone chain is transformer coupled (in case there were any sound degradation).

In the completed unit, no hum is noticeable except from the single coils that pick up all the garbage.

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Delayed Drop-Off

A one-shot (a.k.a. mono-flop) is comprised of a NE555 circuit and additional components to make it re-triggerable. As long as the foot-switch is depressed, the DD-500 receives its hold command, while the relay is activated by the NE555. When the foot-switch is released, the DD-500 immediately begins replaying the frozen content, while the NE555 starts its time-out cycle which holds the relay for another second before it drops it off. This gives the player a moment to mute the strings, before the normal / lead chain becomes active again and prevents the need of choking the signal the moment before the switch is released. This takes a bit of stress out of that situation.

The circuit differs from a stock one-shot implementation by the unspectacular diode between pins 2 and 6/7 that postpones the time-out cycle by clamping the capacitor voltage low as long as the stimulus persists.

The length of this time-out period can be adjusted to taste by modifying the R/C time constant. With growing dexterity you might want to reduce it. I recommend to leave a minimum dead time of 500 ms.

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Switching Without Pops

The SoS routing device needs more freedom of pops than any other circuit, because any transits will be hyper-inflated by subsequent high gain stages and worse, repeated endlessly by multiple echoes. And this will happen multiple times in midst of a song.

A small signal relay was chosen to switch the signal reliably.
However, normal source switching as customary with current relay operated true-bypass switching methods was tried an abandoned, so were several solid state attempts of eclipsing the switching transits.
None of that worked well, besides introducing undue complication. Noiseless switching ICs are unfortunately obsoleted.

The solution chosen, muting with relay contacts, is a forward-thinking one, since relays are unconditionally available. The switching topology was changed into shunt muting, where the DC and AC path never get interrupted even when the respective signals are steered. [ELLIOTT]

This relaxed the situation a lot, but it is only half the truth.

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Concealing The Relay Switching Current Surge

LEDs are well known to create current surges upon switching abruptly, which can intrude into the audio path via the supply [AMZfx]. A relay is no different in this respect. A 40 mA current surge is tough to ignore by other electronics.

It so happens that I had used a HQ signal switching relay from a big stash I had received – which was 5V powered. A suitable dropper resistor made this work in a 9V environment. But the very resistor can be used to serve as a formidable R/C filter, which effectively and efficiently conceals the relay switching.

Pop suppression and signal muting/steering work incredibly well despite using mechanical switches. I believe it cannot get any better. The loudest noise you hear is the mechanical click of the momentary switch. I suggest obtaining a 2p2t soft switch if one is available.

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 Connectivity

All jacks chosen are isolated. This will leave the metal case floating, which is a bad thing. It has to be grounded in order to avoid hum, so it has to be connected to ground somewhere. I cut a thin piece of brass sheet to size that goes between the foot-switch and the case during mounting. A wire soldered to the brass sheet connects to the normal / lead chain’s GND.
Thus the SoS router’s case must not touch any other box’s metal case, at least not from the isolated chain, since this will violate the isolation. In case it happens, it may produce hum,  but it cannot do any harm.

The jacks for the ground-connected side and the isolated side are mounted at opposing sides of the case, so that the plugs’ metal cases cannot touch by accident. All neighboring plugs however may touch without consequences.

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Miscellaneous

• Using an indicator LED was abandoned due to the current inrush problem we just successfully avoided. [AMZfx]
• Can the op-amp cope with the loads?  [KEEN2], [ELLIOTT]
• Although TL074 worked, LM837 has been chosen for better drive capabilities.
• Signal muting could be reduced to one single change-over switch rather then using two by rearranging the contacts. The current schematic reflects the peak of an evolution and the shown  assignment of contacts is historic.
• The foot-switch used (Alpha, likely 32s1n) is supposedly a higher quality switch. They claim 30.000 activations.
  The switch is sold as "soft touch", and while it may require some lower activation energy, it is a clicky one. I obtained a genuine Alpha switch and it is soft touch and quiet.
  Unfortunately, all genuine clickless switches that seem available, are all single pole. It will not be a problem in a normal band context.
  The device could be converted to OptoMOS solid state "relay", but then there is a high chance that these specialty parts will not be available tomorrow (as usual)...
• The principle of this routing device could be expanded to accommodate simpler delay units, but considered how often (or: rather not often) this device expects to gets used...
  

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Programming The DD-500

It is debatable if BOSS wins the cup for the best delay pedal with the DD-500, but they will easily win the prize for the worst and most useless manual. I suggest to look into the factory patches to learn from them.

Luckily, I found a few helpful user patches that got me started. One of them comes from a guy by the name of Brent Hutto [HUTTO] and shows the principle of the freeze mechanism (“hold” as they call it).  His latching version makes no sense, because the frozen signal is intrinsically of invariable nature. I suggest to play with that until it suits your taste.

The drone of the DD-500 can in times sound a bit stereotypical like a mismatched loop with clearly audible “seams” where the samples are stitched together, but in other times beautifully wavering. This is due to the not further disclosed proprietary hold mechanism.
The DD-500 must be instructed to react to an external grounding switch, which is then linked to the freeze patch’s hold function. The aforementioned internet source describes this in detail.

Alone from the perspective of flexibility and ease of programming the DD-500 is hard to beat.

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Should you consider the DD-200, note that it has the hold function externally accessible like his bigger sibling, but as far as I can see, it cannot be switched into simultaneous mode.  It thus cannot make use of the subsequently explained "beautifiers". Seeing the small price differences for used specimen of both units, there is little incent for getting it.

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Sonic Beautifiers

A subsequent “beautifier”, made out of a flanger or chorus preset supplied with the factory presets, obfuscates all that by superimposing its own modulation. You can run both effects in series at the same time by switching the system to simultaneous. In simultaneous mode the DD-500 can run two patches at once, one piped into the second serially or parallel if desired. The display reflects this mode by displaying both patches. In this case, the top one (denoted "A") is funneled into the bottom one (denoted "B").
Further modulation can be applied by assignments.  These are modifiers that work upon parameters like modulation rate to create even more of an ever-changing wavering pattern, which is very appealing and fit for the task. Even stereo effects are possible. You can get wild on that and max out the DD-500's patch space...

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Using The SoS Router

To initiate a new drone chord sequence do:

• mute the guitar signal with your hands and
  
• when the chord change is imminent, depress and hold the SoS router switch
• play the drone chord in sync with the beat
  
• release the SoS router switch
• quickly choke guitar signal to avoid spill into the (potentially) high gain chain. You have less than one second until the normal / lead chain is activated (dead time). Any spill into that will be LOUD and sounds awkward.
  
• await return from dead time and
  
• start lead playing

This sequence requires a little training, but soon it becomes second nature.

Gilmour conceals the transits with a volume swell  by the way. He must have a more complex switching mechanism.

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Sound Samples

A short demonstration of the unit. The video quality has been reduced to keep the file size small. Best listening experience with headphones. SoS router (drone) > DD-500, > stereo output (two amplifiers) SoS router (lead) > Dylan159 bigger muff>proprietary solid state Tube Driver sound-a-like > Strymon Volante The following recording equipment has been used: Stereo room mics Recording device: PC interface into DAW Digital camera Video editing in Mixcraft 10 all recordings untreated

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Reference

    R.G. Keen: Improved Hum Free A/B/Y, http://www.geofex.com/FX_images/humfree2.gif

[KEEN2]

    R.G. Keen on diystompboxes.com: Splitter with a transformer, post#4,

    https://www.diystompboxes.com/smfforum/index.php?topic=121649.0]

[ELLIOTT]
    Rod Elliott: Muting Circuits for Audio, chp.1, https://www.sound-au.com/articles/muting.html

[AMZfx]
    Jack Orman (AMZfx): LED Popping - Stop the LED Indicator Noise, https://www.muzique.com/lab/led.htm

[HUTTO]
    Brent Hutto: Freeze Patches for DD-500; https://brenthutto.com/freeze-patches-for-dd-500/