Monday, 15 November 2021

Very very good article on the inner workings of the Yamaha DX7

I rarely do links to other blogs, but today is an exception! Go to the 'Doing it Right!' section near the end, to see the details of a link to a wonderful article on the inner details of the Yamaha DX7!

Getting it wrong!

The way the Yamaha DX7 synthesizer is treated on the InterWeb is interesting because they are very often full of myths and errors. 

Some articles say it uses FM (Frequency Modulation), when it actually uses PM (Phase Modulation). Some articles say that it sold more units than any other synthesizer, ever, (over 150,000 units - definitive figures are tricky to find and verify), forgetting another digital synth from slightly later, the Korg M1 (over 250,000 units - again, difficult to verify), and several other contenders since... (There again, you can always spiral down into the black hole of clarifications: digital, 12-bit (kind of), hand-built, custom chips...) Some articles say that the DX7, and FM, are difficult to program, but then so is C++ (and many other things) - and yet a lot of people managed to program the huge number of patches/presets/sounds that are available for the DX7 (including a few hundred from myself), so it can't be that tricky! Oh, and some articles say that Yamaha synthesisers do not output MIDI velocity above 100 (instead of up to 127) - which was true for the first few DX7s, and was then fixed, so it not only wasn't correct very soon after the launch in 1983, it hasn't been right since for almost 40 years! Basically, it seems as if many authors of articles about the DX7 and FM don't do their background research properly and just copy the same old fake news and myths. 

(Oh, and hardly anyone ever notes that the original 'number' proposed for the DX7 was the DX5...)

Doing it right!

In total and complete contrast, in every way, a recent blog post from Ken Shirriff is throughly recommended because it not only gets everything right, it also goes very deep into how the DX7 works

Congratulations, my admiration, and kudos to Ken Shirriff for an excellent article!


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Monday, 8 November 2021

The Chase Bliss EXP Pedal...

At the low end of the guitar pedal marketplace, you have budget pedals: small, very affordable, and all very similar: fuzz, overdrive, chorus, phaser, flanger, delay, reverb...

At the opposite end of the market there are what are often called the 'boutique' pedal manufacturers: sophisticated, beautifully engineered and as some marketing people like to say: 'reassuringly expensive' - but most importantly: diverse and unusual. You won't find conventional 'me too' phasers, loopers or delays here. Instead you will get informed, flexible, powerful, eclectic pedals - plus many other words associated with innovation and exclusivity.


A perfect example of this (and actually, pretty much a perfect pedal) is the recently announced EXP pedal from Chase Bliss Audio. Wikipedia describe Chase Bliss Audio as being 'high-end', whilst Anderton's Music Company says: '...not just another pedal company, no...21st Century Trail-blazers...'. Chase Bliss Audio themselves say it very nicely on their web-site: 

Chase Bliss makes pedals that mix digital wizardry with analog goodness to create weird and wonderful sounds.

However, the EXP pedal is slightly different - it is just 'analogue goodness'. (No digital inside at all!)

In these days of 'Do you want a microprocessor with that?' turning up in everything, then this is a bold move, but it is a great move.

The reason why this is such an amazing move is that EXP is the expression pedal, reinvented. 

I will let you think about that for a moment...

Instead of an angled foot plate that you move with your foot to control guitar pedals via the 'Expression' input, the EXP pedal looks more like a very robust, very engineered modulation wheel that has escaped from a synthesizer - maybe someone like Expressive E. So you can use it on top of a studio desk, where it gives a nice, convenient, tactile interface to controlling guitar pedals being used as outboard effects, and the classy black metal case with its minimalistic labelling, plus the 'executive toy' knurled texture on the wheel, plus the glossy numbers wrapped around it, mean that it looks like it belongs there. Alternatively, put it on the floor or on a pedal board, and the rugged metal box, plus the grippy metal wheel that can be moved with your foot, mean that it is right at home there as well. 

And whilst you are visualising your foot moving the wheel on the EXP pedal, here's an interesting practical thought: have you ever knocked an expression pedal whilst feeling about with your foot for a foot-switch on a pedal board? Well, the wheel on the EXP is almost immune to such accidents, because unless you put your foot on it and move your foot forwards deliberately, then it isn't going to get accidentally moved... The reason that you may feel the world wobbling a little at the moment is a large number of guitarists all nodding (with a slightly embarrassed smile on their face at various live 'incidents'), plus more nodding of heads from all the live mix engineers who have had to suddenly pull back the sliders for the guitarist who just jumped up 20 dB in the mix.  

So let's look at this design. Instead of 15 to 30 degrees of angular movement from your foot, you have something like 270 degrees of rotation of a wheel, or about 150mm (6 inches) of linear movement of a foot (or hand). So that's lots of fine control, plus there's a big number that shows from 0-10, so you know where you are in the range. I can't recall ever seeing a foot pedal marked off in degrees...


Now, I have never been able to resist opening up gear, and so that's what I did next...

That's a single piece of bent metal that isn't fastened to either the base of the case (as you can see), nor the top part. It is held in place by the base when the case is assembled, and the electrical connections to the top part of the case are made thru a Molex sparse ribbon cable. As you can see, the wheel occupied all of the available vertical space, nearly touching the base - but it doesn't, of course! What you also get hints of here are some bearings (the thick metal casting in the lower left, and of some mechanics in the central fold hole. 

Sharp-eyed readers (viewers, actually) will probably have spotted that there are two 'stereo' jack plugs connected at the back, and these are for the two electrically separate outputs. The two rectangular hole near the top have two switches just below them, and these can reverse the direction of the outputs, so you can have heel and toe the same or both, or both reverse, or heel and toe opposite in two ways. I set the switches to opposite positions, so that I have two outputs that are the inverse of each other. So whatever I control with expression on one pedal, I can control the opposite on another pedal. But as you can see, just by removing 4 bolts and flipping a switch, I can change that at any time. 

I was pretty impressed with the mechanical minimalism here. I was expecting something much lighter and that the switches would be more awkward to get at. I like good surprises.

(What is interesting here is the forced perspective. You might be wondering why the base looks so much smaller than the case.? The clue is the shadows... Yes, the top part of the case is actually much higher up because I had to stand it on a couple of wood blocks because the wheel sticks out of the top of the case. So the top part is much closer to the camera, which is why the shadow for the top part of the case is diffuse, whilst the shadow for the base of the case is so sharp - it is on the blue background, whilst the top part of the case is about 30 mm above it. So now you know! You eye assumes that the two objects are both on the blue background, and so the base must be smaller even though it is the same dimensions!)

Going a bit deeper was interesting. That single piece of bent metal IS the holder for all the mechanics, and it is not fastened to the top of the case at all. There is just the sparse black ribbon cable. So you just lift out the bent metal...

As I said, the wheel has substantial bearings in those solid blocks of metal. The wheel connects to the pot using a toothed rubber drive belt, so no slipping. and the potentiometer is, as expected, a dual gang linear potentiometer, which explains how the two outputs can be electrically separate - one gang is used for each output. It looks as if the circuit board is held in place by the two jack sockets, so, that's a neat solution as well. The diameters of the wheel gear and the potentiometer gear appear to be pretty much the same size, and so the wheel uses the whole 270 degrees of rotation of the potentiometer, which is why you can invert the ranges for the two outputs.

So the mechanical design is very cool. There are no fastenings needed for the main bent metal assembly to hold it in place inside the case, other than the case itself. The four bolts that hold the base of the case in place, also hold the mechanical assembly securely and solidly in place. The metal is bent to form the supports for the bearings and the potentiometer. I just love it. Minimalistic, with no levers, easy disassembly and it looks like servicing (replacing the rubber belt, or the bearings, or the potentiometer) should be very easy and quick. I'm struggling to find anything to criticise here. 

Even if I was very picky and said that dust might get inside through the rectangular hole in the top of the case for the wheel, then just opening up the case and blowing away the dust is probably going to be all that is required. If liquids get inside, then it is all passive components, and the potentiometer is unlikely to cost a fortune to replace. 

Using it

Expression pedals are made to allow you to control things - to make them expressive. So next, I look at using it...

Well, it works. Wonderfully. It's like an expression pedal, except that you don't rock it from heel to toe with your foot, instead you roll from one end to the other, which seems to be about 270 degrees of rotation, and the end stops feel solid and not sloppy, and not particularly padded. Rather like a lot of synthesizer modulation wheels, actually. Perhaps a little stiffer, but it may loosen up as I use it more. But not a problem. My only criticism is that moving rapidly from heel to toe requires a little bit of practice with your hand (or your foot). 

And there's the amazing thing. It feels right either as a desktop hand operated controller for expression, or on the floor as a foot-operated controller for expression. It is quite heavy, so there's no skittering when you stick the rubber feet on the bottom, and the base is smooth and matt black paint, so Velcro is going to hold it well on a pedal board base. 

There isn't very much room, but I'm sure that someone is going to find a way to put a couple of LEDs inside so that the wheel hole lights up...

(I've tried modding one of my ordinary expression pedals so that they have two outputs, and the problem is that you need a pedal that uses the whole 270 degree range of the potentiometer - and the pedal I used didn't do that... My advice is to only try this on a pedal if you are expert in mechanics, which isn't me...)

With the EXP, it is very cool to be able to seamlessly blend from two very different effects, or even to find interesting points in-between the extremes. As with this type of thing, the more you play, the more you will find. So there's plenty of scope for exploration, and as for robustness, then I would have no hesitation using the EXP live or taking it on tour. 

Expression, and Voltages...

Now I could have used a photo of the EXP connected to the 'Expression pedal' input of a guitar effects pedal, and it would be ever so slightly boring and obvious. Instead, here's a photo where the guitar effects pedal is off camera, way to the left, and instead, the EXP is connected to a little bit of hardware (the 'EXP CV' box) that I threw together, because the design of the EXP also makes it well (maybe even uniquely) suited to also being used as a robust CV controller.

Nope. I haven't gone mad. Expression pedals output a voltage, which used to be somewhere from zero Volts-ish to somewhere less than 5 Volts. These days, processors tend to run on just over 3 Volts, and so somewhere in the range of 0-3V-ish is where you expect an expression pedal to output. 

(What happens is that a guitar pedal 'expression socket' does two things: it provides power to an external potentiometer inside the foot pedal, and then it receives the output of that potentiometer back as an input! So older pedals provided 5V power to the external foot pedal, whereas modern pedals only provide 3.3V. The potentiometer just taps off a percentage of the voltage, so if it is half-way round its whole rotation, then it outputs 50% of the voltage, so for 5V that would be 2.5V, and for 3.3V it would be 1.65V. The guitar pedal knows what voltage it outputs, and so it can calculate how far the foot pedal has rotated. This dual-function: providing a voltage output and a 'rotation' input, is why you need to use a 'stereo' cable for expression. So three (3) wires are needed: Ground (common to both functions, the Power (5V, 3.3V...), and the Rotation CV from the potentiometer - and a 'stereo' jack provides three connections. Note that the foot pedal doesn't care about the voltage - the potentiometer only outputs a 'rotation' CV that is proportional to how far it has been rotated. )

The range depends on the mechanical linkages used in the pedals - levers don't always seem to rotate the potentiometer through the full 270 degrees - as I noted earlier. But the EXP pedal does use the whole range, and so you can have two opposite outputs that both go from heel to toe. This is so cool, especially when you've tried to make a pedal like this and failed! 

So the mysterious box that is connected between the distant guitar pedal and the EXP is a new generation of my expression pedal range tester. It is ridiculously simple: two stereo jack sockets connected in parallel, and a 3.5mm mono jack socket for the CV output. 

To this simple circuit I have added a cheap three-wire voltage display module (a voltmeter) from Amazon, plus four AA batteries in a holder to provide power for the display. One of the things that isn't well publicised about voltage displays is that the two wire versions are powered from the voltage you are trying to display, and they typically need at least 4.5 Volts to work. So you can typically measure (and display) voltages from 4.5V to about 30V. When the display goes off, then you know that it is below 4.5V! 

But a three-wire voltage display module has an extra wire (usually white or grey) that connects to the voltage that you want to measure, whilst the usual black (the common ground connection) and red (power) wires go to a separate power supply - in my case, those four AA batteries in a holder. Anyway, the display is thus powered separately from the voltage that is being measured, and in this case, separately from the expression pedal socket of the guitar pedal. The end result is that the batteries give me a 6V power supply for the voltage display, which is fine for measuring the 0-5V (max) that expression pedals normally output. 

Using this voltage display technique to make some measurements, my M-Audio Expression Pedal can output between 0-2.75V to 2.84-3.14V for a 3.3V expression pedal socket, my Bespeco between 0-3.18V, my Nektar between 0-3.07V, and my Casio VP-1 between 1.68-2.56V. I will leave it as a challenge to you to figure out which pedal I tried to put a dual gang potentiometer into... 

The EXP? It produced 0-2.76V for one switch position, and 0-3.04V for the other position. So, for the first time in the review so far, a not-quite perfect result. But bear in mind that this is not a specified parameter for the EXP, or for any other expression pedal that I have ever bought, so I am being way beyond picky here. And when connected to a range of pedals (Empress, EHX,  TC Electronic, and Keeley), the EXP performed exactly as it should. 

One thing to remember when doing this kind of testing is that most guitar pedals seem to detect when an expression pedal is connected to their expression pedal input only when the power is turned on. So you need to power the guitar pedal down, insert the expression pedal (or EXP!) jack into the expression socket, and then power the guitar pedal up. The expression pedal (or EXP) will then be recognised! 

(If you just plug an expression pedal into a guitar pedal without cycling the power, then it may not be recognised at all - as I found out when doing all the unplugging and plugging...)

Anyways, in the photo above, the mysterious box displaying 1.24 Volts in blue digits is showing the output of the EXP when set to about a setting of 5, with the 'source' voltage from the guitar pedal's Expression socket of about 3.3Volts. That 1.24 Volts is what appears on the orange 3.5mm jack plug.

Yep, not a 'converter', more an extractor, or a rerouter, or an 'in another way' box. But yeah, it takes an EXP and outputs a CV. So what can we do with that?

First off, lets check the voltages and how they correspond to the EXP wheel setting:

Okay, so zero on the wheel is zero Volts. Cool.

Aha! So 10 on the wheel is 3.04Volts. Makes sense. So we have 3 Volts of CV range to play with... So 6 and a bit on the wheel would be about 2 Volts?


One thing we could do with this is connect the orange 'CV' patch cord into something modular. The Rebel Technology 'Witch' is a good example. It is a programmable synthesizer, processor and effects unit that has CV and Gate inputs and outputs (plus audio I/O), is a USB Audio interface and Host, and does way more than you would expect for such a small box. A tiny modular system in a little box, and huge amounts of fun with elastic/flexible capabilities because you can reprogram what it does from a library of patches (and store eight patches inside the Witch for instant recall with those round buttons!). In this case, the CV input is controlling Input A, which sets the pitch of a drone patch. Now if the other EXP expression output was controlling a pedal using the expression socket, we have even more sound-making capability in a tiny space. Wooh!

Getting a CV output from an EXP is nice, but it isn't a MIDI Controller. or could it be? Here's that control voltage (the red cable) going into the CV input of the amazing Befaco VCMC - a Voltage Controlled MIDI Controller. So the MIDI socket in the top left hand corner of the VCMC is outputting MIDI Continuous Controller messages using a CV extracted from the expression control output from a Chase Bliss EXXP Pedal that is simultaneously controlling a guitar pedal off to the top left. Oh, and 8 on the wheel outputs about 2.64Volts...

Now to use this CV output in a larger modular Eurorack (or other modular), then you would need to scale this voltage to make it slightly larger. So some sort of Utility or Maths module would be the first port of call. After that, then you could use it to control just about anything. For me, having those numbers on the wheel means that I have a repeatability of setting a value that I don't get with foot pedal controllers, and being able to control two guitar pedals (in opposition, if I want) plus having a CV available as well, opens up huge avenues to explore...

Voltmeter displays

I need to point out that low-cost Voltage displays from Amazon are not always very accurate in my limited experience (Some may be - your experience may differ. Other voltage displays are available.). I've compared the values displayed here with my own slightly more expensive multi-meters, and whilst the multimeters are 'reassuringly' similar in the value they display, the voltage display tends to be about 0.3Volts lower. Not perfect, and so there should really be a little sticker on my EXP CV home-brew box that says: 'For Indication Purposes Only. Not Calibrated'. But for indicating what sort of voltage is coming out of the EXP, it works fine. And blue displays just look super cool!

Oh, and the EXP CV box that you see here is my own personal home-brew hand-built utility box. Not connected in any way with Chase Bliss Audio. It is not for sale (I don't do Klon pedals either...). If you happen to make a box anything like it yourself then that (and what you do with it) is entirely your own  responsibility - you do it at your own risk. 

The EXP pedal 

And that about wraps it up for this quick look at the astonishing piece of mechanical engineering that is the Chase Bliss EXP Pedal. There's a lot you can do with it, and it puts 'expression' control of guitar pedals into a rather different place when you go beyond the usual 'foot pedal' user interface. I really like having a 'Mod Wheel' for expression, and I intend to use my EXP a lot! Congratulations to Chase Bliss Audio for a very different approach to controlling expression!

I bought the Chase Bliss Audio EXP pedal with my own money. I am not getting paid by Chase Bliss Audio for this review. But I do think they make very interesting pedals. My kind of pedals! (And hopefully, yours too, now!)



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