Monday, December 15, 2014

Sound generation - SS30 Lives! Part 2

On Friday night I made some more wiring fixes and played around with the pitch, detune and vibrato controls.

video

More on the oscillators and pitch control later.

Wednesday, December 10, 2014

Sound generation - SS30 Lives!

Last night I proved that all the key switches on the K boards are working and that G boards are generating tones on all of those notes. In fact I got some sound out of the SS30!

I had to fix one wire on K2 



Then I hooked the scope to the keying circuit of C2, the first note on K1 and the 8L output of G1. 

The voltage on the key circuit rose when I connected it to earth (-7V up to 0V) and then fell back gradually when I disconnected it, showing that the sustain was working well. 

The output from the generator showed something like what I was expecting, although not exactly. It seemed to have a lot more going on that I was led to believe that it would by the circuit diagrams. It looked like a mix of notes.

G1 8U output

I tried the rest of the keys and got the same result. So far so good.

Then I moved to K2 and G2. I got the same result but this time it was unexpected. The G2 8L output should only have C#3 to F#3. On G2 the octaves are divided as part of the keyboard split option. So, keys on the upper half of the octave should not be generating output on the lower half of the generator board. 
 
G2 Outputs

My assumption had been that I would need to check each output from the G boards. This was my thinking because not only do the same key switches drive both the 8' and 16' outputs on each board, but also those outputs then get routed and mixed together in different combinations. Even if I got something audible at the final output for every key there might still be some missing components to the sound. 
I also assumed that because the switching controls, such as keyboard split and the mixing switches were disconnected there wouldn't be any signal making it's way to the output. 

Now that I am seeing what appears to be a mixed signal at every output I needed to check more carefully.

However rather than do that I decided instead to cut to the chase and see if there was any sound coming from the main output. And there was! 

Every key produced a tone and I could play more than one at once (no reason why it shouldn't, but.. y'know, it was nice to confirm). It sounded awful though. Partly because I just grabbed my Korg Monotron and used that as the amplifier. Also, I suspect there is a lot of noise being generated by the number of loose wires and the proximity of the audio output cabling to the power supply. However after looking at the circuit diagrams again I'm convinced that no sound should be reaching the output at all! With no keyboard split switch connected the bottom two G boards outputs should go nowhere and none of the outputs should get through if the mixing switches (Cello 1, Cello 2, Viola, Vioin 1, Violin 2) are disconnected. I can only guess that what I heard was somehow leaking through. All electrical signals create electro-magnetic radiation which can be picked up in wires, so it's possible.  I've seen stranger things happen. 

My next step will be to methodically trace the signals from the G boards through the circuit. Hopefully I will work out where the signal is getting through as expected and is not getting through .  I will also try reconnecting the switches to see if I can get some more consistent behaviour.



  







Friday, December 05, 2014

Owners Manual and other scans


After finding the Yamaha IC guide on the Internet Archive, mentioned here tone generation, I started to think I should share the owners manual and service manual. Actually, I was thinking it would be useful for me to have scans of them anyway. And of course if I was gong to share them they would need to be in pdf format. However, one of the things that irks me about .pdfs is when there are no links or searchable text in them. Most (all) synth manuals I've ever downloaded are just plain scans of the original printed documents and no active content. Fair enough, but I wanted to do better. So, I embarked on the task of creating scans and converting them to .pdf using Acrobat.

There are actually four documents

Owners Manual - User guide, in other words. "STRINGS SS-30 is a keyed instrument to let you enjoy rich tones peculiar to the strings with this instrument."













Service Manual - The circuit and wiring diagrams. This is, obviously, the really useful bit for the project.













Parts List - Including spectacular exploded 3D diagram







































 Block Diagram & Overall Circuit Diagram - No scans yet.


 The service manual folder also contains the parts list and the block diagrams, which are printed on either side of a huge poster sized piece of paper.  I've scanned everything apart from that as I was having problems with the scanner I was using and wanted get on with the rest. It will need stitching together.


I made a start on the service manual and I'm about 80% done.


Work in progress

 It's been a huge job to add all the text though and I got tired of it and decided to knock off the owners manual quickly to show some progress.

So, here it is!


In the pdf all of the text is searchable, the contents section contains links to the relevant pages and the 'How To Use SS30' section has links for the numbered references on the diagram.





Wednesday, November 26, 2014

Benge Studios' SS30

From the It's Full Of Stars blog

Ben 'Benge' Edwards seems to write this blog, although he goes by the handle Zagoba for blogging purposes. It's all about his studio and anything that interests him really. The whole darn thing is a dream for anyone who loves synths and recording technology. He's a lucky devil!

http://myblogitsfullofstars.blogspot.co.uk/2012/05/stringy.html?m=1

This is a nice little SS30 demo he put up. 




Tuesday, November 11, 2014

I Dream Of Wires

A great film and well worth watching even if you're determined not to get sucked into that whole different mess of wires.

http://www.idreamofwires.org/


Also a Garry Numan song and a made up (by the NME) band.

Anyway, as I may have pointed out once or twice there are a lot wires in the SS30. A lot.

I've had a couple of sessions since the G boards were brought back to life just rewiring the K boards.

First off, two of the G boards where disconnected from their corresponding K boards.

De-soldering 
Lots of old, seventies solder. 


Finding a good method for stripping wires as thin as these can be a case of trial and error. Luckily I quickly hit upon using these tweezers to just yank the plastic sheath off.

Strip and twist, strip and twist.
Note the wires are tied with fine string dressing.


Ready to solder

And on the other side, another 49 wires from the K boards to the key switches.


The muck was there when we bought it. Also you can see more loose wires to be reconnected. That yellow one should be attached to AT1.


I had cut all the wires from the key switch PCBs on the assumption I wouldn't need them again. It was also to make handling the whole tangle of boards and wires less difficult. Now I decided to reattach them temporarily to make it easier to test things out.


I removed the switches as they were in a bad way.



And the finished job. A keyboard (if your fingers are earthed).

Monday, November 03, 2014

Why negative voltages and how are the tones created?

This post is just about why the switches use a negative voltage. It's not really important to the whole project but I wanted to understand the circuit and satisfy myself as to why it works that particular way. As I said in an earlier post I was looking for a better understanding of the circuits to help with the key switching problem.

Why are we forced to think about switching a negative voltages anyway?


The way the square-waves are transformed and the switching is woven into that circuit leads to the reason for all this below 0 volts brow-furrowing.

Where is all this negativity coming from?

Envelope

Your actual schematic of the keying drive

Firstly, what is required of the key switch for each note is a method of not just switching the tones on and off but controlling their amplitude. Each note has it's own envelope (or volume contour if you prefer) going through an attack phase, where it increases in amplitude, then a sustain and release. So the key is a switch to start and end the envelope. That envelope is what drives the amplitude of the tones that you hear.

Trace of envelope from simulated keying drive circuit

Let's be positive!


If we had a positive voltage square-wave output from the divider chips and an envelope that goes from ground to positive - which would be more normal - how would we put them together? I would probably reach for an op-amp, but this machine was being designed in the mid-seventies. Maybe op-amps were expensive then.. Then I'd go for a transistor. I'd use the envelope as the control voltage in a simple VCA (voltage controlled amplifier). But what if my boss is already worried about the costs of this thing and transistors tend to demand biasing and all kinds of ancillary components to get a good result. Can I come up with something better cheaper?

Negativity wins the day

There are lots of solutions to the problem  which don't use op-amps or transistors but the actual one used is pretty neat - albeit one that introduces those fiddly negative voltages.

The solution lies in AC coupling and half-wave rectification.

Conscious coupling

Any AC signal, no matter what the DC offset, fed through a capacitor will create a signal on the other side balanced to whatever reference you choose. If it's not tied (i.e. connected through a resistor) to ground, or anything else, the signal will swing (equally) both positively and negatively. In fact, it will do that if it's tied to ground too. It's voltage swing will centre on what ever voltage it's tied realtive to. So, if you tie the other side of the coupling capacitor to a negative voltage, like -7 V,  you will get a signal which is centred on that voltage.
This is exactly what happens in the tone switching on the SS30. The square-wave output from the divider chips is passed through a coupling capacitor. This output is then tied to the envelope generator output which is -7 V in the off state, rising to 0V in the on state. When the key is up, the envelope is off, the output is tied to -7V so the signal centres around the -7 V level. In the trace below you can see the effect of this.

AC coupling - ties to ground


The lower trace is the source square-wave from the divider chip. It is 9Vpp (peak-to-peak) and -4.5 V offset from ground.  It is clearly all below the ground rail. The upper trace is the output on the other side of the capacitor, which is tied to ground. As you can see it's centred around the ground level.

You will also have noticed that the wave has lost it's square shape. I won't go into that in detail here but it has been high-pass filtered when passing through the coupling capacitor. More intuitively this capacitor has the effect of sloping off the neat, flat edges to rising and falling slopes as it charges and discharges.

The next trace shows what you get if the output is tied to -7.5V


AC coupling tied to -7.5V

In this trace the signal is all below the ground rail - like the original signal


The effect of the envelope on the signal is to shift it either completely below or partially above the ground level. That's all very nice, but the signal is never 'off', so what's the purpose of that shift?


Give us a (half) wave!


If you send a DC-balanced square wave through a diode you still get a square wave on the other side. The difference is that you only get the positive half. This is called a half-wave rectifier, which you may already be familiar with. Only the positive side of the signal makes it through the diode and the negative is blocked.
The traces below show this.

Square wave coupled and rectified


The lower traces are as shown before and the top trace shows the half-wave rectified signal.


By now you may have worked out that when the switch is off, and the envelope signal is -7.5V, tying the AC coupled signal well below 0V, the tone will not get through the diode. And furthermore, when the switch is on and the envelope rises to 0V a half-wave signal will make it through the diode.

So, we have a kind of switch controlled by the envelope. It doesn't matter that the signal is chopped and reshaped in this process. In fact we want to reshape it anyway. As noted in the magnificent Sound On Sound Synth Secrets series by Gordon Reid :

The unmodified waveform produced by a real string is similar to a sawtooth wave, so it's no surprise to find that the waveform selected by the designers of electronic ensembles was always a sawtooth or a similarly spiky waveform.
http://www.soundonsound.com/sos/Feb03/articles/synthsecrets46.asp

Circuitry


This is a simplified and cut-down diagram of one of the two circuits which are employed for each tone that goes into to make the note. The two tones share a common switch envelope, each have one of these circuits and are mixed at the end. There's more to follow that diode but this is the essence of the shift and rectifier which makes the notes go on and off.

Simplified tone switch example
Vss is -7.5 volts and the square wave is as described above.

This is the actual circuit for one of the tones from the G1 board.

G1 board tone switch and mixing example

There are two tones being switched and then mixed at the end , on the right of the diagram. The two tones come from the dividers at the top left. C1 is the coupling capacitor. The switching voltage comes in at the bottom left. Note that C1 and C2 values are specified here. These values vary as the tone frequency increases across the four G boards. The capacitors are affecting the shape of the rectified signals, so they are tuned to get the desired wave shape, and therefore sound, of each tone.

Just one last thing on switching negative...


That's nearly it, but there's is one more, unexplained, thing. If the output of the divider chips is passed through the decoupling capacitor then why are they negative going? As I noted above it doesn't matter what DC offset you start with; the coupling capacitor balances the signal to whatever reference you choose on the other side. So, why not start with a positive going square-wave?

Perhaps it's because you need fewer power rails. The amount of wiring in the SS30 is already extreme so limiting the power cables around the place is probably a good idea. The G boards use a single -26V supply with on board regulators to step it down to











Monday, October 20, 2014

More power?

The MIDI interface needs a power supply between 8 and 35 volts. The supply current needs to be 15 mA for the interface itself plus whatever is drawn by the output loads.

The loads are connected to a common positive supply switched to ground by the darlington pair drivers.

If the driver voltage is 15V I'd need the 24V rated Coto reed relay which has a coil resistance of 2KOhm. That would be a current draw of (15 / 2000) 7.5 mA each. Assuming worst case of all 49 relays being on at once that would be approx 368 mA.

That's worst case but the question is this: Can the existing SS30 PSU be used to power the interface?

The SS30 PSU has a +15V rail (this is all carefully worked out, you understand) which is rated up to 500 mA. 


What's not clear, is how much of that 500mA is used by the SS30. I know that the +15V rail is not used for the K boards (-7V) or the G boards (all voltages are derived form the -26V rail) but it is used on the LF, OR and F boards. If the interface took the best part of 400mA (absolute maximum) is it likely the SS30 is using under 100mA?

I'm going to have to get the whole thing working and then measure the current on that rail before I can decide whether I need another power supply.