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YAMAHA
STRINGS
SS-30
RACK-MOUNTED WITH MIDI
MIDI STRINGS

Monday, October 11, 2004

Polyphony

As I mentioned in the requirements there's no real short-cut to triggering the SS30 and every key will have to be addressed individually. I thought it would be a good idea to get my thoughts on polyphony down in writing and put to bed some misconceptions about polyphony in general and the SS30 in particular.

A good part of my understanding of polyphonic synths and their foibles came from the excellent Synth Secrets series of articles by Gordon Reid, published in Sound On Sound magazine. Some of what I'm about say is taken directly from the parts 20 and 21 published in the December and January 2000 issues.


The first question most synth people ask is "how many notes can it play at once". This seems an odd question to an organ or piano player. They can always play all the notes at once (should they choose to do so) but it isn't ever that simple for synths.

In order to cut down on the amount of circuitry required in creating polyphonic synths manufacturers came up with a scanning mechanism using digital logic that meant they only needed a small set of circuits that could be assigned to play notes on the fly. So, one of the early such synths, the Oberheim 4-Voice had four voice circuits and was 4 voice polyphonic. When you pressed the first key the first voice module played. If you then pressed a second key before releasing the first key the second voice module would play and so on till you had four notes pressed down. Many later synths were 8 note polyphonic.

This sort of thing is necessary for synthesizers designed to give you as much programmability as possible as you need a large amount of control, but the SS30 is dedicated to one sort of sound and puts it's effort into that one sound. Instead of trying to be clever the SS30 takes the sledgehammer approach of having an individual oscillator for every note (infact two oscillators for every note! But that's not important right now). So, the SS30 get top marks. It's fully, err, multi-phonic. I've read in some places people saying that the SS30 is 8 note polyphonic but this simply isn't true.

So far the the SS30 is only as good as an electronic organ. However it does have one more polyphonic trick up it's sleeve.

In subtractive synthesis (what most analogue synths practice in) there are two ways to change the sound you hear from the oscillator: 1) Alter it's volume (or amplitude) with amplifiers and 2) Alter it's tone, with filters. In order to bring a sound to life you need to alter these attributes over time. The most obvious way is with an envelope. An envelope determines how the something develops over time from the moment you press the key. Typically they have settings that determine how quickly the envelope contour increases and then decreases.

For a keyboard to be truly polyphonic it must have a individually triggered envelope for each key. Monophonic keyboards only have one tone and they restart the envelope each time you trigger it with a new key, as you'd expect (this can be changed on some monosynths though). On a polysynth you can still just have one envelope which starts when the first key is pressed and finishes after the last key is released but it is rather limiting. It's okay for pads but, with slow attacks for example, overlapping notes wouldn't sound right. If a so-called polyphonic keyboard has just one envelope it's only para-phonic and many synths were built like this including one of the SS30's rival strings machines the Solina Strings Ensemble. To be truly polyphonic it needs to have individual envelope generators for each key. They need not be individually controllable, as setting the envelope for every key would be highly tedious. Instead a global setting is used for all keys.

The SS30 is not a synthesizer in the true sense. It has synth like controls but it is fixed to one type of sound and variations of that basic sound. In particular the oscillators are fixed to one wave form. A real polysynth would have envelopes for each notes volume and each notes filter, possibly even it's pitch too. The SS30 has envelope control over the volume but not the filter. This is simply because bowed string sounds don't markedly change their tone as they develop. There's just no need for a filter envelope and it would mean more circuitry. The volume, or amplitude, envelope attack is locked to two settings - Fast and Slow and the sustain is variable .

The envelopes are generated by the the K boards which you can see in the picture below. Look! There's a circuit for every key.


You can see a bunch wires attached at one end and then another bunch leading off to the other. These are the connections from the physical key switches, (some of which you can see at the bottom of the picture) each going through a circuit and then leading off to the G (Generator) boards. The circuit is an envelope generator with settings for attack and sustain. Over on the top right of the board are some more wires and these run back to the attack and sustain controls. There's also a trigger line which switches the vibrato circuit in and out. I'll maybe write in more detail on these boards later

So, is the SS30 a truly polyphonic synth? Yes and no. To be a truly polyphonic synth the SS30 would have to have a individual VCA and VCF envelope for each of it's 49 keys. But, then again, it's not a synth it's strings machine and it does score over a few other strings machines and even proper synths with it's polyphonic envelope generators.



Friday, October 08, 2004

Pictures.

I now have a digital camera so here's a few snaps of the box of junk that used be functioning SS30. Annoyingly some of these came out a bit blurred but you get the idea.

The box of junk.



Examining the entrails. Note the key switches at the bottom.



Switches.



Pot's sans knobs.



These are the key driver boards. I've used PCB pillars to stack them up whilst I did some investigations. Originally they were laid flat under the keyboard which flipped up to allow you access.



Here are the 4 voice generator boards laid out with the 3 mixing/FX boards. This is how they were laid out in the original case except they were mounted vertically rather than flat like they are here. Check the 30 cm rule for scale.



And here's a shot of the back of the generator and mixer boards.



I've put the power supply in a case so I could work safely with the mains on.




I fitted the back panel too.






I've tracked down the user and service mannuals from Yamaha. The service manual is original whilst the user is just a photocopy.




The service manual pages fold out!



Tucked in the back of the service manual is this double-sided sheet with a block diagram.