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Synthesis Techniques For Emulating Vintage Sounds

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Synthesis Techniques For Emulating Vintage Sounds

Postby ramthelinefeed » Fri Jul 24, 2020 4:06 pm

Re: Rory Dow's "Synthesis Techniques For Emulating Vintage Sounds" article in the latest SOS
https://www.soundonsound.com/techniques ... age-sounds
Interesting article, and I didn't disagree with any of it,
but here's another oft-overlooked contributor to the 'vintage' sound (at least, in my hot take :ugeek: ):

On most old monosynths, so that they'd play 'in tune', you'd typically 'range and scale' the oscillators. This would usually be done by calibrating the keyboard CV at the bottom and top of the keyboard (something that would often involve much cursing and fiddling about with 'trim pots' on the circuit board).
So for instance, with for your typical 1 volt-per-octave monosynth with a three octave C-to-C keyboard, you'd tweak the trimpots to get 0V on the bottom C and 3V on the top C (i.e. perfect octaves). Same for Hz/V synths, only different ;)

The keyboard was designed with the correct resistor values to that each key tapped a proportional amount of voltage along the keyboard to give 'equal temperament' (i.e. each semitone was a twelfth-root-of-two ratio to the next, all equal).

My hot take is that vaguaries of resistor tolerances along the keyboard would be such that some of the semitones would be fractionally bigger or smaller than they were supposed to be for equal temperament. We'd only be taking a few cents in either direction.
(When discussing such matters of tuning and temperament, semitones are conventionally subdivided into 100 'cents'.)
This would be a static sharpening or flattening of each scale degree along the keyboard, not something that would drift (well, unless over a timescale of years). And it would be characteristic of a particular individual instrument (e.g. no two minimoogs being identical in that respect).

It is subtle thing, and generally (on a well manufactured synth) much less of a deviation from equal temperment than what you would require to achieve well known alternate temperements such as 'Just intonation' or 'werkmeister' (where the deviations would be more like a couple of dozen cents). And you probably wouldn't perceive it directly just from playing the synth on its own.
But I'd argue that it it most definitely adds a certain 'character' to any given instrument that will make itself felt.

To investigate whether I'm talking rubbish or not,
take an old analogue synth, set it to a very simple unmodulated basic patch (one VCO would be best), and play it into a tuning or pitch-detection plugin on your DAW (e.g. 'ReaTune' on Reaper).

First you need to make sure the VCO is ranged and scaled so that it is giving a true octave interval between the top and bottom of the keyboard.

Then work your way up and down the keyboard, playing scales.
If I'm right, you'll see from your tuner plugin that quite a few of the individual keys are a few cents off the ideal equal tempered value.


Modern digital synths generally have absolutely perfect spot-on equal temperament (unless they come with esoteric menu options to select other temperaments and tunings).
These preset alternative temperaments can be great fun to experiment with in themselves, but if your synth also offers the ability to set up custom temperament (usually by programming in different offsets for each scale degree in cents), you could experiment with adding some very small deviations of just a few cents, for more 'vintage' character.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby desmond » Fri Jul 24, 2020 4:31 pm

Well, that's just another source (among many) for slight pitch instability and/or randomness, which is easily replicated in modern synths with decent modulation facilities.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby ramthelinefeed » Fri Jul 24, 2020 4:37 pm

desmond wrote:Well, that's just another source (among many) for slight pitch instability and/or randomness, which is easily replicated in modern synths with decent modulation facilities.

NO, Desmond, it is *not* a source of pitch instability or pitch randomness.

It's a systemic static off-set in the temperament of particular keys on a synth's keyboard. Which would always be the same, not random or drifting.

It will potentially imbue a certain subtle 'character' to a particular given synth.

A more crass and obvious example of larger static variations in temperament was on particular mellotron tape sets.

For example, as every schoolboy knows, Tony Banks wrote the intro to Genesis' "Watcher of the Skies" using a selection of chords that 'sounded good on his MkII' (using a blend of MkII brass and 3violins, and accordion bass).
He would bemoan that it 'never sounded quite the same' on the string tapeframes of this Melltron M400.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby desmond » Fri Jul 24, 2020 4:42 pm

ramthelinefeed wrote:NO, Desmond, it is *not* a source of pitch instability or pitch randomness.

It's a systemic static off-set in the temperament of particular keys on a synth's keyboard. Which would always be the same, not random or drifting.

Sure. I understand that. And, like all the other things helping to destabilise the overall pitch or any given played note (temperature variations, modulation paths, component tolerances and age etc etc), it will add up with those things to give whatever pitch on whatever note you play - you can't decouple it's affect on pitch with everything else that's not giving perfect pitch.

That's what I mean by overall pitch instability per note, which is not a deterministic thing, even if the keyboard generated voltages as you describe might be somewhat deterministic within a given set of conditions (which itself may be variable, eg temperature variations, component tolerances and age etc etc).
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby Eddy Deegan » Fri Jul 24, 2020 5:13 pm

ramthelinefeed wrote:My hot take is that vaguaries of resistor tolerances along the keyboard would be such that some of the semitones would be fractionally bigger or smaller than they were supposed to be for equal temperament. We'd only be taking a few cents in either direction.
...
This would be a static sharpening or flattening of each scale degree along the keyboard, not something that would drift (well, unless over a timescale of years). And it would be characteristic of a particular individual instrument (e.g. no two minimoogs being identical in that respect).

Much as I'm sure there were/are tiny differences in the intervals between successive keys on old analogue gear, specifically because of it's fixed nature I don't buy that it really had much, if anything, to do with 'that analogue sound'. Apart from anything else, they sounded 'as good' while holding a single key down (which would have nothing to do with the tuning of that specific key) as they did while playing melodies or chords.

Far more significant is the dynamic instability and inconsistency inherent in those legacy analogue systems. LFO speed variances, oscillator drift, filter component differences, noise and so on all add up to a system with just enough 'randomness' in it to hit a sonic sweet spot.

At least in many cases; in others it could be a pain in the neck - I've heard Rick Wakeman bemoan the instability of some early Moogs and others bemoan the instability of some early Prophets.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby N i g e l » Fri Jul 24, 2020 6:34 pm

on the note variation from scale value, I have found two usefull methods to implement this on digital synths which have good mod matrixes

[1] LFO = random, frequency = changes when new note pressed.
if LFO is applied to a note then not only will the note be off scale but it will be different every time the note is pressed (eliminates "machine gunning").

[2] modulate the note with the midi note number. Same "error" for a particular note but obviously as the MIDI note number increases the error increases.

This can be overcome with a Modulo maths function - MIDI note mod 12, would mean there would be consistancy every octave. Use further functions, offsets and finer Mods to randomize/tune the note inconsistancies.

These are perhaps quicker to implement than a custom "wonky scale" but more difficult to fine tune if you want a specific "error" to your notes.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby ramthelinefeed » Sat Jul 25, 2020 10:49 pm

desmond wrote:Sure. I understand that. And, like all the other things helping to destabilise the overall pitch or any given played note (temperature variations, modulation paths, component tolerances and age etc etc), it will add up with those things to give whatever pitch on whatever note you play - you can't decouple it's affect on pitch with everything else that's not giving perfect pitch.

That's what I mean by overall pitch instability per note, which is not a deterministic thing, even if the keyboard generated voltages as you describe might be somewhat deterministic within a given set of conditions (which itself may be variable, eg temperature variations, component tolerances and age etc etc).

No, it's not a source of pitch instability. It's *completely* deterministic. It would be a static adjustment to the instruments' temperament. Resistor values won't drift except over a timescale of years, and any overall variation or instability in keyboard CV with temperature or dodgy voltage regulation generally would still always be off-set in the same direction and proportionate degree for each note.

If general VCO pitch stability was awful enough, it would swamp the temperament offset effect, yes.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby ramthelinefeed » Sat Jul 25, 2020 10:53 pm

Eddy Deegan wrote:
Much as I'm sure there were/are tiny differences in the intervals between successive keys on old analogue gear, specifically because of it's fixed nature I don't buy that it really had much, if anything, to do with 'that analogue sound'. Apart from anything else, they sounded 'as good' while holding a single key down (which would have nothing to do with the tuning of that specific key) .

Well yes, it's an effect that would only be noticeable in a musical context when played or multitracked with other instruments. It is not something you would notice if all you do with the synth is play one key for 'woo! cool sound!' effects. I appreciate this rules out about 95% of everything that's ever done with modulars these days :bouncy:
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby BillB » Sat Jul 25, 2020 11:06 pm

Interesting. The Ensoniq MR76 has the following amongst its various pitch tables such as Pythagorean, Turkish etc:

RandomDetune - Each note has been ‘tweaked’ by + or - up to 10 cents, giving chords a chorused effect which is different for each note.

Now I know why it is an option. :D
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby Folderol » Sat Jul 25, 2020 11:46 pm

ramthelinefeed wrote:Resistor values won't drift except over a timescale of years, and any overall variation or instability in keyboard CV with temperature or dodgy voltage regulation generally would still always be off-set in the same direction and proportionate degree for each note.
I've got news for you.
Resistors do change instantaneously with temperature, and the worst are carbon ones - which were used exclusively in older kit as there was no alternative. Capacitors also are temperature dependent, and again it depends on the type. Really good designers try to mix resistors and capacitors with opposite temperature characteristics to compensate. This works to some degree, but a resistor's coefficient is always linear, a capacitor, not so much.

Just to make things more interesting, transistors will change their gain, depending on temperature, and a change in gain means a change in apparent impedance.

On top of all this there will be to some degree changing air currents inside the kit, so some bits will get cooled while others are warming up. Then the air currents shift a bit.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby ramthelinefeed » Sun Jul 26, 2020 8:25 am

Not really - if you think about it, the resistors on the keyboard will all be subject to the same temperature variation and most likely be all of the same type, so they'll all be subject to the same temperature coefficient for their resistance, so the net effect at any given point in time would still be the set of offsets from equal temperament in the same directions.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby desmond » Sun Jul 26, 2020 9:07 am

ramthelinefeed wrote:Not really - if you think about it, the resistors on the keyboard will all be subject to the same temperature variation and most likely be all of the same type, so they'll all be subject to the same temperature coefficient for their resistance, so the net effect at any given point in time would still be the set of offsets from equal temperament in the same directions.

I'm not sure you understand my posts from your responses, or perhaps I'm communicating poorly regarding "pitch stability" which perhaps I might have better phrased as "pitch accuracy" or "pitch variability".

I understand, and acknowledge the more or less static nature (at any given point in time) of this one effect you describe on pitch.

My point though is that this is only *one* source that potentially moves the pitch away from true pitch - there are many other sources that also affect pitch accuracy on an analog synth (as described above), so the *overall effect* is not just this one source of pitch variation - it combines, and varies accordingly with the sum of all voltages that combine to decide on the exact pitch a given oscillator reads out.

Your C# might (always) be 3 cents sharp as generated from the keyboard voltage, but might end up with the note *sounding* only 1 cent sharp, as some other source, say temperature dependent components might detune that note, at that particular instant, by -2 cents. And perhaps the next time you play the same note, the keyboard generated voltage is the same 3 cents sharp, but a different voice is allocated to play the note, and that voice board has a slightly different temperature and calibration, so perhaps now the sound heard triggered by the same note is now 5 cents sharp. This is the overall result that I described above as "pitch instability".

*Overall* this means that in analog synths that have VCOs, you get note pitch changes away from perfect pitch when you play notes, and this is *one* factor for that "alive" analog sound - and it's a factor that is fairly easily replicated on modern synths. There are other factors too of course...

ramthelinefeed wrote:If general VCO pitch stability was awful enough, it would swamp the temperament offset effect, yes.

Indeed, but it doesn't just have to swamp it, to contribute to it...
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby ramthelinefeed » Sun Jul 26, 2020 2:05 pm

Well obviously Desmond, yes, if you start getting into polysynths with voice cards, that would likely swamp any inherent temperament offset from the keyboard. That's why my initial post talks about a monosynth and advises using a single VCO and no modulation if you want to measure the effect.

However having said all that, I think you're getting too hung up on random picture fluctuations listened to on a one-off key press, rather than when listened to over a time frame of minutes when playing actual pieces of tonal music.
Most of the pitch variations you're thinking about are quasi random or periodic about a zero point, so the ear's sense of pitch will still take the zero point as the note's actual pitch and perceive the variations and instability as vibrato.
That isn't the case for a static offset in temperament. For instance if you're playing a tune and the 5th scale degree is always 5 cents sharp compared to the other instruments, you'd be a pretty tone-deaf not to hear that.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby N i g e l » Sun Jul 26, 2020 9:52 pm

desmond wrote:
ramthelinefeed wrote:
*Overall* this means that in analog synths that have VCOs, you get note pitch changes away from perfect pitch when you play notes, and this is *one* factor for that "alive" analog sound - and it's a factor that is fairly easily replicated on modern synths. There are other factors too of course...

ramthelinefeed wrote:If general VCO pitch stability was awful enough, it would swamp the temperament offset effect, yes.

Indeed, but it doesn't just have to swamp it, to contribute to it...


Non - VCO synths with static pitch offsets:

The WASP synth uses a master oscillator which is frequency divided down to produce the note frequency (controlled by a note_number & octave number).
The division is only 8 bit, so the note pitches are not exact.

This is similar to organ technology of the time where note pitches were derived from a master oscillator & a top octave divider IC. This was also used in String Synths and also I think some monosynths too.

I dont know the accuracy of the organ chips in terms of pitch.
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Re: Synthesis Techniques For Emulating Vintage Sounds

Postby Folderol » Sun Jul 26, 2020 10:11 pm

On some 1970s kit they used phase locked loops with a ratio of 196:185 which gives an interval of 1.05945945946
Actual interval is 12 root of 2 which is 1.059463

I know about this because I was going to design and build one myself... till I added up the cost!
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