Re: [wiardgroup] Mini-lecture on VCO "Attack-Release" generator and "Player Piano" technique
2008-12-27 by maldoroar@yahoo.com
Grant,
this is really interesting. I've never heard of using tape to record control voltages before, although i vaguely remember hearing about it being done on VHS. Maybe Dan Sandin did this?
I'm wondering, if you had a multi-track tape player, could you layer cv's to create complex shapes, and mix them externally (or in the Mixolator), and send them as a mono signal into the VCA eNV jack of the VCO?
Do you use the Coarse knob of the VCO to determine speed of the cv's, or the tape player's speed?
And you could also make tape loops with one's favourite cv's on them!
I guess this was how things were done in the early days.
I have some old reel to reel tape machines, but i've only ever made audio loops.
Thanks for bringing this up!
One question about the Mixolator- can you ring modulate cv's? Would you get different results if you ring modulated the cv from the tape deck before increasing them to audio rates, than if you brought the cv's up to audio rates first, then ring modulated them?
Matt
--- On Sat, 27/12/08, Grant Richter <grichter@asapnet.net> wrote:
> From: Grant Richter <grichter@asapnet.net>
> Subject: [wiardgroup] Mini-lecture on VCO "Attack-Release" generator and "Player Piano" technique
> To: wiardgroup@yahoogroups.com
> Received: Saturday, 27 December, 2008, 3:39 AM
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> Hello Mark,
>
>
>
> > A quick couple of questions, for which there is no
> rush for an answer....I saw in the hints
>
> a tips Grant mentioned that a 5V gate on the Classic VCO
> and WC won't fully open the
>
> VCA on those units.
>
> >
>
> > 1. I have a Kenton convertor that gives 5V or 15V (no
> 10V). Is 15V safe for the
>
> oscillators gate?
>
>
>
> The "Attack-Release" generator on the Classic VCO
> and Waveform City is a hybrid of an
>
> envelope follower and an envelope generator. If a gate is
> just a step function, 0 to 10 volts
>
> and back to 0 again. Then by slew rate processing that
> signal, it will naturally make an
>
> attack release envelope.
>
>
>
> To make an envelope follower, you need something to remove
> the negative portions of the
>
> analog audio, otherwise that average of the + and - is
> zero.
>
>
>
> The Gate input has a current limiting resistor and a 10
> volt zener diode at the input. When
>
> processing a normal 10 volte gate, the device is not even
> activated.
>
>
>
> When processing audio, it removes the negative portions of
> the audio signal, in what is
>
> called "have wave" rectification. Not an ideal
> envelope follower, but a usable one essentially
>
> for free.
>
>
>
> In you case of a 15 volt gate signal, it will "chop
> off" (limit) the signal to 10 volts, and the
>
> output envelope will be 10 volts, just what you want. There
> should be no problem using
>
> the 15 volt gate signal, it will be converted safely to a
> 10 volt envelope.
>
>
>
> >
>
> > 2. Presumably the Envelators (with the envelope
> generated into ZMOD of the Mixolator)
>
> Â are fine with just 5V?
>
>
>
> The ARP 2600 was such a well thought out synth that I
> actually divide synth history to
>
> before and after the 2600. It is not completely perfect,
> but it was the first one to get
>
> almost everything right. Two well know problems are
>
>
>
> 1. DC offsets on the audio which causes
> "thumping" with the filter and VCA
>
>
>
> 2. The envelope trigger voltages (once you modded the unit
> to get at them) had six volt
>
> thresholds. So you could not take your shiny new 808 or 909
> or 606 and run the
>
> envelopes from the rhythm box external triggers. There were
> work arounds but it was
>
> clumsy and a repeated pain. And you really did want to link
> your rhythm box and
>
> sequenced synthesizer together, oh yes you did. Just trust
> me on that.
>
>
>
> So in the design of the Wiard modules, trigger threshold
> were set to work with TTL (5 volt)
>
> technology like the rhythm boxes from the 80's, and
> other modern stuff that uses 5 volts.
>
> The thresholds are nominally set to TTL "High"
> which is around 2.5 volts.
>
>
>
> So a 5 volt gate or trigger will work fine with the
> Envelators. All the Sequantizer inputs
>
> also have this 2.5 volt threshold, Step, Reset, Glide,
> Octave + and Octave - should work
>
> just fine with patched out 606's and other rhythm
> boxes.
>
>
>
> They will also accept clock, gate and trigger sources from
> Serge modules in general and
>
> the TKB in specific.
>
>
>
> But that is only the beginning of the story. Here is the
> deep stuff.
>
>
>
> Morton Subotnick worked out an advanced synthesizer
> technique on the Buchla he call the
>
> "player piano" technique. Normally a tape
> recorder can not store the very low frequencies
>
> that are the control signals humans generate as control
> signals when we "play" a
>
> synthesizer.
>
>
>
> Subotnick brilliantly came up with a work around using AM
> (amplitude modulation). All
>
> that requires is simply controlling the amplitude (volume)
> of a simple "carrier" wave and
>
> the very slow signals are stored as variations in the
> volume of the carrier, and can be
>
> recorded on magnetic tape.
>
>
>
> Upon playback a simple "decoder" recovers the
> control signals from the amplitude
>
> variation of the tone recorded on the tape.
>
>
>
> To "encode" the signals you need a sine wave
> carrier from a VCO followed by a Voltage
>
> Controlled Amplifier (VCA) then going to the tape recorder.
> The control signal to be
>
> recorded is used to control amplitude (volume) of the
> carrier (VCO sine wave) as it is
>
> recorded to tape.
>
>
>
> In the Classic VCO, to record a control signal with a 0 to
> 10 volt range, patch the SINE
>
> output to the VCA1 input, set the "VOL" knob to
> zero, patch the VCA OUT to the tape
>
> recorder input, patch the control signal to be stored to
> the VCA ENV jack.
>
>
>
> To record control signals that have negative voltage
> portion, just advance the VOL knob
>
> until the most negative excursion, just shuts the VCA off.
>
>
>
> Most any VCO frequency (pitch) will work, but since we only
> have "half wave" envelope
>
> following, a higher frequency like 1 kHz will give the best
> results in terms of the "ripple" or
>
> error signal generated by the "decoder" during
> playback.
>
>
>
> As you have probably guessed, the "decoder" for
> an AM encoding technique is an envelope
>
> follower. The "Attack" and "Release"
> controls are used to modify the control signal after
>
> recording. Normally, another unit called a "Gate
> Extractor" is needed to interface the
>
> incoming envelope follower signal to trigger onboard
> envelope generators.
>
>
>
> The low threshold voltage on the Envelators and
> "forgiving" design of the Envelator GATE
>
> input, allow direct connection of the AR ENV output to the
> GATE inputs with only some
>
> tweaking of the Release control needed to get a repeatable
> envelope trigger. A "Gate
>
> Extractor" is optional for use of the AM encoding
> technique on the Wiard instrument.
>
>
>
> I hope this explanation will help to clarify why the
> "AR" generator on the VCO modules
>
> does not precisely follow the trigger thresholds and
> behavior of the Envelator and
>
> Sequantizer modules. Some compromise was needed to allow
> the Classic VCO to be used
>
> as a self contained "channel" for the play piano
> control voltage storage technique.
>
>
>
> The ability to record and store long complex controls
> signals, which can then be brought
>
> back into the instrument and used like "live"
> playing signals is extremely powerful. It
>
> allows you to use all the modules to generate the complex
> control shapes, and then the
>
> same modules can be repatched for use at audio frequencies,
> this doubles the
>
> effectiveness of the the instrument. It also allows the
> player to have both hand free for
>
> "knob twizzling" and other techniques which add
> dynamics and expressiveness during
>
> playback. Essentially the ability to "conduct"
> the stored performance.
>
>
>
> Thanks for reading and have a Happy Holidays
>
>
>
> Grant Richter
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