[sdiy] SSM2164 panning curve

Fri Jul 24 16:40:08 CEST 2009

One reason I started this thread was that I built a voltage controlled 4 
to 1 mixer right from the schematic in the SSM2164 spec sheet and 
controlled it with a Doepfer A-174 Joystick module. It always felt like 
it had a "dead spot" in the middle where it wasn't as loud as it should 
be. A slightly different application than quadraphonic panning, but this 
experience is still in my mind as I plan out this new module.

D.

Jerry Gray-Eskue wrote:
> <<In the dead center, both channels will be mixed equally at some level less
> than unity, but which when added gives the same loudness as a single
> signal at unity.>>
> 
> Assuming an ideal speaker setup (equal distance from listening point and
> room acrostic matching), yes with this setup the joystick will "move" the
> quadraphonic sound in direct relation to the joystick position.
> 
> I believe the "Magic" level is 1/2 the power (~= -3db) in center position
> which is why one of the "panning laws" uses -3bd for the center position.
> 
> - Jerry
> 
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Tom Wiltshire
> Sent: Thursday, July 23, 2009 1:34 PM
> To: Derek Holzer
> Cc: synth diy
> Subject: Re: [sdiy] SSM2164 panning curve
> 
> 
> Derek,
> 
> Ok, cool. Now I'm with you. I think. Famous last words.
> 
> Basically you want the sum of the outputs to remain constant. I'm not
> bang up to date on the latest in the panning law debates, but I
> understand that some people prefer the "equal power" version whilst
> other people go for "equal level" or some such distinction.
> 
> Let me just think this through aloud...
> Presumably the X-axis and the Y-axis are identical? So we can look at
> one of them independently of the other. So if you solve the problem
> for one direction, you just repeat it for the other.
> 
> On that basis, let's have a look at the Left/Right axis. On the hard
> left side, you'd want only LHS signal, at unity gain. On the hard
> right, you'd want all RHS signal, again at unity. As you move away
> from these positions, the volume of that channel decreases whilst the
> other increases, following the log response curve of the VCA. In the
> dead centre, both channels will be mixed equally at some level less
> than unity, but which when added gives the same loudness as a single
> signal at unity. Am I still on the right lines?
> 
> Given that the SSM2164 does the complicated linear CV to logarithmic
> response part for you, I'd have thought you just want a circuit that
> gives you a linear voltage for the CV, say 0V (unity) to +4V (>120dB
> attentuation). This needs to be inverted and offset for the opposite
> VCA, to give 4V to 0V instead. This isn't difficult (a lot simpler
> than that electromusic schematic). The joystick pot can provide one
> CV directly (with a bit of tweaking) and then the opposite one needs
> an inverting op-amp mixer, fed with the CV and an offset voltage of
> -4V. And then same again for the other channel.
> 
> Hope this helps,
> Regards,
> Tom
> 
> 
> On 23 Jul 2009, at 14:21, Derek Holzer wrote:
> 
>> Hi Tom,
>>
>> sorry don't have an image right now, but let me try someone else's
>> explanation:
>>
>>> Some words about panning and crossfading:  If you simply have two
>>> VCAs connected together such that varying the control voltage
>>> brings the gain on one up and the other one down, you have a
>>> rudimentary panner, but the panner has a problem.  At dead center
>>> (both VCAs equal), the loudness will be less than full right or
>>> full left.  You need some flattening of the response curve to make
>>> this complete.  The Serge VCAs provide this equalized left-center-
>>> right response curve, so panning sounds fairly natural.
>> From: http://www.serge-fans.com/wiz_vca_mix.htm
>>
>> Does that explain my idea a bit better? At center position, all
>> channels should be at unity, and should never get louder than that.
>>
>> D.
>>
>> Tom Wiltshire wrote:
>>> Derek,
>>> I'm afraid I don't understand what you're trying to do. I need to
>>> see more diagrams for my visual head to get it. So long as you're
>>> clear about it, that doesn't matter.
>>> However, I have played with the SSM2164. 15V is too much for the
>>> control port. The datasheet gives the CV response as -33mV/dB.
>>> Hence you need 3.3V for fully off -100dB, and -0.66V for 20dB
>>> gain, with 0V giving you unity gain. I don't know of a way to set
>>> the SSM2164 up for attenuation only. You just make sure that you
>>> only have positive control signals. I don't think just a simple
>>> diode would work that well, since the diode won't conduct until
>>> the CV is significantly negative, which isn't what you want. You
>>> could modify the op-amps in the Electro-music schematic to
>>> precision half-wave rectifiers, which would give you a more
>>> accurate result.
>>> Regards,
>>> Tom
>>> On 23 Jul 2009, at 11:31, Derek Holzer wrote:
>>>> I've got a hopefully simple question about using the SSM2164 as a
>>>> joystick controlled quadraphonic spatializer/panner. I have been
>>>> looking at Marc Bareille's Quad VCA/CV-Mixer as a good model for
>>>> this project:
>>>>
>>>> http://m.bareille.free.fr/modular1/quadvca/quadvca.htm
>>>>
>>>> with some additions for the joystick control:
>>>>
>>>> http://electro-music.com/forum/viewtopic.php?t=24705
>>>>
>>>> My concern is this: when the joystick is it dead center the gain
>>>> on all channels should be unity. However, for this particular
>>>> gain/panning curve I don't want more than unity gain, only
>>>> attenuation. I am aware that this nulls the effects of half the
>>>> range of the joystick... however from what I have read this is
>>>> the panning curve used by the Serge and Buchla spatializers, and
>>>> it makes sense acoustically to me.
>>>>
>>>> So let's assume that set up the SSM2164 as in the datasheet,
>>>> where 0V is unity and minus voltage is attentuation. And I wire
>>>> up my joysticks as voltage dividers between +15 and -15 (or
>>>> should it be +/-5V?). But since I don't want any amplification, I
>>>> use a diode to block the positive voltage coming from the
>>>> joysticks, making sure the diode is rated not to break down
>>>> anywhere near 15V (or 5V?). Am I on the right track? Or is there
>>>> a way to set up the SSM2164 itself so that it only attenuates?
>>>>
>>>> Thx+best!
>>>> Derek
>>>> --
>>>> ::: derek holzer ::: http://blog.myspace.com/macumbista :::
>>>> http://www.vimeo.com/macumbista :::
>>>> ---Oblique Strategy # 52:
>>>> "Do nothing for as long as possible"
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>> --
>> ::: derek holzer ::: http://blog.myspace.com/macumbista ::: http://
>> www.vimeo.com/macumbista :::
>> ---Oblique Strategy # 107:
>> "Look at the order in which you do things"
>>
> 
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-- 
::: derek holzer ::: http://blog.myspace.com/macumbista ::: 
http://www.vimeo.com/macumbista :::
---Oblique Strategy # 80:
"Go to an extreme, come part way back"