# [sdiy] Panning with LM13700

Dave Manley dlmanley at sonic.net
Sat Jan 14 21:16:47 CET 2017

```I'm thinking out loud, always a bad choice, but:

1. the input voltage is linear

2. the output of the BPC (tired of typing break point circuit) is
non-linear

3. if the CVL goes 0->5 then CVR goes 5->0 (as you state)

4. the chopper makes S/H Left, or SHL = f_BPC(CVL),  and SHR =
f_BPC(CVR)

Isn't this what is needed?

-Dave

PS (CVR typo corrected below)

On 2017-01-14 12:01, Tom Wiltshire wrote:

> I don't understand how this reflects the CV in the vertical direction. CVR = 5 - CVL is going to give you a horizontally-flipped copy.
>
> On 14 Jan 2017, at 19:35, Dave Manley <dlmanley at sonic.net> wrote:
>
> How about a weird chopped version to allow only a single breakpoint circuit:
>
> 1. Rename CV = CVL (left)
>
> 2. Derive CVR = 5 - CVL
>
> 3. Add a switch before the breakpoint circuit to switch between CVL or CVR at a rate TBD
>
> 4. On the output of the breakpoint circuit add two simple switch+cap+opamp S/H where the switches are synchronous to the input switch.
>
> One S/H controls the left VCA, and the other the right VCA.
>
> The switching can be done with a 4016 with a couple of inverters configured as an oscillator to generate the chop frequency and control the switches.
>
> On 2017-01-13 17:57, microtonalsynthesis at yahoo.com wrote:
>
> Dave,
>
> The input CV is 0 to +5V and that needs to be split at zero volts. One CV is positive only and the other is negative only. If you look at the chart in the THAT AN120, you'll see that you cannot simply derive one CV from the other. They are symmetric across the vertical axis, not the horizontal axis. The breakpoint circuit is almost symmetric but there are additional offsets applied for the left CV generation. Every offset adds to the DC error and therefore adds to the panning and volume error.
>
> John
>
> On Thursday, January 12, 2017 1:58 PM, Dave Manley <dlmanley at sonic.net> wrote:
>
> Is there a separate diode breakpoint circuit for the left and right channels? Is that the cause of the mismatch?
>
> If so, can you use one circuit for the left and then derive the right from that?
>
> -Dave
>
> On January 9, 2017 8:17:05 PM PST, microtonalsynthesis at yahoo.com wrote:
>
> Tom,
>
> I think you are on the right track with the microcontroller version. I've been simulating the THAT AN120, which is just a simple diode breakpoint generator. But the amplitudes never match up for the given circuit, with differences of 50% or greater depending on the setting and the left or right channel. In reality I think this circuit will need multiple trimmers to get everything right. Considering it is modifying CVs that go into an exponential control circuit, just small errors can be significant.
>
> John
>
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