[sdiy] RE: cross-fades (was ...)

[mark verbos]

On Jan 15, 2009, at 5:25 PM, John Mahoney wrote:

> At 04:26 PM 1/15/2009, David G. Dixon wrote:
>> This talk of VCFs has led me to another newbie question:
>>
>> What is the preferred method for implementing a cross-fade  
>> circuit?  I'm
>> assuming one channel's gain is increased from 0 to 1 while  
>> another's is
>> decreased from 1 to 0, and they both add up to one at any given  
>> setting?
>> How exactly is this done (it doesn't involve dual-gang pots, does  
>> it?  That
>> wouldn't be elegant at all!)?
>
> Passive approach requires a ganged pot.
>

or run both signals through resistors and after them have a single pot  
with it's ends on the signals, post resistor and wiper to ground.  
After this section, the two signals can be passively combined with 2  
resistors. It would help to combine them with an opamp stage.


> Active approach uses VCAs. One channel gets an inverted-and-offset  
> form of the control voltage.
>
> Vactrols may be another possibility.


As would 2 analog switches, a comparator and a high frequency  
oscillator. Post filtering necessary. See the Buchla 257.

Mark