Cross Fader

[Gold]

I need to make a four channel into four channel crossfader. I would like to have an "auto" mode with "fast" "medium" and "slow" buttons as well as a dedicated manual crossfader.

What are the ways to accomplish this? I've looked at some application notes from Silonex optocouplers. VCA's? I think an active panning circuit is out because there couldn't be an auto mode. Any ideas and pointers to application notes are welcome. I have a feeling I'll end up hiring someone to do this but I'd like to give it a whirl if there is something close to a paint by numbers solution.

 

[sodderboy]

As I understand it, in the mastering world less is more- less cable, connections, circuitry, etc. To me that would say passive design. VCA's or DCA's, or optos would require much support circuitry and, much, much more tweaking.

Imagine 4 high quality stereo P&G faders, lined-up side by side, with two travelling opposite two. The wipers can be linked with a single bar that has one knob. The whole assembly would be about 2" wide, able to fit in a double fader slot. This would be unbalanced unless you added balancing of your choice- iron or sand. I guess you could double the fader count for balanced passive, but you are buying 4 more $175 faders.

I think that the log taper would work for a crossfade, and P&G can give you any law you wish. Maybe it is more like a pan-pot law? Prototype the idea with chincy Mouser faders and see if you get the results you need. I think that they sell cross-faders for DJ mixers. For $20 you can try the idea and tweak it before buying the P&G's.

There would be no automatic or selectable cross-fade speeds, but I think that doing it by ear would be superior anyway. Every fade would be unique to the program material.
Mike
PS: I just re-read the TapeOp interview with Malcolm Cecil. The BBC had 3 foot long splicing blocks with a diagonal slice for doing cross-fades. They must have been designed by the guy with the brother with the splicing tape factory!

 

[Gold]

I want to be able to both "instantly" switch and cross fade between sources depending on the situation. I don't see how that can be done passively. On the early Neumann consoles there was only a relay switch, the later ones had the option to crossfade too. I haven't seen the schem (I know where one probably is) but I assume it's active. If the fader thing is going to be really difficult I'd rather have a silent switch than a crossfader.

I'm looking to make a 4x4 of this http://www.maselec.com/products/mxf-1/index.html

I've never looked inside but I doubt it's passive.

 

[Larrchild]

Buy a cheap video switcher that has automated cross-dissolve and drive the VCA's with the ramp signals normally used for video.

You can hit the square , lit, switcher buttons to hot switch, or activate the ramp-gen.

 

[Gold]

Thanks guys. I think this is over my head. Maybe I'll start out with a switch and after six months of staring at the documents I'll put my feet in the shallow end.

 

[SSLtech]

Wayne,

For a pan application, you USUALLY go with something between -3dB and -6dB. -For a 'summed-common-source' crossfade, such as you may find in mastering) you would usually go with a -6dB crossfade.

Paul, This is eminantly doable, and -using mechanical potentiometers- it's basically the circuit which I drew up for the "crush-n-blend*"... -ASSUMING that I'm guessing your application purpose correctly...

If I guess right, you're considering taking a common source of audio, splitting it to two paths, then sequentially alternating between the paths, setting the 'inactive' path up for its next 'call', while the other path is active, then making the transistion seamless. -Is this correct?

If so, then consider minimum difference between two paths: with the IDENTICAL signal in both paths, the mid-point will combine +6dB, hence you'll need a -6dB center to keep the level constant, and a fully-linear curve either side of that also... other wise slower crossfades -even with no processing- would produce dips and/or surges in signal level.

So...

the THAT application could be used, but it would require a little bending of the law... maybe we need an attorney/accountant!

:green:

*The idea behind the Crush-n-blend is to allow parallel-path processing and a 'blend' between the two paths, more commonly to blend between unprocessed and processed (for example "smashed" drums and direct, using the user's free option of compression tools) but the fundamental idea is essentially similar, and BOTH paths can also be processed; neither one HAS to be 'clean'.

Keith

 

[Gold]

[quote author="SSLtech"]If I guess right, you're considering taking a common source of audio, splitting it to two paths, then sequentially alternating between the paths, setting the 'inactive' path up for its next 'call', while the other path is active, then making the transistion seamless. -Is this correct?[/quote]

That is correct. The only wrench in the monkey works is that I need the ability to 'switch' as well as fade. The swich will actually be used more than the fade. The Neumann console I have uses a transistor switch. I said it was a relay above but it's a transistor. I think I will fool around with an opto isolator as a switch first. Neumann used them in the VG66 which I have and Sontec uses them as a silent in/out switch.

 

[SSLtech]

The SSM2402 is a very nice way of cross-switching, or a VCA could have a perfectly silent 'switch' setting as well as a user-definable cross-fade setting.

Switcher:
http://www.analog.com/UploadedFiles/Application_Notes/35650192AN121.pdf

(stereo as shown, but double-up for quad...)

When you say "four-channel", do you mean two, 2-channel paths (for stereo) or two 40channel paths (for quad, eg)

Keith

 

[Gold]

[quote author="SSLtech"]The SSM2402 is a very nice way of cross-switching

When you say "four-channel", do you mean two, 2-channel paths (for stereo) or two 40channel paths (for quad, eg)

[/quote]

Now yer talkin' ! A single chip solution. I can do that.

I mean quad into quad. (A)program/preview into (B) program/preview

 

[mikep]

IMO VCAs would be the most practical approach, but I think you could do it most transparently with Optos. It could be done more simply, but Ive had this idea for several years in my head, I think it is relevant: for each signal use 2 two-element optos, for instance the VTL5C2/2 (series/shunt). in each package one element is for signal, the other is read by an a/d for feedback. use a microcontroller with built in A/Ds and drive the vactrol LEDs with PWM outputs.

do you really need to crossfade the preview path post-delay?

 

[mikep]

SSM2402 is a nice SWITCH. it will not fade. well, I guess it always "fades" to avoid popping, but it is way fast to be a crossfader.

 

[Gold]

[quote author="mikep"]

do you really need to crossfade the preview path post-delay?[/quote]

No delay. Straight from tape. I know I'm nuts, no need to rub it in. I want to do it because it's a unique capability and will be fun to operate. I just nod and smile when they say analog is better.

My immediate need is a switch. The only time a switch won't work very well is when there are radically different settings and no space between songs. I do have two preview decks for live crossfades when I can do it.

 

[mikep]

in that case I think you really need a honking big manual fader. IMO automation is moving away from the original intent. perhaps rig up a couple of linear taper 100mm P+G faders with a nice machined bar to gang them. mount it on a 4U panel. linear taper with slugging resistors from wiper to gnd can achieve the right amount of loss at mid-fade. you will need one buffer stage post fader.

 

[SSLtech]

[quote author="mikep"]SSM2402 is a nice SWITCH. it will not fade. well, I guess it always "fades" to avoid popping, but it is way fast to be a crossfader.[/quote]
Understood.

And therefore only suggested thusly:

The SSM2402 is a very nice way of cross-switching, or a VCA could have a perfectly silent 'switch' setting as well as a user-definable cross-fade setting.

Keith

 

[Gold]

[quote author="mikep"]in that case I think you really need a honking big manual fader. [/quote]

See above. I need a switch more than a fader. The crossfader in the Neumann SP79 wasn't passive. The switch in the SP75 wasn't either. Nor were many switches in the VG66 or SAL74 drive racks. If you can think of a way to have the option of both switching and fading passively I'm all ears.

Could I use a shorting switch after a transformer for silent switching?

 

[Gold]

BTW I have a feeling opto's are the way to go. The ones optimized for fading have higher distortion for switching but in a switch situation I don't think it matters.

I'll get some SSM2402's because I'm pretty sure I can get that to work. Then I'll get some optos and play around with them as time permits.

I've learned that when I need to do something beyond my current capabilities it's best not to have a time frame.

 

[SSLtech]

Paul,

PM sent.

 

[PRR]

> a way to have the option of both switching and fading passively

Define "switch".

In this case, it surely means "fade too fast to hear, too slow to click".

When car racers want a fast yet controllable shift, they use an air cylinder to push the shift lever.

This has the practical advantage of NOT needing any understanding, even belief, in magical sub-atomic particles or aether or influence at a distance or Ohm's Law. It is just wind, trapped in pipes.

So use an air cylinder to push a standard passive cross-fader.

In the associated image: fader is "all A". Apply air to cylinder port B. Piston moves to the other end.

http://i6.tinypic.com/7xvsnbo.gif

Select which way ("A-B" or "B-A") by which way you throw the standard "3-way" valves. (Yeah, you could auto-select the direction with another valve, but you probably know which way to go, and K.I.S.S.)

Ports in the cylinder wall ends bleed all pressure just before the piston slams.

Select the speed of throw with wide-open pressure or pressure through an orifice. You can calculate the speed, if you spend a day measuring inertia and friction. You can buy ordered sets of orifices. You can jam a stick in a hose and drill it with jeweler's drills (hey, it was good enuff for Ford!). Or you can use a needle-valve and trim to taste, probably best.

For maximum speed you want light piston and large ports and hoses. The upper limit is the speed of sound, 13,500 inches per second. You may not approach 1/10th of that with practical ports/hoses, 1,000 inches per second. Then a 4-inch throw is 4 milliSeconds, which is just scarcely audible, and would generally be considered a "switch" not a "fade". If you insert an orifice 1/10th the diameter (1/100th the area) of the port, speed is roughly 1/100 or 400mS or half a sec. Much smaller orifices are probably possible for several-second fade. I'd think that a fade THAT slow ought to be hand-handled.

If you have vacuum, this works with vacuum. You need larger piston, perhaps diaphragm (though you can't end-port a diaphragm as easily as a cylinder), and much of the plumbing is reversed. My Willys' windshield wiper used vacuum to cross-fade rain across the glass. (You can do better than the tired wheezy warped old Trico hardware on my Willys.)

 

[Gold]

[quote author="PRR"]So use an air cylinder to push a standard passive cross-fader.
[/quote]

Holy cow! That's some creative thinking. It's just crazy enough to work and it will look cool too.

 

[Gold]

Hmmm. Since the SSM has 16 inputs I could use two as a switch and use a third for the output of a crossfader. That would give me all the combinations that I need.