What's the Difference: Polarity vs. Phase

I’ll be honest, I’ve been putting off writing this one. Mainly because this could go down the rabbit hole very quickly and I don’t want to do that. So I’ll say at the start that I’m going to keep this fairly simple and not delve into a deep treatise on phase. I would suggest you listen to our podcast with Bob Heil if you want to learn more about phase. He’s pretty smart in that area. With that out of the way, what’s the difference?

Phase has a time component, Polarity does not.

That’s about as simple an explanation as I can give. Polarity is the reversal the positive and negative terminals in a balanced circuit. As you may recall from our previous discussions of balanced and unbalanced circuits, you’ll recall that a balanced circuit has a positive pole, a negative pole and a ground. It is said to be balanced because the voltage that exists on the positive side is the same as the negative side; it’s just that one is + voltage, the other is - voltage. 

When you press the polarity button—which is often labeled with a or symbol—what you are doing electrically is swapping the positive and negative poles of the input. That has the effect of flipping the phase 180° relative to itself. If you were to have two mic’s right next to each other pointed at the same source, flipping the polarity on one would cause a near total cancellation if you brought both channels up. 

Down the Rabbit Hole

Here is a pretty vast simplification of what we’re talking about. Thanks to the desmos graphing calculator for the visuals. Below are two signals that are out of polarity with each other; that is, we’ve swapped the + and - making them 180° out of phase. For every + voltage on one signal, there is a corresponding - voltage, which would cause total cancelation of the signal. 

Below is the same thing, only the polarity of both signals is the same. It’s hard to see but both signals are overlaid on top of each other. In this case, the resulting signal would be twice as loud as one of them alone because they would add.

To further our discussion, below is one with one of the signals shifted over in time. This causes a phase shift.

Phase is Time

When I took the Rational Acoustics Smaart course last January, Jamie Anderson said, “Phase is the most demonized and BS term in the industry today.” He also said that, “Filters don’t have phase shift; filters are phase shift.” Phase has a time component, whereas polarity does not. I said that again in case you missed it the first time. 

So don’t say, “Flip that channel out of phase, would you?” Instead the correct phrase is, “Flip that channel out of polarity, please.” 

Why Change Polarity?

You may want to change polarity for a few reasons. When I mic up a Leslise 122 rotary speaker cabinet that is being driven by a Hammond B3 (we’re getting really technically correct here at CTA today…), I like to put two mic’s on the top horn. I place them 90* to each other and flip one out of polarity with the others. That results in a really wide stereo image of the top horn.

I also find when I have an interview situation on stage polarity reversal can come in handy. Let’s say you have a pastor on a headset mic interviewing someone with a handheld mic. The headset mic may also pick up the other person, but because they’re a foot or two away, it will be out of phase with the handheld mic (phase is time, remember?) Sometimes flipping one of them out of polarity will minimize the phase interaction. Basically, that shifts the phase offset by 180°, which may make it less destructive.  

When mic’ing a snare drum on the top and bottom, you’d want to flip the polarity of the bottom mic. The wave front coming from the bottom of the snare is 180° away from the top of the snare, and you need to flip polarity so you don’t get cancellation. That’s a bit of a simplification, but try it sometime. The reason you don’t get complete cancellation is because is because rarely are the mic’s pointed straight up and straight down at the snare heads, so you’re not technically 180° out on either of them. But if you don’t flip the polarity on the bottom mic, there will be some cancellation, which will make the resultant sound very thin. 

 Well, we ended up down the rabbit hole after all. I made a video a while back that demonstrates some phase shift concepts that may help further explain the concept. Check it out if you want to learn more.

Roland

Today's post is brought to you by CCI Solutions. With a reputation for excellence, technical expertise and competitive pricing, CCI Solutions has served churches across the US in their media, equipment, design and installation needs for over 35 years.

What's the Difference: VCAs and Groups

So far in our What’s the Difference series, we’ve considered AFL/PFL and Pre-Fade/Post Fade. Today, we’re going to look at another pairing that I see confused all the time. That is the difference between Groups and VCAs. 

Here we have a couple of VCA's (blue channels), a group (red) and input channels (grey).

Here we have a couple of VCA's (blue channels), a group (red) and input channels (grey).

A Group is a Mix Bus

A group is a place to send channels post-fader. To make this clearer, the LR main output on your console is technically a group. So is the Mono output if your mixer has one. When you send a channel to a group, it is after all processing and the fader, so it is truly the final step on the way out of the console. You can assign channels to as many groups as you want; up to the number of groups you have. The level of the channel going to the groups will always be the same. In this way, groups are different from auxes. With an aux, each aux send goes out at its own level. Sending a channel to two groups sends the same exact signal to both groups. 

A VCA is a Remote Control

A VCA (short for Voltage Controlled Amplifier) is really a way to remotely control the level of a channel or group of channels from a single fader. When you assign a channel to a VCA, you can add and subtract gain from that channel using the VCA fader and/or the channel fader. Moving the VCA master up by 5 dB will have the same effect on the channel as moving the channel fader up 5 dB. Turning off the VCA master will effectively mute the channel(s), making it easy to turn entire groups of channels on and off with one fader move. 

When to Use Them

I wrote a much longer series on this topic some time back, but here’s the shortened version. Groups are useful for applying the same processing to a group of inputs. Clever, huh? For example, if you want to do some parallel compression on the drums, you can assign all the drum inputs to a group and insert a compressor on the group. Mix that with the uncompressed version and you have parallel compression. Or perhaps you want to subtly compress all the BGVs. Same thing. Only don’t assign them to the main LR bus; send them to the group, compress then send the group to the LR mix. 

VCAs are useful for mixing similar types of instruments. On digital consoles,  you may not have the faders on the surface for all your inputs. Really large analog consoles may be a long reach. So, you can combine channels into one to make it easier to manage. For example, you may set up the mix for the drum kit, then assign all the drum channels to a VCA. Because the drums are one instrument, you can adjust the level of the drums with the VCA. Some engineers like to put the bass and kick on a VCA and move their level together. Others will assign all the keys to a VCA and all the guitars to another. 

It’s important to note that a VCA is not better than a group, nor is a group better than a VCA. They are different. Not all mixers—especially small ones—have VCAs so you have to make do with groups. But when you have both, use them for what they are good at. 

VCAs and DCAs

On some digital consoles, Yamaha for example, VCAs are called DCAs. DCA stands for Digitally Controlled Amplifier. The function is the same, but the underlying technology is different. For all practical purposes, they are the same. 

This is a pretty simplified explanation. For a lot more detail, check out some of the posts below. 

Other posts with more detail:

CTA Classroom: Understanding VCAs and DCAs

CTA Classroom: Defining Auxes, Groups, VCAs & Matrixes Pt. 1

CTA Classroom: Using Groups

Groups, VCAs and DCAs

Groups, VCAs and DCAs Part Two

Roland

Today's post is brought to you by Heil Sound. Established in 1966, Heil Sound Ltd. has developed many professional audio innovations over the years, and is currently a world leader in the design and manufacture of large diaphragm dynamic, professional grade microphones for live sound, broadcast and recording.

CTA Classroom: Making Mono Sources Sound Stereo

This is a video I've wanted to do for a long time. I finally got around to getting it done. You'll want to listen on headphones or decent studio monitors to really hear the effects; it won't really sound much different on laptop speakers. 

“Gear

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What's the Difference: Pre-Fade vs. Post-Fade

Today, we’re back to our What’s the Difference series. These are going to be short posts where we look at commonly misunderstood terms in the tech world. Nearly every audio console offers aux sends with a pre-fade or post-fade option, but what does that mean?

It’s the Pick Off Point Again

As we discussed in the last episode (AFL/PFL), pre-fade and post-fade are really all about the pick off point. That is to say, at what point in the channel strip is the aux send being picked off. A pre-fade aux takes the signal before (pre) the fader. So, the level of the fader has no impact on the level of the aux send. A post-fade aux takes the signal after (post) the fader so the level of the fader does impact the level of the send. Sometimes, it’s really that simple. See, I told you these were going to be short posts.

Options, We Have Options

Back in the days of analog consoles, it was often possible to change the pick off point. I remember reading the manual of our old Soundcraft Series 2 in which it described breaking solder jumpers to move the pick off from pre-fade, post-EQ to pre-fade, pre-EQ. Sometimes, it could be done with jumper blocks on the board.

With the advent of digital consoles and DSP, it’s now easier than ever to change the pick off point. For example, Digico allows for pre-fade, pre-mute; pre-fade, post-mute; and post-fade, post-mute options. Pre-fade and pre-mute are both pre-processing while post-fade is after the processing block. Even the Behringer X32 allows for each aux of each channel to be set pre-EQ, post-EQ, pre-fade, post-fade. 

I especially appreciate the signal flow diagram showing your aux options. 

I especially appreciate the signal flow diagram showing your aux options. 

You’ll have to break out the manual to see what options your board has.

Why Use Them?

Generally speaking, we use pre-fade sends for monitors and post-fade sends for FX. Post-fade sends are also useful for things like broadcast mixes, and feeds to ancillary rooms. We want monitors to be pre-fade because we don’t want to be changing the musician’s mixes each time we make a house mix adjustment. If you’re getting complaints from musicians that their mixes keep changing, make sure you’re set to pre-fade auxes. 

For FX, we want the level going to the FX processor to be tracking with the dry signal going to the mix. If you sent pre-fade signals to an FX processor, even if you pulled the channel down, the FX would still be in the mix. Similarly, if you’re using a post-fade aux bus to mix broadcast, you want the fader changes of the mix to track to the broadcast mix. 

Adding to the Confusion

Some manufacturers make analog boards with a few pre-fade auxes and a one or two knobs labeled FX. The FX knobs are simply post-fade auxes that often feed an internal FX system. Typically, there is an output on the board to use an external processor with those FX sends, so don’t be limited to the internal FX (which may or may not be any good). 

Many analog boards will let you switch the send for each aux or a pair of auxes to pre or post. Again, be sure the switches are in the right spot if you want to keep your musicians happy.

Pre-fade and post-fade is one of those concepts that is really quite simple, but can cause a lot of problems if not implemented correctly. Hopefully, this post helps with that.

“Gear

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