It's Just Logistics: September 2013

Saturday, September 28, 2013

Open Your Test Booklets

All right gang...you were warned last week. And here it is, the official "It's Just Logistics" Quiz. The first person to email the results of the test back and get 'em all right will get an iTunes gift card...a small one, but a gift card none the less.

In the event that nobody gets a perfect score, the gift card will go to the highest score with the earliest time stamp.

Good luck. And....begin!

When does the show start?
1. On the downbeat of the first song
2. At the last rehearsal before the gig
3. At load-in
4. After everyone has had a shot of Patron or Jack Daniels

What should you do before hooking up to power at a new venue?
1. Check that the outlets are wired correctly and have a functioning ground
2. Determine what outlets are on the same circuit
3. Both a. and b.
4. Get everyone a shot of Patron or Jack Daniels

How should you "build" your stage setup?
1. Back to front, top to bottom
2. Stage Left to Stage Right
3. From the middle out to the edges
4. The same way you play Jinga

You've determined that two normally well behaved pieces of equipment start humming as soon as they are connected together. The cause is most likely:
1. A power supply failure
2. A faulty cable
3. A ground loop
4. A fruit loop

When performing at private parties, the key to a successful gig is:
1. Remaining patient and flexible
2. Getting half the money up front
3. Loading in and out through the kitchen
4. Doubling up on your Prozac before leaving the house

The function of a compressor to:
1. Limit the dynamic range of a signal
2. Re-equalize a signal
3. Correct the pitch of a singer
4. Make up for a crappy pack in the trailer

When should you use sub-groups?
1. When you need to control the level of several sources without changing the balance between them
2. When you need to apply processing or effects to several sources
3. Both a. and b.
4. When the main group is late to the gig and you need a replacement

What is "Power Alley"?
1. 20Hz to 50Hz
2. 200Hz to 500Hz
3. 2000Hz to 5000Hz
4. Where most of the crack dealers in Austin hang out

When your band picks a song to cover, make sure everyone learns:
1. The same version and key of the song
2. All of the lyrics so anyone can sing it
3. English and Spanish versions
4. The entire Phil Collins catalog

Generally speaking, when assembling a pedal board which effect should go last in the signal chain?
1. Compression
2. Distortion
3. Delay
4. The on/off switch for the fog machine

When discussing "American" watts and "British" watts, it's important to remember:
1. American watts are louder
2. British watts are louder
3. It has to do with the differences in line voltage
4. The person talking about the two terms is a idiot

When EQ'ing monitors, stop when:
1. All of the sliders are at the top or bottom of their travel
2. You've moved about one-third of the sliders
3. The peak detect light comes on
4. There's blood coming from the lead vocalist's ears

From a reliability and noise stand point, the best way to power a pedal board is:
1. Batteries
2. A switching power supply (like a 1-Spot)
3. A linear power supply (like a Voodoo Labs Power Plus)
4. A tank of electric eels being aggitated by a trained monkey

You've just fired up your PA. The drum and instrument mics are working, but none of the vocal mics are coming through. The problem is likely:
1. The main power amps did not get turned on
2. The monitors are not connected
3. The vocal sub-group on the board is muted
4. President Obama's fault

The #1 enemy of electronics is:
1. Humidity
2. Temperature
3. Vibration
4. A 1st semester electronics student with a Radio Shack soldering iron.

When should you learn your parts to a song?
1. On your own time
2. During rehearsal so the rest of the band can help you
3. During sound check
4. On the way to the gig

When working with a house sound man or hired PA, the most helpful thing you can give them is:
1. Verbal instructions on how your band is set up
2. Nothing...they're pros and will figure it out
3. An up to date stage plot
4. Weed

PA speakers should be aimed:
1. Straight ahead to minimize bleed onto the stage
2. Toward the main audience area
3. At the bar
4. Right at the guy yelling for "Freebird"

When booking an extended tour, the first thing you should look at is:
1. Routing
2. Accomodations
3. Getting plenth of merchandise to sell after the shows
4. Having a "connection" in each city you're playing in

Keep the meters out of:
1. The green
2. The red
3. The black
4. Your corn hole

OK...put your pencils down...or however you're doing this. Don't forget...the first person to get 'em all right and email their answers to itsjustlogistics@gmail.com gets the glory and the iTunes card.

The answers will be posted next week.

Until then....you know.

Ken

Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com

Saturday, September 21, 2013

Phantom Of The Microphone

I've never really liked the term "Phantom" power. It makes it sound like it's not really there, or that it's just a figment of someone's imagination. But since I don't have a better suggestion (and apparently neither has anyone else), we're stuck with it.

If you're using condenser microphones in your set up, you're going to need phantom power to them. Condenser mics will not work without some kind of power source. Period.

Condenser mics aren't the only thing that can take advantage of phantom power. Some active DI boxes can use it as their power source, as can a few preamps and effects pedals (the Sansamp Tech 21 series being a good example). So lets start the discussion today with exactly what phantom power is.

Special Delivery
Phantom power is a way to deliver DC power down a balanced audio connection without causing problems for the audio coming back the other way. The original specification was written to allow 1mA (.001 amps) per connection, but as with everything else electronic, power demands have increased over the years, and now that power budget is 10mA.

Thinking back last week to our discussion of balanced and unbalanced connections, keep in mind that phantom power can only be done through a balanced system. The positive voltage is applied to pins 2 and 3, and the shield carries the ground return current. At the mixer (or voltage source end), the DC is blocked using capacitors as a high pass filter (DC is essentially a 0Hz signal, so we can filter that out long before we get to audio frequencies).

Electrically, here's what it looks like:

At the microphone end, the transformer that's built in keeps the DC from messing with the output of the preamp (DC can't cross a transformer, only AC / audio can). At the mixer input side, you see the blocking capacitors I talked about. Capacitors don't pass DC after they've charged (which only takes a few microseconds in a circuit like this).

So now you can see "how" we get the phantom power down to the mic and get the audio signal back. I suppose you could think of it like a double-decker highway. The audio is riding down the upper deck, and the DC the lower deck.

Did You Want Medium, Large, or Extra Large?

I was brought up thinking that phantom power was always supposed to be 48 volts DC. When you're using phantom for condenser mics, that's where they operate best.

However, it's a well know fact that most consumer grade electronics don't put out that much...it's typically 15 volts or so. Why? Easy...it saves them the cost of putting another power supply in their product (as most audio electronics run off of +/- 15 volts or less). And in all fairness, you're not going to hear the difference with a mid-grade mic in a live situation.

It's interesting to note that the first mic to use phantom power (the Schoeps CMT-20) was built in 1964, and was designed to use 9-12 volts. It was built for French radio, and that value was picked because that's what was easily available in the studio.

It was in 1966 that Neumann Audio of Berlin built a mic with a transistorized preamp for the Norwegian Broadcasting Network. That mic was designed for the now accepted 48 volts. Again, why? Because the emergency lighting in their studios ran off of 48 volts. Again..it's what they had available. And you thought there was a bunch of math and egg heads involved in picking these values.

There was a 24 volt standard proposed in the 1970's, but it never really caught on.

But I Don't Want Any...

So, what happens when you hook a plain old dynamic mic (that doesn't require phantom) up to a cable that is providing it? Nothing. Since the same voltage is being applied to both sides of the mic's transformer, there's a net zero potential across it. In other words, it doesn't need it so it ignores it.

Big Bang Theory

Now, it's never a good idea to connect or disconnect an audio cable unless the channel is muted. But with phantom power on a cable, it's worse. Even if you're working with something that doesn't use phantom power, connecting or disconnecting the cable with the channel unmuted is going to sound like a .22 caliber weapon going off. You won't damage the device you're hooking up to, or the mixer. But, there's a good chance you'll blow a high frequency horn out. This is why (when the equipment allows it) I only turn phantom power on the channels that need it. Most digital and higher end consoles allow this. But for less expensive gear, it's typically all or nothing.

You've been warned.

No, You Can't Do That

I mentioned earlier that some active DI boxes and preamps can use phantom power instead of their internal batteries. This is cool (as anytime you can take a battery out of the equation is a good thing in my book). But, there's one major drawback. That is, when using phantom power you can not use the ground lift if you need to. Phantom counts on the ground being there as the return path for the current. If you have to use the ground lift, you'll have to run the device on batteries.

We've been at this for almost a year now, and I've wondered how much of this stuff I've been writing about has stuck with the readers. So next week...we're having a quiz! Yeah, you read that right. It will be multiple choice, and all of the questions come from what we've covered here.

And just to give you a little more motivation, the first person to email me with all of the correct answers....heck, I'll send you an iTunes gift card. In the event no one aces the test, the highest score with the earliest time stamp wins.

Study up and get some rest.

Ken

Thursday, September 12, 2013

Lost My Balance...

There's a long list of buzzwords in audio that people toss around, but really don't understand what they mean. Two of those terms are "balanced" and "unbalanced". Now, if you're talking about your band mates...then everyone knows what you mean. But if you're talking about an audio connection or interface...that's where a lot of people's understanding gets a little murky. So, this week we'll try to bring some understanding to these terms. I'll attempt to keep it light on the math and heavy on the practical applications.

First off, lets start with an unbalanced audio connection. Every time you hook up a guitar to an amp, an iPod to a computer or mixer, or anything else that just uses a 2 conductor cable, you're using an unbalanced connection. This is what that type of cable looks like:

A balanced connection requires three conductors. If you're hooking up a professional mic to a mixer (especially if it's going down a long cable), then you're likely using a balanced connection. Here's what that type of wire looks like:

OK, we've identified the physical difference between the two types of cable. But what's going on here electrically, and why is one better than the other.

Let's look at the unbalanced (also called single-ended) connection first, since it's a little easier to understand. With unbalanced connections, the shield is performing two functions. It's serving as the return path for the signal (the "negative" side of the audio if you will), and it's shielding the center conductor (the "positive" side) from external noise. It does a pretty good job, but if the interference is strong enough, it's going to start bouncing the ground around...and you're going to hear that in the audio.

Here's a very basic premise...the shield is only going to do so much to protect the signal from interference. Unless you're using wire with a solid outer shield (which would make the cable extremely stiff), then some amount of noise is going to get through. And the longer the cable, the more chance for garbage to get in. That's one of the reasons why you rarely see unbalanced connections longer than 30 feet.

OK, so why is a balanced audio connection so much better? It has to do with the fact that there's a second line of defense against noise that's built into the architecture. It's a concept called Common Mode Rejection. No, I'm not talking about when you got turned down for dates back in high school. This is far more useful and much less painful.

Before we talk about common mode rejection, we're going to have to have to touch on a mathematical concept here (sorry...I've put it off as long as I can). That concept is Vector Addition. OK...let that sit for a moment...the sting will go away.

The classic (and simplified) definition of a vector is a ray with magnitude and direction. Putting that concept into a real world example...a car heading north at 50 MPH could be considered a vector. You have both magnitude (50 MPH) and direction (north).

So far so good, but how does vector addition work? For that example, let's use a tug of war game. Let's say that there are 4 people on each side of the rope pulling. Pulling from the left side, it's 4 Marines. On the right side, it's 4 members of the Austin Botanical Society. When we say "go" there's force being applied from both directions, but guess which way the rope is going to go. Yeah, I'm betting the that vector force coming from the Marines side is going to be a little greater.

Now, let's even this up. We'll send the Botanical Society home and bring in 4 Army Rangers to stand in for them. And now let's assume it's a dead even match and both teams are pulling with the same force. Which way will the rope go? Right, it doesn't move because the 2 forces are cancelling each out. (Now, in reality I know that one team will probably prevail...but I have friends who are ex-Marine Corp and ex-Army...so I'll just let them talk trash to each other).

Back to the world of electrons. A balanced audio connection works by running the same signal down 2 wires, but they're running opposite of each other. If you've heard the term "out of phase"...this is it.

By the way, here's another tech term you can throw around and impress people. Whenever you have two signals like this that are compliments (opposite) of each other, it's called a Differential Pair. That's going to turn some heads at your next beer bust.

I know...you're thinking "if those signal cancel out, then how do we hear anything?". Well, you're right, but this is just to show you what happens to two signals that are out of phase with each other when they're added together. I'll show you how we recover the signal in a bit.

Now, we've got our two out of phase signals running down the cable. All of a sudden, noise decides to show up and join the party. Now, our otherwise pristine signal looks something like this:

I want you to notice something very important here, because it's key to understanding how we're going to get rid of the noise. While our audio signals are out of phase with each other, the noise is the same (or in phase) on the 2 lines.

So what do you suppose would happen if we could flip one of those audio signals over so that is was in phase with the other one. What would happen to the noise?

Now our two audio signals are in phase with each other, but the noise is now out of phase! When we add those two signals together...the noise is cancelled out. And just to throw another tech term out there for you, the specification that describes how well a piece of equipment does this job is called the Common Mode Rejection Ratio, or CMRR. That's one of those specs where the bigger the number the better.

The "phase flip" and signal addition was done with a transformer in the olden days. Now, it's typically done with a ....wait for it...differential input amplifier. Electrically, what's going on looks like this:

Given this significantly improved method of getting rid of noise on audio lines (along with a couple of other electrical characteristics), we're able to run much longer cables without degrading our signal. How much longer? If you've got a good pro mic and you're plugging it into a decent mixer, you can typically run up to 2000' feet of cable before you start messing the signal up. Yeah, you read that right...about 4/10ths of a mile! This is why you have to used balanced audio connections when you have a console out front to mix the band. In that application, it's not uncommon to have 120 feet of wire between the mic and mixer.

So, can unbalanced and balanced connections ever mix? The short answer is yes. You can unbalance a balanced connection through wiring:

You would use a cable like this if you wanted to plug a balanced microphone into a guitar amp. You loose the noise cancellation and some signal, but it works. When you do this, though, the rules for unbalanced wiring apply...especially the length of cable limits.

Going the other way from unbalanced to balanced can't be done through wiring alone. You're going to have to have some electronics or a transformer. If you're a bass player or keyboardist, you've like already used a device that performs this function. You just probably know it by it's common name...a Direct Injection (DI) box.

The transformer does the job of converting the signal from single-ended (unbalanced) to differential (balanced). It also electrically isolates whatever you're hooking up to prevent ground loops. And again, this can be done with electronics to...which would make this an active DI box (as opposed to a passive one shown above).

I want to leave you with one final thought here. If you happen to have a balanced cable that's had one of the signal conductors fail, you will continue to get a signal through it, because that that point you've got an unbalanced connection. It's going to be quite a bit lower in level, and likely have more noise on it (since you've lost the ability to cancel it out). So, if you're setting up your PA and happen to notice that a mic is quieter than you expected, it's probably a good idea to bust out the handy dandy cable tester that I'm sure you have now and check that cable.

This is has been a pretty heavy installment, and I've only covered a small part of theory behind all of this. If you have additional questions, you're always welcome to email me.

Next week, as long as were talking about these connection schemes we might as well tackle another "mystery subject" to a lot of people, and that's phantom power.

Until then, keep the meters out of the red.

Ken