Tips & Techniques - March 1989

Production Basics

Mono Dubs: Avoiding Alignment Problems

by Jerry Vigil

Every now and then you'll get an agency dub with tones on the front of it. These tones are dual in purpose. First they are there to set levels. Secondly, they are there to adjust the alignment of your playback head to match that of the machine that recorded the dub. (The high frequency tone is used for this.) Unless you want an engineer screaming at you, you'd better leave the heads alone, but there are ways to deal with the alignment problem.

With a stereo spot on reel, a difference in alignment will result in a phasing problem and a loss of some high frequencies when it's heard in mono. Take a stereo spot from an agency, mono it out, and listen for a drop in highs.

The same holds true for a mono spot. Mono one out and listen for a drop in highs. This is what happens to the spot on a mono AM station or if the listener is using a mono FM receiver for your stereo FM signal. Unfortunately, unless you want to eliminate one channel of a stereo spot, this can't be avoided, however, you might be able to avoid the problem with mono spots.

Check the box label first to determine if the spot is mono or stereo. If it doesn't indicate so either way, use your ears and listen to the spot. Use headphones, if necessary, to determine if the spot is in stereo or mono. Once you've determined that the spot is a mono spot, mono your console out and listen for a drop in highs. If there is none, the alignment is OK. If there is a loss of high frequencies, you can do a couple of things. How your 2-track is wired into your console and patch bay will dictate which method will work for you.

Let's assume you have the 2-track coming up on a stereo input at your console. Go to the 2-track machine and shut off one of the channels. (Drop the output level down to zero.) Now, locate the stereo/mono switch (if you have one) on your console and switch the console to mono. You've taken just one channel of the tape and split it to left and right. Since both channels are in phase with each other, there will be no cancellation of highs should the dub be heard in mono. After making the dub, listen to it with the console in stereo, then in mono. The audio should sound the same both ways. If it doesn't, you're cart machine may need to be re-aligned.

If you're using a console with mono inputs and the 2-track comes up on 2 different faders, turn off one of the faders. Now assign the other channel to both left and right channels of the console and make your dub.

Make it a habit to dub all mono spots this way and you won't have to bother checking for alignment problems. You'll save time and can rest assured that the mono dub is fine.

If the alignment problem on a stereo spot is real bad, you have the option to make a judgment call on your own and decide whether or not to drop one of the channels out completely. With some stereo spots, you might get away with it.

This kind of attention to dubs may seem like petty stuff to some. It is seldom that these problems present themselves to FM listeners because most FM receivers are stereo, but let's not forget about the many portable FM radios and clock radios in use today that are mono. If you're dubbing spots for your AM station, and it's not stereo, attention to these details should be standard procedure; it can clean up the sound of your stopsets tremendously.

Low-Pass & High-Pass Filters – A Simple Definition

by Jerry Vigil

As technology brings new toys into our production studios, it also brings new terminology we must deal with if we are to fully understand the equipment we're working with. In the "Basics" section of Tips & Techniques, we will better acquaint you with your machines by defining some of those terms. Let's look at these basic filters you'll find in many audio processors.

First of all, understand that a high-pass filter is essentially the same as a low-cut filter. The high-pass filter passes high frequencies and rejects the low frequencies. The low-cut filter passes the highs and cuts the lows. Many processors offer these filters as parameters you can modify. (By the way, low-pass = high-cut.)

The adjustment made to a high-pass filter is the frequency at which you want the filter to begin passing high frequencies and rejecting lower frequencies; the higher you set it, the more "tinny" the output will sound as it loses more and more of the lower frequencies. Some processors offer the word "THRU" as a parameter setting of the filter. In this case, all frequencies are passed THRU the filter.

The low-pass filter acts in the same manner. Imagine a door that allows fewer and fewer high frequencies to pass as you slowly close it. A "THRU" setting here again passes all frequencies or leaves the door open.

Then you have a band-pass filter. It neither cuts highs nor lows, but passes a band of frequencies. Crossover networks in speaker systems will use a form of band-pass filtering to send the mid-range frequencies to the mid-range speaker. A high-pass filter will then be used to send just the highs to the tweeter, and a low-pass filter will send the bass notes to the woofer. These filters represent the basic circuits of an equalizer.


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