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  1. #21
    Hokay. That's an easier question to answer.

    The combined response of the mics, electronics, speakers and room is never flat. There are peaks and dips all over the place. If you want the sound to be natural, you want to correct for those peaks and dips. This would mean setting the graphic EQ curve to the inverse (opposite) of the room response.

    I say "would" because actual system/room equalization of this type requires some special equipment you don't have. This does not make the graphic EQ useless. The other use for a graphic EQ is to increase the system's available gain before feedback.

    Regarding those response peaks and dips, as you turn up the system gain, it will feed back first at the frequency where the largest peak is. If you can EQ that peak out, you can turn up the gain. This doesn't require test equipment. Start with the graphic EQ set flat. Turn up the gain until the system is just barely into feedback; then try pushing each graphic EQ slider down in turn until you find the one that stops the ringing. (Remember to return each slider back to zero before proceeding on to the next one.) When you have found the correct slider, set it to about minus 6. Then turn the gain up until the system feeds back at another frequency and repeat the process.

    This is a fairly crude way of killing feedback ringing, especially with a 9-band equalizer. (30- or 31-band equalizers are usually used for this purpose.) You will only be able to kill two or at most three feedback frequencies before you hit the practical limit. Even so, that practical limit represents a very useful increase in the amount of gain you can get before feedback. Or, instead of running the system at higher gain, you can run it where you've been running it but now with cleaner sound because the worst resposne peaks have been tamed.

    If you tell me the frequencies for each of those nine sliders, I can tell you what musical pitches they represent. This can speed up the process a lot.

    You know, you could probably become very good at this. Were you the one who said you're blind? Blind people tend to compensate by developing very acute hearing. You've heard of people with "golden ears." That's much more due to practice and development than to any genetic gift. You can teach yourself to have something close to perfect pitch, and that's really handy in the audio biz. Try it.

  2. #22
    And now I try again on the levels thing. Let's start at the end and go backwards, 'cuz the end is what really matters. What we're talking about here is voltages.

    The standard level for professional gear is about 1.2 volts. If you put a tone into the mixer and set it so the meters read "zero," and then put a voltmeter on an output jack, the meter should read fairly close to 1.2 volts.

    The standard level for consumer gear is about 0.3 volts.

    A balanced input or output, especially if it appears on a 3-pin XLR connector, is almost always professional level. An unbalanced line, especially if it appears on an RCA connector, is almost always consumer level. 1/4-inch phone plugs can be either; check the equipment manual.

    The difference between 1.2 volts and 0.3 volts is a 4:1 ratio, which is about 12dB. This means that, when you feed a consumer-level signal into an input that is set up for professional-level signals, it will be 12dB lower than you expected. It also means that when you feed a professional-level signal into an input that is set up for consumer-level signals, it will be 12dB stronger than you expected. On a pot, the gain difference between halfway up and all the way up is usually about 15dB, so you can see that a 12dB difference is significant. Not a deal-breaker, but you will find yourself with some odd control settings.

    When going from a professional balanced output to an unbalanced consumer input, you get a reprieve. Assuming the output is an XLR connector, and is electronically balanced (that is, no transformer, which is almost always the case nowadays), the proper way to wire it is, XLR pin 1 to shield, XLR pin 2 to the unbalanced signal input pin, XLR pin 3 not connected. This throws away half the output signal, which changes the level discrepancy from 12dB to 6dB. 6dB isn't enough to cause much trouble.

    Now, about those other numbers - and don't worry if you don't "get" this, since you already know the actual voltages.

    Consumer-equipment levels are referenced to one volt. That isn't the voltage that's on the line, it's just the voltage it's referenced to. One volt was chosen because it's simple. The actual voltage, as we've said, is 0.3 volts, which is about 10dB lower than 1 volt. That's where the "-10" comes from. It should properly be stated as "-10dBV," which means minus 10dB relative to one volt.

    Pro-equipment levels are referenced to 0.78 volt, abbreviated dBu - u, not V - for historic reasons we won't get into. 1.2 volts is 4dB higher than .78 volt, so when somebody says "plus 4" they mean "+4dBu."

  3. #23
    Since we're talking about levels, here's another useful factoid:

    Ipods and battery-operated CD players can't be expected to give you even 0.3 volts output. They usually operate on two AA or AAA cells, which is about 3 volts. Maximum peak-to-peak output is limited to the power supply voltage. Maximum sinewave RMS output voltage is a bit less than 1/3 the peak-to-peak voltage, or about 0.9 volts.

    Conventional wisdom says you need at least 10dB of headroom above average output to handle short-term signal peaks. An output of 0.3 volts should just barely do it, since 10dB above 0.3 volts is 0.9 volts.

    But with today's dynamic music, 10dB of headroom isn't acceptable any more. You need 15-20db of headroom to completely avoid clipping, which means the output must be set to deliver an average of 0.1 to 0.15 volts.

    This means an Ipod output will be as much as 10dB lower than what a consumer-gear input is expecting, and more than 20dB lower than what pro gear wants. You might actually have to buy an amp to match the Ipod output to a line input. Or pad it down and feed it into a mike input.

    Just a heads-up.

  4. #24
    So should I keep the eq on the channels at u to?
    Ivan Fegundez. Recording Technician, live sound technician, and mastering technician.

  5. #25
    I assume you mean keeping the channel eq's at 0.

    Graphic eq is all about setting up the whole system so it's as flat and linear and neutral as possible, and so you can get as much gain as possible before running into feedback. This is objective.

    Channel eq is about changing individual mics, either to deliberately depart from neutral for the sake of a special sound, or to correct something that's wrong in that channel only. For example, a cardioid mic gets very bass-heavy when you speak into it at close range. Male speakers in particular like that effect, but it can get to the point where the sound is nothing but a big jumble. You want to cut out some of the bass, but only on that mic. You may also want to do things to make sound warmer, or crisper, or clearer, or whatever. This is subjective.

    Adjusting the graphic EQ is setting up the easel you're going to paint on. Channel EQ is painting. And do use restraint, okay?

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