Transcript
Chapter 7
MAX DEMO
Equalization In Chapter 7, we talk about equalizers and their use in manipulating the frequency response of an audio signal. This Max patcher demonstrates a parametric equalizer, allowing you to manipulate and visualize its properties and hear the effects it has on an audio signal, using examples from real audio projects. Figure 1 shows the Presentation Mode of the Equalization.maxpat file. You can open this file in the Max runtime as well as the full Max program.
Figure 1. Main patcher window
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This material is based on work supported by the National Science Foundation under CCLI Grant DUE 0717743, Jennifer Burg PI, Jason Romney, Co-PI.
Audio Setup Begin by verifying your audio setup. Click the Setup button under the loudspeaker icon to open your DSP Status window as shown in Figure 3. Make sure that the audio interface you have connected to your monitors or headphones is selected in the Driver menu, as shown in Figure 2. Also make sure the Output Destination is set to the correct audio output on your interface and that Audio is turned on. Since we manipulating frequency response in this demo, it's critical that you're listening to the sound through loudspeakers or headphones with an even frequency response over the entire audible frequency spectrum. The built-in speakers on your computer are not good enough to do EQ work. Use near field monitor loudspeakers or headphones for this tutorial.
Figure 3. Audio setup button
Figure 2. Select audio driver
Parametric EQ Parameters Before we get into the various demonstrations, let’s review the controllable parameters found in a parametric EQ. A single filter on a parametric EQ will have as many as five controllable parameters. These parameters are shown in the Max patcher in Figure 4. The Filter Type control found on the right of each EQ band in our demo selects between five different filters. A Peak-Notch filter will control a range of frequencies by boosting or cutting those frequencies. A Low Pass filter will completely cut all frequencies above a certain point and allow frequencies below that point to pass unchanged. A High Pass filter is the opposite of Low Pass in that it completely cuts all frequencies below a certain point and allows frequencies above that point to pass unchanged. A Low Shelf filter will boost or cut all frequencies below a certain point by a controllable amount. A High Shelf filter is the
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opposite of Low Shelf in that it boosts or cuts all frequencies above a certain point by a controllable amount. The next parameter is the Frequency (Hz) knob. In the case of a peak-notch filter, this knob controls the center frequency of the filter. For High/Low Pass and High/Low Shelf filters, this knob controls the cutoff frequency of the filter. The Bandwidth (BW) knob, sometimes called Q, controls the range of frequencies affected by a peak-notch filter. With a bandwidth parameter, a higher value represents a larger range of frequencies. In EQs that have a Q knob, a higher value represents a narrower range of frequencies. A Q value is much easier for a computer to understand, but the Q value is difficult for the human user to visualize. Bandwidth is much easier to understand, especially for someone with a background in music, because bandwidth values are in octave units. A filter with a bandwidth of 1 will affect one octave of frequencies between its 6 dB down points. For High/Low Pass and High/Low Shelf filters, the Q knob controls filter resonance. For more information on filter resonance, see Chapter 6. The Gain knob adjusts the amount that the filter boosts or cuts the defined frequencies. As High Pass or Low Pass filters only cut frequencies, the gain knob is inactive. Finally, each filter includes a toggle that will turn the filter on or off. This is helpful when programming a filter because you can easily do an A/B comparison to hear the effect of your filter without losing the filter settings. It is also good to turn off any filters that you’re not using to avoid any unintended effects on your audio signal.
Figure 4 Parameters for a single EQ filter
Layout and Display Let’s take a moment to go over the layout of the patcher and the nature of the displays and what they'll show you as we dive deeper into the demo. Figure 5 gives a closer look at the demo interface. The patcher gives you a three-band parametric EQ using the three rows of controls at the bottom of the screen. Above these controls is a filter graph that provides a visualization of the filter parameters. Since manipulating an EQ can have implications on gain structure, the Digital Sound and Music
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This material is based on work supported by the National Science Foundation under CCLI Grant DUE 0717743, Jennifer Burg PI, Jason Romney, Co-PI.
PPM meter to the right of the filter graph shows you the audio level after the EQ. Keep an eye on that to avoid clipping your signal. Above the filter graph is a master bypass button for the EQ. You can use this to perform quick A/B comparisons when you have more than one filter band programmed at a time. The Audio Select menu allows you to choose between several included audio signals. You can even add your own audio by saving it as a WAV file in the same directory as the patcher file. Name the file MyMusic.wav and restart the patcher to access your file through the Audio Select menu. We’ve also included some sample EQ settings for the various demo audio files. You can recall these demo settings by selecting them from the settings menu. If you come up with your own settings that you want to save, you can store them using the “Store My Settings” button and then recall them from the “My Settings” item in the menu.
Figure 5. Closer look at the demo interface
Equalization in Action Before we get into the specific demo tracks, take some time to familiarize yourself with the EQ controls. Select the “Pink Noise” item in the Audio Select menu. Pink noise contains equal energy per octave that allows you to easily hear any frequency processing you apply to the signal. Try all the various filter types and become familiar with how the gain, Digital Sound and Music
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bandwidth, and frequency controls affect each filter type. One thing to notice in particular is how boosting a band of frequencies can easily cause the pink noise signal to clip. This situation can be seen in Figure 6. When it comes to equalizing an audio signal, for some reason most people find it easier to identify parts of the frequency spectrum that seem to be missing and feel too quiet, rather than the frequencies that are too loud. For this reason most beginning users tend to focus on boosting frequency bands that they would like to hear more. The problem with this strategy is that in a properly gain-aligned system, even a small boost in level can cause the signal to clip. It is a much better strategy to reduce the frequencies that are too loud rather than boost the frequencies that are too quiet. In the end, the resulting frequency response is the same with both strategies, but reducing loud frequencies will help you avoid clipping the signal. All of our examples in this demo will focus on this gain reduction strategy for applying EQ filters.
Figure 6 A filter with a gain increase can easily clip the audio signal
Once you are familiar with all the controls, select the “Reset EQ” option from the EQ Settings menu to clear out all your changes. The first EQ example we will look at is a sound cue from a play called Fully Committed. The play is a one-man-show where one actor plays all the various characters in the story. The main character, Sam Peliczowski, is the telephone operator for a busy New York City restaurant. In this scene he calls his own home to retrieve the messages on his answering machine and hears a message from the Digital Sound and Music
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telephone company who is trying to collect an unpaid bill. In the spirit of the one-manshow, all the various voices on the answering machine are also performed by the same actor. In this case the actor is Sam Lloyd, Jr. The various voices were recorded from Sam in a recording session and then edited together to create the full sequence. The final task here is to EQ the track. You can listen to the track by selecting “Answering Machine” from the Audio Select menu. Right now, it sounds like a voice that was recorded using a Shure SM58 dynamic microphone. We need to make it sound like something that was recorded from a telephone signal on an answering machine. Telephone signals are band-limited to remove frequencies above approximately 3 kHz and below 300 Hz to enhance intelligibility and signal propagation in microphone and speaker systems that vary in quality. This effect can easily be mimicked with an EQ. Engage the first filter band and set the filter type to “LowPass." Now adjust the frequency knob so that frequencies above 3 kHz or so are filtered out. Engage the second filter band and set the filter type to “HighPass." Now adjust the frequency knob on that filter band so that frequencies below 300 Hz or so are also filtered out. You should now have something that sounds much more like a signal on a telephone. You can compare your EQ settings by toggling the “EQ On” button. If you have trouble figuring out the right settings, you can recall the “Answering Machine” preset from the EQ settings menu to recall the settings shown in Figure 7 to see the settings and hear what the EQ should sound like.
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Figure 7 Answering machine EQ settings
The next example is a voiceover for a DVD menu. The actor here is Wayne Bills, and one of the signature qualities of Wayne’s voice is the deep bass resonance. To hear the initial recording, reset the EQ from the EQ settings menu and then select “voiceover” from the Audio Select menu. The recording here is already in pretty good shape, but in an effort to enhance the interesting features of Wayne’s voice, you might want to boost the low frequencies by creating a notch filter centered somewhere around 130 Hz with a 6 dB boost. Since enhanced low frequencies tend to harm speech intelligibility, you may also decide to compensate with a similarly boosted band centered around 3 kHz as shown in Figure 8.
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Figure 8 Voiceover EQ with frequency boosts causing a clip
This sounds pretty good, but the boosted bands are now causing a clipped signal at the beginning of the recording. If you look at the filter graph you might notice that your two boosted bands have effectively created a 6 dB reduction in a wide band centered around 500 Hz. You can avoid the clipping and create a very similar effect using a single filter band with that same reduction as shown in Figure 9. Don’t let yourself be fooled that this new setting doesn’t sound as good as the first one; it’s simply quieter. The frequency response is essentially the same. If you turn up your speakers by 6 dB, you’ll have something that sounds very similar to the sound you had using the two boosted filter bands. If you need help, this EQ strategy can be recalled from the EQ setting for “Voiceover.”
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Figure 9 A similar voiceover EQ using a cut instead of a boost to avoid clipping
The next example is a loop of music composed by Jason Romney for a radio drama. To hear the initial music, reset the EQ and select “music” from the Audio Select menu. Suppose you want to enhance the presence of various parts of the mix in this music track. There’s a midfrequency synthesizer pad sound that is pulling a lot of focus and masking the lowfrequency bass and the definition of the chimes. This is not so much a mix issue as a harmonics issue. You can’t use an EQ to re-mix a track, but sometimes when several instruments get mixed together the harmonic content of the various instruments can mask other sounds. Turning up or down various instruments in the mix may not always work in this case. After all, we’re not talking about making one instrument louder than another; we’re talking about enhancing various harmonic frequencies to satisfy a certain aesthetic. This is part of what is accomplished in the mastering process of music production. Mastering engineers take something that is already mixed and manipulate the various harmonic frequencies with an EQ and/or a multiband compressor to enhance the sound of the track.
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To begin you might try boosting the low frequencies around 80 Hz, and again a boost at around 5 kHz. Now the bass and the chimes are given more prominence in the mix than before. This technique is shown in Figure 10.
Figure 10 Music EQ with two bands boosted
While this particular EQ boost may not be enough to cause a clipped signal, it is still a good idea to see if you can achieve a similar result though cutting frequency bands instead of boosting them. One example of how this might be achieved is by using a peak-notch filter, a high pass filter, and a low pass filter together as shown in Figure 11. These settings can be recalled using the “music” preset in the EQ Settings menu.
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Figure 11 Music EQ settings using cut filters
Our final example uses a recording of a small kick drum in a drum kit. Reset the EQ and select “Kick Drum” from the Audio Select menu to hear the initial recording of the drum. This particular drum was recorded with the microphone very close to the drum, and there is much more high frequency content than is typically desired in a kick drum. The drum is supposed to be the low thump in the drum mix. To get this low thump sound you could use an EQ to reduce the level of all the high frequency content in the recording. This can be accomplished with a single high shelf filter as shown in Figure 12, and found in the “Kick Drum” preset in the EQ settings menu. If you want to remove the high frequency content entirely, you can replace the high shelf with a low pass filter.
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Figure 12 High shelf filter for the kick drum recording
Conclusion In this demo we have seen several examples of using an EQ to shape the frequency response of an audio file. Knowing what frequencies to process involves training your ears to recognize frequencies, and practicing using the EQ parameters and controls. To start training your ears to recognize the various frequencies processing them appropriately, see our Max demo on ear training. Most importantly, remember our rule of thumb: don’t boost the frequencies that are too quiet; reduce the frequencies that are too loud. As always, if you’re curious how we’ve programmed this Max patcher, you can switch from Presentation Mode to Programming Mode to see how most of the objects go together. If you have the full version of Max, you can unlock the patcher and see all the details and copy and paste stuff from our patchers into your own projects.
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Figure 13. Programming Mode
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