As we roll forward with our look at the importance of digital signal processors in car audio systems, we’ll talk about the need for proper equalization. Even if you’ve purchased the best speakers available and invested in having them integrated into your vehicle with the utmost precision – compensating for the acoustic characteristics of the vehicle is necessary to achieve great sound quality. Let’s look at how the equalizer in a digital signal processor works, explain its importance, and offer some purchasing tips.

What Is an Equalizer?

An equalizer, often called an EQ as an abbreviation, is an electronic device that changes the frequency response of an audio signal. The simplest and most common equalizer would be the bass and treble tone controls built into a standard radio.

In the digital domain, there are four types of equalizers that we’ll discuss: graphic, parametric, semi-parametric and shelving.

Graphic Equalizer Basics

A graphic equalizer comprises a number of adjustments (called bands) that are set at specific frequencies. The range of frequencies each band affects is determined by the total number of frequencies in the equalizer. A 31-band EQ is often referred to as a 1/3-octave equalizer because there are three adjustments available per octave. For a 31-band equalizer covering the entire audio range from 20 Hz to 20 kHz, each adjustment has a Q of roughly 4.32. If you have a 20-band EQ, then that would be a half-octave EQ, and each adjustment would have a fixed Q-factor of 2.87.

Each band of the equalizer can add to the signal (called boosting) or remove information around a specific frequency (called cutting). There are myths around the maximum amount of boost you can add to a signal that was based on analog processing and noise issues. Properly designed 32-bit processors have enough digital headroom that significant boosts don’t cause distortion. It should be noted that if there is a need for a significant amount of boost, there’s possibly a cancellation issue in the vehicle that can’t, or shouldn’t, be resolved with tuning.

Parametric Equalizer Basics

A parametric equalizer is more flexible than a graphic EQ in that you have control over the center frequency and the bandwidth of each equalization band. As such, each band has three values associated with it in the configuration screen. This flexibility allows the technician who is calibrating the system to hone in on specific issues. Since the concept often isn’t understood, we’ll provide a couple of examples to demonstrate how adjusting the Q-factor changes the range of frequencies that each band alters.

Semi-Parametric Equalizer Functionality

Though rare in the car audio industry, a semi-parametric equalizer (sometimes called a paragraphic EQ) allows you to pick a center frequency for each band, but you can’t adjust the Q-factor. For example, most 1/3-octave EQs have bands centered at 800, 1,000 and 1,250 Hz. A semi-parametric EQ will let you pick any frequency. In the example below, we have a 1/3-octave EQ with a fixed Q-factor of 4.31. The center frequency of the band is set to 1080 Hz and the boost remains at 10 dB.

Shelving Equalizer Applications

Very few digital signal processors include shelving equalizers. With that said, they can be handy in terms of shaping the acoustic response of a mobile audio system to suit the listening preferences of the vehicle owner. A shelf EQ boosts or cuts frequencies above or below an adjustable frequency. If the technician needs to tame a set of efficient tweeters in a passive system, shelving equalization is a great way to make that adjustment and maintain a smooth transition to adjacent frequency bands.

All-Pass Filters in Two-Seat Systems

If you are having an audio system designed and installed such that it will be configured to provide a stable relatively center image from both the driver and passenger seat, then you will need a signal processor that includes the capability of adding an all-pass filter. An AP filter doesn’t affect the frequency response of the signal, but it improves the way the left and right channels interact with each other.

Why Does Your Car Audio System Need a DSP Equalizer?

As we mentioned at the beginning of this article, equalizing the output of your speakers to compensate for changes in frequency response because of reflections and resonances is crucial to obtaining great sound in your car or truck. As the sound produced by your speakers reflects off of different surfaces in your vehicle, it will be reflected toward the listening position. The variances in arrival time wreak havoc with the perceived frequency response of the system.

The graph above shows the frequency response of a four-way audio system. The green trace highlights just how bad this system would sound. There’s more than 12 dB of variation between 220 Hz and 700 Hz, and another dip of 11 dB at 2.8 kHz. There’s also no bass and too much high-frequency energy. The violet curve shows how some equalization can smooth the response of the system. After calibration, the system is flat within 4 dB from 200 Hz to over 15 kHz. The boost in the bass and the roll-off of the high-frequency information demonstrate that the system was calibrated to a commonly-used, quite enjoyable reference (or target) curve.

Shopping for the Right DSP Equalizer

When it comes time to buy a digital signal processor for your car audio system, you’ll want to ask your product specialist a few questions. If you’re investing in a processor, then you likely have an audio system designed to use active filtering. You will want to ask about how many EQ bands are available for each output. If it’s a graphic EQ and you have an active system, 15 bands per output, assuming they are at least semi-parametric, is adequate. If the system is going to use passive filters between a midrange and tweeter, you may want more bands – 30 would be ideal. These numbers drop dramatically if the system uses parametric filters – 10 is likely adequate for a properly designed system.

Global DSP equalization is important to maintaining correct phase through the crossover region. The DSP you choose should offer the ability to adjust multiple bands simultaneously.

Presets are another great feature. You may want to have one configuration that’s tuned close to a reference system response curve for critical listening and have another with a little more bass and midbass for those times when you want to rock out at high volume levels.

Experience How a Digital Signal Processor Can Improve Your Car Audio System

Even if you have a simple car audio system with a four-channel amplifier and new front and rear speakers, a digital signal processor can, once calibrated for your vehicle, transform the listening experience into something that’s truly remarkable. Drop by your local specialty mobile enhancement retailer today and ask for a demonstration. You’ll be blown away!
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As we continue our deep-dive into the functions of car audio digital signal processors, we’ve reached the topic of crossovers. Many people overlook the importance of configuring crossovers properly to achieve a smooth frequency response. Let’s take a look at what a crossover does and why your technician must set them correctly to protect your speakers.

What Is a Crossover?

All speakers, no matter their size, have limits in terms of their ability to reproduce low-frequency audio information at high levels. For small speakers like a tweeter or midrange driver, attempting to produce bass is a recipe for disaster. Crossovers are used to limit the audio information being sent to a speaker above or below a set frequency.

High-pass crossovers allow information higher than the set frequency to go to the speaker. Low-pass filters do the opposite – they pass audio information below the crossover frequency.

High-pass filters are used to prevent bass and midrange information from being sent to tweeters. Low-pass filters are used to prevent midrange and high-frequency information from being sent to a subwoofer. For midbass and midrange speakers, we combine a high- and low-pass filter to create what’s known as a bandpass filter. A bandpass filter has limited low- and high-frequency information.

Crossovers have three defining characteristics: the crossover frequency, the attenuation rate and the type of crossover response curve.

Crossover Frequency

Depending on the response curve chosen, the crossover frequency defines the -3 dB or -6 dB point for the filter. When setting crossovers between midbass woofers, midrange speakers and tweeters, we want the output to sum flat – as though there were no crossover, and we had a single speaker that would play through the entire audio range. To achieve this, we need the crossover frequencies for both speakers to be the same.

Crossover Slope

The slope describes how fast audio signals in the stop-band are attenuated. A crossover isn’t a brick wall or a switch. Say you set a high-pass crossover at 3 kHz for a tweeter. You still get output at 2.5 kHz and 2.0 kHz. The rate at which that output attenuates is the slope. The slope is described by how much the output is reduced per octave away from the crossover point. The most commonly used slopes are -12 and -24 dB/octave.

Crossover Response Options

Some companies describe the shape of the filter response using different terms – alignment, damping or simply crossover type. These options describe the behavior of the filter around the crossover point and how the output sums with an adjacent filter. This is a topic that could fill a textbook, but suffice it to say that some types work better in car audio applications than others. Your technician should know what to use to achieve the flattest response and best speaker radiation pattern around the crossover frequency.

Choosing the Right DSP

Most of the high-quality digital signal processors on the market have very flexible crossover frequency, slope and response options. Where you need to pay attention is in choosing a DSP that has enough channels for your system, and in ensuring that any channel can be configured with any type of crossover. In some cases, manufacturers link channels together. While usually acceptable for conventional systems, linking presents limits when you want to drive a center channel speaker.

Lastly, and most importantly, you want to choose a technician who has the tools (a calibrated RTA) and the training to ensure that the crossovers that he or she configures for your audio system protect speakers adequately. Of course, the system also needs to be designed using speakers that complement each other and deliver smooth sound distribution throughout the vehicle. Start with your local specialty mobile enhancement retailer. Ask to hear some of their demo vehicles. Ask what RTA they use, how long they take to configure a system, and how they ensure it’s adjusted to suit your listening preferences.

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As vehicle manufacturers work to reduce the number of wires that need to be run in a vehicle, more and more have switched to using the vehicle data bus to control amplifiers. Using data commands in concert with digital or fixed-level audio signals also helps to ensure that these audio systems deliver noise-free performance. Finally, with the volume control and signal processing built into the amplifier, manufacturers can use similar system designs across a variety of vehicles by loading different calibrations within the amplifier. Having your installation technician upgrade these systems with new amplifiers isn’t typically difficult, but it requires extra hardware.

Conventional Car Audio System Design

In classic analog audio systems, the volume control for the system is built into the source unit. When you turn the volume up, the voltage of the audio signal going to the amplifier increases. The drawbacks of these systems are that there is a relatively low voltage of that audio signal between the source unit and the amp, and that they are susceptible to picking up unwanted noise.

Data Bus Audio Systems

In most new mid- to high-level audio systems, the CAN bus is used to send digital commands from the source unit to the amplifier. These commands include changes to volume level, fader changes and instructions to load different presets depending on vehicle conditions. For example, Ford Mustang convertibles load a different configuration depending on whether the roof is up or down.

In terms of the audio signal from the source unit to the amp, these are passed as moderate voltage, fixed-level analog signals or as digital signals using copper wire or fiber-optic connections.

When you adjust the volume on the source unit, a command is relayed to the signal processor in the amplifier. The amplifier adjusts its output level to turn the music up or down.

Upgrading Audio Systems with Data Bus Communication

To upgrade these audio systems, you’ll want your installer to add new amplifiers, a digital signal processor, new speakers and a subwoofer system. The DSP is pretty much mandatory since the factory-installed systems are often carefully calibrated to deliver good imaging and neutral tonal balance. Forgoing equivalent calibration will result in a reduction in perceived quality, even if the system has more bass and plays louder.

The first thing you’ll need is an interface that can understand the commands from the radio. Companies like Nav-TV, iDatalink and PAC offer solutions for these applications. The Nav-TV ZEN Audio and PAC AmpPRO 4 modules provide RCA analog or Toslink digital outputs that can be fed into your processor and amplifiers.

The Maestro AR module adds the option of communicating directly with a signal processor from ARC Audio, Audio Control or Audison so that commands from the radio are executed in the digital domain inside the processor itself.

Because many of these interfaces are universal in terms of their hardware, they have to be combined with appropriate interface cables and flashed with vehicle-specific software by your installation technician before installation. As such, this isn’t a do-it-yourself kind of project.

Benefits of Audio Upgrades Using the Factory Radio

If you drive a late-model vehicle that includes an audio system that is integrated into the dash in a way that prevents the addition of an aftermarket radio, then you have no choice but to add a DSP and new amplifiers to power the speakers. Likewise, using the factory source unit helps to maintain the overall appearance of the vehicle, which reduces the potential for theft. Finally, all elements of the interface, including touchscreens, instrument cluster displays, heads-up displays, steering wheel controls and center console controls, will continue to function normally with the factory-installed radio as the source for your audio system. In some cases, you can even add smartphone connectivity solutions like Android Auto and Apple CarPlay to your factory infotainment system.

If you want more output, better bass performance, improved imaging or increased clarity from the stereo that came with your car or truck, then drop by your local specialty mobile enhancement retailer today and ask them about upgrading the audio system with new equipment and a bus interface.

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If you’re planning on having someone upgrade the stereo system in a late-model car or truck, you need to know about all-pass filters. Sometimes called phase EQ or phase rotator filters, this type of signal processing is used in multichannel audio systems to help deliver improved frequency response at the listening position. The concept can be complicated for those who aren’t intimately familiar with how signal phase works, but that doesn’t change the need to understand how all-pass filters affect aftermarket audio system upgrades.

What Is an All-Pass Filter?

An all-pass filter passes audio signals without affecting the frequency response. What the filter does do is change the phase of the signal.

Phase is a complex topic. In simple terms, phase is the relationship between two waveforms. If you’re sitting at home in your living room, listening to a nice stereo audio system, the sound coming from both speakers is, for the most part, equal in amplitude (volume) and phase (start and stop times).

If one of the speakers were to be wired backward, we call that “out of phase,” or more correctly, “reversed polarity.” When the sound coming from one speaker is backward, the sound waves cancel the sound from the other speaker. In most cases, this eliminates the bass and dramatically diffuses the center image.

Phase in a Car Audio System

Our car stereos are quite different than a home audio system. As drivers, we sit very close to the left-side speaker. As such, we naturally hear sound from that speaker before hearing the sound from the right side. Because we are much closer to it, we also perceive the left-side speaker as being louder. If the audio system isn’t calibrated for these conditions, most of the sound will appear to come from the left side of the vehicle. You can test this at home. Sit equidistant from the speakers in your home stereo system. Pay attention to where it sounds like the music is coming from. Now move closer to the left-side speaker so that the distance between it and you is a quarter of the distance to the right-side speaker. You’ll barely hear the right channel. That’s how most basic car audio systems work.

Phase and Frequency Response

The differences in pathlength from one speaker to the other do more than just change the amplitude and arrival time of the signal; they change the perceived frequency response.

All sounds have specific wavelengths. When two sounds coming from different sources are in phase, the signals add. When the sounds are out of phase, the signals subtract. When we sit closer to one speaker than the other, some frequencies add, and some subtract. You can use simple math to calculate which frequencies subtract, but we aren’t going to get into that here.

How All-Pass Filters Fix Frequency Response Problems

The goal of the companies designing audio systems that come with the cars and trucks we purchase is to deliver the best sound possible, given the restraints of the vehicle design and budget. These companies have the tools to analyze not only the output of each speaker but to understand how each speaker in an audio system interacts with the others.

The goal of these audio systems is to deliver a smooth frequency response and good imaging. If they know there is a dip in the frequency response at the listening position because of pathlength differences between the speakers, they have two choices to resolve the issue: signal delay or all-pass filters.

Delaying the output from the closest speaker so that the sound it produces arrives at the listening position at the same time as the sound from the other side speaker works great. If the output level is also matched, it can seem as though you are sitting exactly between the speakers. The drawback is that the person in the passenger seat ends up with a mess – the passenger experiences terrible imaging and poor frequency response. We refer to a car audio system that uses signal delay to create a soundstage as having a “one-seat tune,” as only one seating position will sound good.

Option two is to use all-pass filters at the frequencies that are canceling. By shifting phase at the frequencies where cancellation occurs, the sum of the left and the right signals add instead of subtract. More and more factory-installed audio systems are using all-pass filters to deliver a good listening experience for both the driver and the passenger.

All-Pass Filters and Audio System Upgrades

If you’re planning on having the stereo in your car or truck upgraded, one of the many things your installer needs to check for is all-pass filters. Once they have confirmed whether or not your stereo has this feature, they can tailor the audio system upgrade to work with it, or they can choose to work around it. If you are bypassing a factory amplifier with a Zen Audio, AmpPro or Maestro AR interface, then the filters won’t be part of the new system. Your installer will have to apply an appropriate amount of calibration with a DSP to recreate a good soundstage and smooth frequency response.

If there’s no way to bypass the amp in the vehicle, then your best bet is to add a DSP that will accept the output from the amp, then feed the output of the DSP into a new amplifier and new speakers. The installer can fine-tune the frequency response of the system. What he or she can’t do is use signal delay to change the imaging. Doing so, without bypassing the filters (by removing the factory amp), will make a mess.

There is a third option. Some signal processors claim to be capable of removing all-pass filters used in factory amplifiers. The Audison bit ONE HD Virtuoso and the new Kicker KEYLOC can remove all-pass filters. More likely, they add additional filters to the opposite channel – but the net effect is the same. You end up with a signal that can be used almost any way your installer wants.

Choose Wisely When Deciding Who’ll Upgrade Your Factory Car Stereo

As we’ve mentioned numerous times, upgrading the audio system in a modern car or truck is no easy feat. There are potentially equalization, signal delay and all-pass filter processing issues to worry about from an amplifier. Those are just the tip of the iceberg. Choosing the right shop to work on your vehicle is more important than the products you choose. Shop wisely and ask a lot of questions. Start with a local specialty mobile enhancement retailer and see just how amazing your car stereo can sound.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

When it comes to feeding a signal into a digital signal processor, most high-quality units provide your installer with a few options. While even the most basic include RCA inputs, some add the ability to accept speaker-level signals or digital signals in a variety of formats. Once an audio signal is inside the processor, the input mixer stage will let the technician combine and route information to create the sound system of your dreams. Let’s take a close look at some of the options that are available and why they’re essential.

DSP Signal Input Types

When you’re shopping for the perfect DSP for your vehicle, you’ll want to make sure there are enough inputs to provide the control you want or to deal with the channels that are needed. If you’re having the processor connected to an aftermarket source unit, then feeding signals from the front, rear and subwoofer channels is common. This configuration will let you use the subwoofer level control function built into the radio and, depending on the system design, provide you with fader control capabilities.

You’ll want to make sure the processor uses balanced differential inputs to keep noise out of the system. Likewise, the processor should be able to accept at least 4 volts of signal on the RCA connections.

Factory Audio System Integration

Connecting to a factory stereo isn’t as simple as it seems. First, amplifiers produce a lot more voltage than would be seen on the preamp output of an aftermarket radio. If you’re having a factory-installed audio system upgraded, then the processor will need to accept these high-voltage speaker-level signals. Some units have separate connections for speakers, some have built-in attenuation that can be controlled through software or a physical switch, while others include external adapter modules.

The output of some radios or, more importantly, amplifiers, needs to see a specific load in order to deliver smooth frequency response. Some processors have small load resistors built in. If you want genuinely flat frequency response, your installer will need to add external loads that simulate the presence of a speaker.

More and more factory-installed amplifier replacement modules are including TOSLINK SP/DIF digital outputs. These fiber optic connections are impervious to EMI/RFI noise and contain two-channel stereo audio signals. If your audio system is going to use something like a Zen Audio or AmpPro interface, then choose a DSP that has a digital input.

Another feature you may need is the processor’s ability to combine digital and analog signals simultaneously. This capability is especially important to ensure that chimes, warnings and voice prompts function properly after an upgrade. Some processors can automatically change presets when a digital signal is available.

If you are upgrading an audio system like a Bose Centerpoint or a JBL or Lexicon system that uses the Logic-7 processing, you’ll need a processor with enough inputs to accept signals from all the amplifier channels in the vehicle. This type of upgrade can easily be 10, 12 or more channels.

Signal Routing and Mixing Functions

Once the audio signal from your source unit is inside the processor, your installer needs to determine how it will be routed to feed the various output channels. Most high-quality processors can combine signals from multiple inputs and supply that information to several outputs. Say your factory stereo has a two-way door speaker configuration – your installer may need to recombine these signals to drive a set of three-way speakers. Of course, the technician working on the car will have to check for signal delay and the presence of all-pass filters before he or she does that. Some processors do this integration checking automatically and can often handle the correction or compensation, if required, using programmed algorithms. Other models include manual time-delay settings that perform similar features but need manual configuration.

When shopping, look for a signal processor that has the ability for the input from any channel to be routed to any output. Likewise, you want each output channel to have the same crossover, signal delay and equalization capabilities as the others. Some processors are limited in this regard, making compatibility with future applications impossible.

It’s important to note that audio systems with an upmixer (like the Bose and Harman examples we mentioned earlier) can not be converted back into two-channel stereo audio without replacing the amplifier. Proper testing of the frequency response, phase response and signal steering audio signals are mandatory before upgrading one of these audio systems. Without a thorough understanding of how the factory stereo system works, the performance of the upgraded system is unpredictable.

Not All Digital Signal Processors Are Created Equally

While on paper, it seems as though most of the signal processors on the market are similar, the reality is that their features and signal routing flexibility vary a great deal. Be sure to ask about the flexibility of the solution you are purchasing for its suitability in a future application. If you’re upgrading a factory audio system, then you may also want to ask about compatibility with integration solutions like the Maestro AR. The AR module relays both audio signals and operational commands from the factory radio into the processor, so the audio system will function like as though was installed on the production line, but with fantastic sound. Your local specialty mobile enhancement retailer can provide additional information about the DSP upgrade options available for your vehicle.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.