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.

When it comes to choosing sound deadening, not every option on the market is the same quality. Different materials, varying thicknesses of butyl and aluminum, and options for foam or lead mean that some research is required before you settle on a solution. In this article, we’ll explain what to look for and what to avoid.

Material and Design Considerations

We are going to assume that you are shopping for a constrained layer damping (CLD) material that you’ll have installed on the doors, fenders or floor of your vehicle. In most cases, these products use a dense butyl or vinyl core that is good at absorbing sound energy and converting that energy into small amounts of heat. These materials are bonded to a thin layer of aluminum to provide additional structural stability to the damping sheets. The inclusion of aluminum allows the sheets to span large openings.

Some companies market their damping materials based on thickness. Thicker materials will absorb more energy. At the same time, they also add more weight to the vehicle. Though the information is nearly impossible to acquire, the quality of a damping solution is based on the composition of the butyl layer. The mixture needs to have a perfect balance of flexibility and strength. You also want a solution that won’t turn rock hard in cold weather or become extremely soft when it gets hot.

Adhesive Quality and Strength

The performance of the adhesive that holds the damping material in place is far more important than the performance of the damping material itself. If the sheets fall off the doors or roof, you don’t get any benefit. Likewise, if the material melts, it will not only release from the mounting surface, it can damage the carpet and upholstery in the vehicle.

We can’t stress enough that you want the best quality sound deadening, not the deadening with the lowest price.

Sound Deadening Solutions That Don’t Work

We’ll openly admit that we’ve experimented with inexpensive alternatives to proper sound deadening over the years. Our first attempt used vinyl floor tiles. We bought these from a local hardware store and applied them to the outer door skins and the inner door panel of a vehicle. As soon as the weather got hot, most of the panels fell off. The adhesive simply isn’t strong enough to keep them in place, nor is it designed for high temperature environments.

Another old-school alternative to name-brand deadening was to use a product called Ice and Water Shield, also from the hardware store. This a rubberized asphalt material designed to be used along the edges of roofs to prevent water from damaging the wood underneath. While it does come with a pressure sensitive adhesive, it’s not strong enough for use on vertical surfaces in a vehicle.

The time wasted in having to remove a gooey mess from the floor of your vehicle negates any potential cost savings that could come from using a product not designed and tested for automotive applications. If the carpet or upholstery is stained or damaged, well, replacing it could be costly.

Invest in Quality Sound Deadening Materials

If you’re upgrading the sound system in your car or truck, this is a perfect time to have the comfort of your vehicle improved by adding sound deadening. Visit your local specialty mobile enhancement retailer today and ask them about the products they recommend.

Lead-In Image: SoundShield sound deadening in the cab of a Ford Raptor by Elevated Audio in Denver, Colorado.

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.

Having a digital signal processor in your car audio system is a necessity if you expect your music to sound accurate. We’ve told you many times that speakers are essential to getting great sound. While 100% true, so is the need to calibrate the output of those speakers to work with the acoustics of the environment. Proper equalizer configuration is the only way to make your audio system sound genuinely realistic.

In this series of articles, we’ll look closely at the features included in a digital signal processor and explain how each of those functions improves the performance of your mobile audio system. We’ll also give you some tips to help you find a retailer who has the tools and training to optimize your stereo efficiently and reliably.

What Is Signal Processing?

For decades, car audio systems used analog crossovers and equalizers with sliders or knobs to allow installation technicians to configure and calibrate the signals going to each amplifier. Tens of thousands of hours have been spent tweaking AudioControl, Phoenix Gold, Precision Power and Image Dynamics analog equalizers over the years.

The problem with analog processing is its accuracy. The values of resistors, capacitors and potentiometers in analog circuits varies by temperature and sometimes by age. Another problem is accessibility. Since these devices are somewhat large, tuning a vehicle typically involves repeated trips from the driver or passenger seat to the trunk to make adjustments. This dramatically increases the time it takes to configure and calibrate the system. Finally, there is the issue of repeatability. If you want two different configurations, you can’t reliably change the settings, then hope to return them to their original configuration and have the system sound exactly the same each time. Potentiometers are by nature, nearly infinitely variable. Even a difference of only a few degrees can dramatically change the value of an EQ or filter setting.

According to the definition provided by Analog Devices (a company that manufactures DSP integrated circuits), a digital signal processor (DSP) is a device that takes real-world signals like voice, audio, video, temperature, pressure or position that have been digitized and mathematically manipulates them. A DSP is designed to perform mathematical functions like “add,” “subtract,” “multiply,” and “divide” very quickly. Many companies manufacture DSP chips, and some car audio product manufacturers will note the specific unit they use. Products like Blackfin, SHARC, Sigma from Analog Devices, Symphony from NXP and Velvet Sound from Asahi Kasei Microdevices get name-dropped quite frequently.

In a DSP, the interface between the person configuring the processor and the system is computer-based. Once a system is calibrated, the settings can be saved to a file on the computer. Every time the processor turns on, the software applies the exact same signal modifications. The results are repeatable and predictable.

Processors with adequate memory can store multiple presets. You can have a system calibration for use when only the driver is in the vehicle, then another for when the car has people in the driver’s and passengers’ seats. You may even want a third calibration that raises the crossover points a bit so you can play the system at higher levels without fear of damaging speakers.

When to Use a DSP in a Car Audio System

If you want your music to sound realistic, then you need some form of equalizer in your mobile audio system. Praying that a new set of speakers will magically compensate for the reflections and cancellations in your specific vehicle is a fool’s errand. They won’t, and they shouldn’t. To compensate for the acoustics of the vehicle, you need a DSP with at least 30 bands of graphic equalization on both the left and right channels. You can get away with fewer bands if you are using more channels. For example, 10 to 15 bands of EQ are more than enough on a woofer or a midbass channel. We’ll explain equalizer functions in detail in Part 4 of this series.

Most car audio systems use a dedicated amplifier for the subwoofers and a separate amp for the midrange and high-frequency speakers. Getting the crossover settings between these amplifiers right is crucial to ensuring the bass and midbass in these systems sound good. If the system is more elaborate, it may have dedicated amplifier channels for the tweeters, at which point the DSP needs another set of filters. If you have midrange and midbass speakers, then you need yet another set of amp channels and filters. Most high-end car audio systems use a four-way system with dedicated subwoofers, midbass, midrange and tweeter channels. We’ll explain more about crossovers in Part 3.

The technician setting up your vehicle will likely want to apply some delay to the speakers closest to the listening position, or delay the front and rear speakers so that the sound from them arrives at the same time as sound from the subwoofer. We’ll explore signal delay details in Part 5.

Beyond equalization, crossovers and signal delay, many processors include signal mixers to combine signals from different channels or sources into something that can be used in the upgraded system. Finally, companies like ARC Audio, Audison and AudioControl have processors that will work with the ADS Maestro AR amplifier replacement module to allow your installer to integrate your system directly to your factory radio while maintaining proper mixing of warning chimes and navigation prompts. We’ll look at this more in Part 2.

When Not to Use a DSP

There are cases where you shouldn’t invest in a digital signal processor. If your installation technician doesn’t have a high-quality microphone to calibrate the system, there is no way to make the car sound good. Likewise, if the technician hasn’t installed several processors in the past, they likely don’t have a defined process in place that will ensure that your vehicle will sound good. Since you’ll be paying for the calibration process, you should know how long it’s going to take before work begins.

Speaking from experience, configuring basic settings like crossovers, signal delay and output levels within a DSP is easy. The process of equalizing each speaker in the vehicle and knowing how to deal with the anomalies that pop up can be much more challenging. It’s crucial that the technician you choose to calibrate your system understands what he or she is doing. They should have experience tuning many systems in the past. While tuning isn’t an art form, it is a complex process that requires formal training and hours of practice. It’s not something that can be figured out on the fly or that can be done by ear.

Upgrade Your Car Audio System with a Digital Signal Processor

If you’re interested in having your car audio system upgraded with a digital signal processor, start your shopping process by auditioning systems that have been tuned by the technician who will be working on your vehicle. You want to know that your audio investment in an audio system upgrade will yield similar results. Not everyone is well-versed in calibrating digital signal processors – even some technicians who are great fabricators.

As always, we suggest you start with a local specialty mobile enhancement retailer. In this case, it might be beneficial to shop around a bit. Listen to as many demo vehicles as are available. Listen to the same music in each one. Learn what you like, then talk to the Product Specialist you are dealing with about those criteria so they can be recreated in 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.