When 99% of car audio enthusiasts think about a digital signal processor (DSP), they associate it with equalizers, crossovers, and signal delays. In more advanced solutions, a consumer-grade car audio DSP might add the ability to include all-pass filters, an upmixer for a center channel and signal summing. However, at an engineering level, much more is available. Let’s look at a handful of DSP features that make audio systems at home, work, and on the road sound better.
What is a Digital Signal Processor?
Before we discuss some of the hidden features in digital signal processors, we should define them. A DSP is a microprocessor designed specifically to perform high-speed numerical calculations to process signals. Digital signal processors are used in video transmission, radio frequency systems, and audio systems. They are also used to interpret and manipulate sensor data in commercial, industrial, and research applications.
In short, a DSP can take a stream of digital data and manipulate or extract information from it extremely quickly. Our smartphones, smart speakers, digital cameras, drone quadcopters, smartwatches, and security cameras use this technology to provide the features, functionality, and performance you want.
Audio DSP Development
As a little peek behind the curtain, we want to introduce you to an audio DSP development suite called Analog Devices SigmaStudio. The technicians and engineers who develop car audio signal processors from scratch use this tool as part of the development process. It works like a flowchart. The designer can drag and drop elements into the project and then link them together. They would then write code in their interface to control the different elements in the DSP configuration. That’s a greatly simplified version of how it works, but you get the general idea. Writing a new DSP software package from scratch takes over ten thousand man-hours, making it a very expensive and time-consuming proposition. Testing that software also takes thousands of man-hours.
We are by no means experts in working with SigmaStudio, but its basic functionality is simple to follow. Looking at the image above, there is a stereo input on the left. One channel from that input feeds a volume control, which can be thought of as the gain. Then, the signal goes into a limiter, which we’ll discuss shortly. After that, there are three parametric equalization modules, followed by a filter module. Finally, we have an output. In theory, this setup would serve as a three-band equalizer and an adjustable crossover.
Hidden DSP Features – Audio Limiters
You might have noticed that the latest generation of factory-installed amplifiers in cars, trucks, and motorcycles are much less prone to damaging speakers. Is it that the speakers are now better? That’s part of why, but not the most significant factor. Many of these modern amplifiers have a limiter built in. A limiter will reduce the amplitude of a signal if it exceeds a certain threshold.
For example, if your installer turns up two bands of an equalizer with similar frequency centers, that might try adding 24 dB of signal at a specific frequency. A boost of 24 dB would take a 0.5-volt signal and increase it to 7.93 volts. That’s likely far more signal than an amplifier can accept.
We talked with our friends at Rockford Fosgate about the amplifier used on new Harley Davidson motorcycles. They incorporated several limiters into the design. As such, the amplifier won’t clip (overdrive) the outputs and add huge amounts of distortion, even if all the equalizer bands are boosted to their maximum levels. Similar features are integrated into some car audio amplifiers.
As a side note, anyone trying to measure power output on this amplifier with a device that looks for distortion will result in horribly inaccurate results. The output signal never reaches 1% THD, so units like the SMD DD-1 and D’Amore Engineering AMM-1 or AD-1 won’t accurately measure power. Audio analyzers like those from Audio Precision or QuantAsylum can measure output level and distortion. More importantly, these devices determine when the signal stops increasing in amplitude regardless of the harmonic and noise content.
Noise Gates
Many DSP solutions include a feature called a Noise Gate, which operates at the opposite end of the audio amplitude scale. A noise gate turns off the audio output circuitry when the signal drops below a preset level. This suppresses any background hiss or noise. As the music fades out, just when you might hear noise, the outputs turn off, leaving silence. Most modern recording studios use gating like this to help isolate a performer’s voice.
ARC Audio uses a similar approach to noise-gating with the LR1 remote level control in its signal processors. When the remote’s level is set to its lowest setting, the output devices are muted by a digital signal from the microprocessor.
Bass Processing
If you’ve been around the block for a while, you might remember the Waves MaxxBass processor. This processing algorithm analyzes harmonic content in an audio stream and then filters out the low-frequency information. Yes, that’s right—it removes bass information. It then modulates the upper bass and lower midrange frequencies to make it sound like the deep bass is still there. It’s a very cool way to produce the perception of deep bass from a small speaker with limited excursion capabilities. Smartphones and smart speakers—we’re looking at you!
If we can remove bass information, then could we not add it? If you’ve ever experienced the AudioControl Epicenter or Wavtech bassRESTOR, you know what we’re discussing. Imagine a system that can analyze the harmonic content of an audio stream and then add audio information that’s an octave or two lower. It would be like having a super-grand piano capable of playing a fundamental of 13.75 or even 6.875 hertz. Your subwoofers might not like it, but it would be fun to try! Subharmonic generators are easily added functions already built into the SigmaStudio.
Stereo Width Expansion
By now, you’ve realized that signal processors are capable of much more than just equalization and filtering. Way back in the day, many portable speakers—called boom boxes or ghetto blasters—had a switch that made the sound coming from them seem much wider. The SigmaStudio includes a stereo expander control as well.
Some research shows that Philips Semiconductors used to offer an IC called a Spatial, Stereo, and pseudo-stereo sound circuit. This was introduced in 1985, which coincides with our memory of these functions.
More Features Require More Space
The goal of this article is to provide some insight into how digital signal processors are used in different audio systems. Some devices you might think are simple are, in fact, quite complex in terms of audio processing. One that caught us off-guard is a smart speaker, of which the Apple HomePod is a perfect example.
Anytime you have microphones and speakers, you can measure the sound in the time and frequency domains. In the case of the HomePod, the unit can use its microphone array to evaluate the acoustics of the environment it’s used in. For example, if the speaker is 12 inches from a wall, frequencies around 283 and 849 hertz are likely to be attenuated.
Sound Reflections Can Cause Cancellations
Sound emanates from all speakers in a spherical pattern below the frequency where it starts to be directional. The audio information that bounces off the wall behind the speaker will eventually mix with the sound coming directly to the listening position. In our example, we have a total distance of 24 inches added to the signal path—the distance from the speaker to the wall, then back to the speaker. Where the audio wavelengths match, but are inverted, the amplitude (volume) decreases around those frequencies.
Now, back to the HomePod and its signal processing. The system will have a benchmark for the time it takes for the sound to leave its speakers, arrive at its microphone, and then be processed. Let’s call this two milliseconds, to keep the math simple. If we have the HomePod in the middle of a table, it might be the aforementioned 12 inches from the wall. It takes sound 0.0008886 milliseconds to travel 12 inches. As such, it would take 1.777 milliseconds for the sound from the speakers to bounce off the wall and return to the microphone. Let’s add that processing time, and the DSP might measure a delay of 3.777 milliseconds. The math, calibrated in controlled testing conditions, knows there will be a dip in frequency response at 283 and 849 hertz. It can then apply equalization to those frequencies to produce a much smoother overall response for the listener.
Automatic Equalization
The system will also be able to measure the frequency response of the sound it hears. If it detects a constant increase in bass frequencies due to room resonance, it could theoretically adjust for this. We’ve heard many times that HomePods sound mediocre for the first few minutes they play. Then, they mute the audio for a second, load new equalization parameters, and continue playing. Everyone who’s heard them says they sound exponentially better after they recalibrate.
Many car audio digital signal processors have have the ability to make measurements, or work with external hardware to automate the process of setting signal delays and equalization. This is achievable thanks to the processing modules available for the DSP chips. We will note, it takes a LOT more code to make these work well. Add another ten thousand man-hours to that software development time.
Vehicle Presets
A simple DSP feature is the ability to load an entirely new calibration quickly. This is the same as we described above with the Apple HomePod. For example, if you drive a newer Ford Mustang convertible, you might notice that the audio pauses for a moment as you are raising or lowering the convertible top. This is the system loading a new audio system calibration. Your music should sound similar, at least in the midbass, midrange and high-frequency ranges. However, the settings used to achieve what you hear will be very different with the roof up or down.
Real-Time Noise Cancellation
The last feature we’ll talk about is active noise cancellation. Many new cars and trucks come with an array of microphones integrated into the vehicle interior. The signals from these microphones are sent to a DSP for analysis. The DSP works out the frequency response of the sound from the microphones, then sends a signal with the opposite polarity to the audio system amplifier. When this new signal mixes with the road, exhaust, and wind noise in the car, it cancels. Again, the system is much more complex as timing is crucial to making this work. The result is a vehicle that’s quieter to drive, and that doesn’t incur weight penalties from massive amounts of sound deadening. Adding weight reduces fuel economy.
This same noise-canceling technology is used in headphones and earbuds.
DSP Features Improve Audio and Listening Experiences
We’ll step back to our discussion about car audio DSP features. Not all processors have all the technologies we’ve mentioned. Some solutions might use a chip that costs $5, while others might be $30. Every function added to a DSP increases the amount of memory required. As such, you might find that some inexpensive solutions have limited equalizer bands, whereas others have more than you might ever use. Further, you can’t just call a car audio company and say, “I know the Analog Devices chip can do this. Can you add this feature?” Having been on the other end of that, I guarantee it won’t happen quickly, if ever. It takes exponentially more time to develop and test the software than you can imagine. Even small changes require extensive lab and field testing. However, the lack of a feature is often attributable to parts costs and the coinciding lack of memory, or the fact that the company doesn’t develop their DSP in-house.
With that said, if your DSP has an upmixer for a center channel, bass restoration, automatic equalization, an RTA display, stereo width expansion or a whole slew of other features, you can thank the impressive processing power of modern digital signal processors.
Upgrade Your Car Audio System with a DSP, Today!
Digital signal processors are everywhere these days, often in devices we think are much simpler than they actually are. We hope learning about how digital signal processors work in general terms has been enlightening. If you are looking for a way to improve the performance of your car audio system, drop by a local specialist mobile enhancement retailer and ask them about adding a DSP to your audio system. Assuming the system is designed, integrated, configured and calibrated properly, the DSP upgrade will be stunning!
