DSP stands for Digital Signal Processing, a very fast form of computation that can be used in many products. Radar detectors, cruise controls, and sound systems all use DSP chips today. Let’s talk for a moment about how DSP works in car audio.
Take the Take the great-sounding Audison Thesis speaker. Now, put it in a car.
Wherever we install it in a car, it’s not a perfect speaker location. It might be in a door, or it might be in a dashboard, or it might be in a trim panel. It might be on axis or off axis, but we know the sound almost certainly will be affected by the place that we installed it, and the reflections in the vehicle. DSP lets us manage the speaker’s response with equalization, so that it sounds better - in spite of the fact that we installed it in a car.
We can safely say that this one speaker won’t be able to play all the sounds we want to play to reproduce music. One speaker won’t play all the way from the upper treble all the way down to the lowest bass. We usually need speakers of different sizes (tweeters, midranges, woofers, subwoofers) when we want to do a good job of reproducing all these different notes.
Once we have one of these multi-element speaker systems - a two-way system or a three-way system or a four-way system - we’ve gone from one speaker to 2, or 3, or 4! The individual speaker drivers will interact with each other at any frequencies where they overlap - that is, wherever they both contribute to the sound. We want the result of this interaction to sound good, but any time you have two speakers playing the same note, they might cancel each other out to some degree (sometimes, to a great degree).
This can happen even if all the speaker drivers are installed into a cabinet (think, “home speaker”), but it’s even worse when the various speaker drivers are installed in different locations around the vehicle, at different distances from the listener. DSP helps us manage our speaker system so each speaker plays only the notes it should, and lets us ensure our multiple speakers play well together without bad cancellations.
On top of that, we are talking about car stereo. Stereo uses two channels, left and right, and when it’s set up properly we hear an illusion of musical performers between the speakers, in front of us. To do this, we need both left and right speakers in front of us, for every kind of driver we are using - tweeters, midranges, and woofers (subwoofers are exempt from this requirement). Suddenly, we need almost twice as many speakers as we needed in the previous paragraph! Again, whenever multiple speakers are playing the same sounds, they can interfere with each other, and stereo speakers play the same sounds left and right any time the performer is in the center of the stage - so that’s another big potential problem.
DSP helps us manage the left and right speakers so that everything is in alignment and works well together, and then the performer recorded in the center of the stage sounds the way they should.
Car sound systems often need to be louder, because there is a lot more background noise (and sometimes because that’s what the client prefers!) So, we often add rear speakers. Well, when we install rear speakers, we create another opportunity for speakers playing the same notes to interfere with each other! Now the rear speakers can interfere with the front speakers.
DSP helps us manage this potential problem - we can make the rears and fronts play nicely together, without ruining the sound - or the stereo illusion - up front.
So, DSP helps us get better sound from an individual speaker (with equalization). It helps us get more output and better sound when we use multiple speaker drivers to cover the audible range (with crossover filters, level controls, and cancellation management using delay and phase processing). It also helps us when we use left speakers and right speakers to play stereo sound, or to get louder by adding speakers in the rear without interfering with the front speakers (cancellation management tools again). While the biggest cancellation management tool we have is delay, it’s not the only tool we have for managing cancellations.
OEM system designers have the same access to DSP processing we do - maybe even more, since they can add it to their OEM head units or amplifiers in the design phase. If they decide to use equalization, crossover filters, delay, or phase manipulation, they’ve got the capability. It seems that most OEM head units have a basic DSP functionality today - we see basic head units with all those types of processing in many mid- and entry-level cars today.
The thing is, none of that processing is intended for our new speaker system. If there are crossover filters, they are probably not the crossover filters that would make our speakers sound great. If there is equalization, or delay, or phase manipulation, none of it was done for our speaker system (often, it’s a watered-down tune averaged for both front seats, rather than the driver-seat-optimized systems we often sell our customers). What’s worse, OEM sound processing often foils the techniques we plan to use to manage our new speaker system. If we don’t correct the OEM processing, we can run into great difficulty in getting the sound we expect from our new speakers. If we want to use delay, we need all the signals in phase with each other at all frequencies before delay will work the way we expect.
DSPs with OEM Integration capabilities have specific features intended to let us correct for OEM processing before we use DSP to manage our new speaker system. Not all DSPs have OEM Integration functionality, and most are pretty limited in this regard - but a DSP with good OEM Integration can make it much simpler to get good sound.
I thought that DSP was for high-end.
Depends on what you mean by “high-end”. If you mean, “we care enough about the resulting sound that we want it to be good”, then maybe it is for “high end”? But OEMs are using it to make base-level deck-and-four systems sound better - and get louder. It’s not just for “SQ” cars - it can make any speaker system sound significantly better!
DSP helps you get louder?
Well, when multiple speakers interact, they often lose output at various notes. Using a DSP to manage cancellations can result in a system that has more output across the board.
What do we do with a DSP to make stereo work in cars?
Basically, we make the left and right sides sound the same, and we manage the various speakers so they don’t interfere with each other too much. Once you’ve done that, stereo reproduction just happens - it really is a side effect of playing stereo recordings on systems that meet those two conditions.
Isn’t there a lot more involved?
Well, yes and no. There are indeed some things that experience helps us with, but making stereo work in a car is indeed making left and right match in level and frequency response, and putting the speakers in phase with each other using the cancellation management techniques listed above. That’s what makes stereo work in your living room. With OEM Integration jobs, the most important thing is that you start with a good signal.
Do you recommend a certain DSP processor?
Yes - the new AF Forza family of DSP amps have industry-leading OEM Integration features, very powerful equalizers, and a great set of system- and cancellation-management tools.
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