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Automotive rear fill “surround sound” with Boss DD-3

Lately I’ve been experimenting with rear fill in my car. Similar to matrix “surround sound”, but not a discrete multichannel system where sounds are meant to originate from the sides or rear; rear fill is intended to improve the front soundstage in a car which is normally pretty poor due to proximity to the left and right speakers as well as cabin reflections. Specifically this is implemented as a delayed, bandwidth limited, mono L-R difference signal for the rear speakers.
There is quite a bit of variance in implementation of this method, such as amount of delay, filter corner frequencies and Q, etc. After spending much time reading and not being confident picking some values, I decided to build a fully variable test setup.

To handle bandpass filtering and create a mono L-R difference signal I started with two op-amps to first buffer the left and right signals and then another op-amp follows as a differential amplifier to create the mono difference signal. This signal is then fed to a twin T cell notch filter with constant in-band gain but adjustable Q to provide variable bandwidth around a center frequency I chose, about 925Hz. The bandwidth of the filter is adjusted by feedback through a potentiometer without affecting gain so relative cutoff frequencies can be changed in real time. To turn the notch filter into a bandpass filter the output of the notch filter is compared to the input by a difference amplifier.

Probably the most important part of this is adding a delay. The effect relies on psychoacoustics to trick your brain into thinking you are in a bigger space since all reflections in a car are early reflections. Due to the speed of sound there just isn’t time in a car to produce late reflections which give your brain a sense of “space”. These reflections are different from echoes which the brain interprets as a separate event. The Haas effect (or precedence effect) defines that if a sound and its reflection arrive at the listener within a sufficiently short period of time the listener will perceive them as the same event. The period of time is different for different sounds, but is generally accepted to be between 20 and 30 milliseconds. The goal with rear fill is to add enough delay to simulate a larger room with later reflections but not produce an echo.

I bought a Boss DD-3 digital delay guitar pedal for this because it is perfect for the test. You can isolate just the delay signal, the delay is continuously adjustable, and one of the adjustment ranges is 12.5-50mS (which is something the MiniDSP and PT2399 can’t do). It has defeatable feedback so there will be no echo and it is digital so the signal should be accurately reproduced. It can also be instantly bypassed with the footswitch which is essential in identifying whether a change is better or worse.
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Tags: audio, Electronics