Its amazing how much difference .5ms can make in stereo imaging. Any decent home audio receiver has the ability to set the distance of each speaker individually so that you can compensate for physical placement constraints.
If you get a stereo pair perfectly locked in on delay to eardrums, you can produce an extremely compelling listening experience for those in a very specific region of the room.
Finding out about all this can be revolutionary for some music enthusiasts. Once you get a good listening setup (or headphones), you start going through old things to see how the "stage presence" sounds, or if you are now able to physically place each instrument in the virtual space.
My dad bought an Isuzu Impulse in 1985. It had a fancy OEM stereo system with Technics branding, and a button for the driver to toggle the imaging. The button was on the 7-band graphic EQ that game with the car. (I don't think I've ever seen another factory stereo with physical EQ sliders like that.)
It was magic, if you were sitting in the driver's seat. Toggling that button made the sound switch from "okay" to "magically spatial."
I'm pretty sure that lead to a conversation about how we locate sounds. Also wonder how it was implemented in 1985. I doubt there were DSP chips in there. What's the "simple" way of adding delay to some speakers?
> I'm pretty sure that lead to a conversation about how we locate sounds.
So, psychoacoustics is incredibly complicated. There's something like 13 different mechanisms that co-operate in sound localization.
However, the bulk of it was known quite a bit before 1985, and had nothing to do with the "spatialize me" button on a specific car stereo.
There's no simple way to add a delay to some speakers unless you're working in the digital domain. In analog you have two basic choices. With passive components, you build a ladder filter, which is as the name suggests, just a long chain of low pass or all pass filters. Each "rung" only adds group delay on the scale of a couple usec, so these get very big and expensive fast. They also suffer from accumulated imprecision issues. With active components you can create a feedback loop through an op amp. This is how guitar delay pedals work, but the more delay you have the more distortion you introduce.
Technically there's a 3rd way: extremely long wires, but that's basically never practical.
Thankfully these days everything starts out in the digital domain, so you just need a controllable fifo before the DAC. Entry level home theater receivers have had this since circa 2000.
Edit: re: extremely long wires, this is how some of the physical layer testing is done for networking equipment. Have rolls of hundreds of miles of fiber sitting on the ground to simulate large distances between switches.
> Technically there's a 3rd way: extremely long wires, but that's basically never practical.
Did the math, ~170km (assuming 300,000 km/s and speed of light in copper been 90%) - that's a long wire, would also have to be superconducting or very high voltage ;).
Whatever room temperature superconductors end up costing Audiophiles will be the early adopters ;).
Hell, even if the premise of the conversation was wrong, I still learned something that day :)
DSPs were a thing in the 80s, right? I guess the question is: were they so expensive that it was unlikely to find one in an OEM stereo of a mid-priced car? A sibling comment mentions that this button may have just been a stereo expando kind of thing, rather than localizing the sound stage through signal delays. I'm thinking that may be right, and that I'm misremembering the feature. It would be awesome to find some original owners manuals for the 1985 Impulse that have any mention of this feature. My DDG-fu is failing me right now.
Now, which one of those 13 mechanisms is failing on me when the damn cricket keeps "moving" around the room as I try to follow the chirp?
DSPs were around, but there's no way that car stereo was digitizing the signal, delaying it, then converting it back to analog.
I get it was an impressive experience, but it's essentially certain it's what the other poster said: just boosting the out of phase content between the channels. This was a very in vogue effect at the time. I remember listening to the top 40 on the radio one time and Madonna had some new song where they were hyping it as surround sound and turning it into a whole event. In any case this effect can be more effective than you might assume. After all, the first consumer version of Dolby Surround was just this out of phase content run through a bandpass filter and sent to surround speakers.
I knew a family friend with the 90s version of the Impulse. Neat quirky car from what I remember. As a kid I definitely thought it was very cool.
They way they work by design, clock noise needs to be filtered out of the final signal, so relatively heavy low pass filtering is standard. The result isn't very hi-fi.
At that time, the most common way to do stereo enhancement was to decrease the L + R component of the stereo signal. [0] L+R/L-R was (and still is) a common way to encode stereo signals, including for FM radio. [1]
The impact on the stereo field by just changing the mix of these two components is profound. No signal delay needed.
For signals that are simple L and R, sum them to get L+R and difference to get L-R. So you can use this technique on any stereo source.
I may be misremembering—I was a kid at the time—but I distinctly remember the "wow" effect only working if you were sitting in the driver's seat. To me that suggests that the signals were delayed to the nearer speakers to center the sound stage around the driver rather than some arbitrary point above the center console.
But looking up photos online, I see the button I was talking about labeled as "Ambience", which kind of suggests the method you're describing.
It also turns out that using the Internet to find technical info about a 40-year-old car that was never popular to begin with is very hard!
The simple way is to add some cross-channel antiphase to both channels. So R = (R - cL), and L = (L - cR) where c << 1.
This is very cheap and easy with opamps.
It does indeed sound magical, and makes the stereo image expand beyond the speakers.
It also does weird things to a mono mixdown and if c is too big you get a hole in the middle. But if you keep c small and use it as the final effect just before the speakers that's not a problem.
You could also use BBD[1] chips to add a ms or so of analog delay, but that's less likely because it would have been more complex and expensive.
Digital audio delays had appeared in studios by the late 1970s, but they were still more expensive than BBDs in the mid-80s, so unlikely for in-car use.
I built a pair of full-range speakers and was therefore introduced to that effect — an effect I had only experienced before with headphones.
No special modern receiver in my case, just simple speakers. It seems the 2-way, 3-way speakers I grew up with kill stereo imaging. (Never mind the crossovers eat power and diminish the efficiency of the speaker — requiring a higher current amp, etc... Lovely what a small ½ Watt tube amp and a pair of full-range drivers can sound like ... and throw in a sub.)
> it seems the 2-way, 3-way speakers I grew up with kill stereo imaging.
Yeah there are ways to build crossover networks that can minimize these issues (phase shift) across the frequency range. The most ideal crossover would dissipate 100% of the undesired acoustic power as heat rather than storing it as reactive energy in inductors, but the frequency domain be a tricky beast to dance with.
The best overall approach is probably the 4th order Linkwitz-Riley filter:
This is why both sets of speaker cables to my amplifier are the same length.
It does make me curious about the recording process and effects if one microphone has lets say 50 feet of cable and another on a different musician has 100 feet of cable.
It's easy to do the math and show this is nonsense.
If a foot is a nanosecond for c, then 50ft difference is 50ns or about 10000x times smaller than the smallest difference. Even if the speed in a cable was 1%c, it's still under the proposed threshold.
Even so, try convincing my dad that it does not matter.
Thinking it's the cable length is a decent hypothesis, but it's almost completely due to the different times it takes for the sound waves to travel from the speaker cone to your ears.
But maybe you meant that the cable lengths helped place the speakers symmetrically.
If you get a stereo pair perfectly locked in on delay to eardrums, you can produce an extremely compelling listening experience for those in a very specific region of the room.
Finding out about all this can be revolutionary for some music enthusiasts. Once you get a good listening setup (or headphones), you start going through old things to see how the "stage presence" sounds, or if you are now able to physically place each instrument in the virtual space.