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Recently, I read about some major musical concert held somewhere in the USA to a record-large audience.

Since there were so many people, not all could see or even hear the music from the main loudspeakers.

Therefore, they put additional loudspeakers far away from the scene, connected with cables from the main equipment at the stage (presumably).

Since sound travels slower in the air than electrical signals inside cables, the music coming out from the far-away loudspeakers would have been out of sync with the actual sound waves, unless they did this "clever trick".

The clever trick apparently consisted of them spending a huge amount of money ("the cost of a new car per loudspeaker") on then-cutting edge electronics which delayed the signal exactly so that it would match the sound waves, and thus not cause disharmony.

While this made me think of how clever that sounds, I don't understand why exactly this would be so costly, even if it was in the early 1970s. In fact, I don't understand why it had to be so technical at all. It seems like this could've been accomplished in some analogue manner, very cheaply. The cost of a new car, many times over? Really?

What made it cost so much money? And had nobody ever held a big enough concert before this relatively late date, which would warrant a similar solution?

Did they have concerts for many years which sounded bad for the people far away, because the real audio waves were mixed (unsynced) with the local loudspeakers?

And if they barely heard the music so far anyway, did it really matter at all? Or was this more of a way to sell tickets by claiming a "perfect hi-fi-quality experience for every single participant"?

(Sorry I can't remember the name of the concert; I thought I had it bookmarked.)

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    $\begingroup$ Engineers capable of designing that work for free? Or do you think they should be paid? $\endgroup$ – Solar Mike Nov 3 '20 at 6:42
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    $\begingroup$ State of the art means they had to create it, build it, test it, ship it, set it up, operate it and make a profit. All made from a bunch of discrete transistors. A computer was an air conditioned room. And a new car was a couple of thousand dollars. So it makes perfect sense. $\endgroup$ – StainlessSteelRat Nov 3 '20 at 11:20
  • $\begingroup$ The Wikipedia article on delay lines lists a bunch of patents on analogue delay technologies, but all of those patents would have expired by 1970, so there is a puzzle as to why cheap generic copies weren't becoming available. On the other hand, the problem might be mutually-conflicting specifications for getting just the right delay time, impedance-matching with the speaker, safety of the audience (trip hazards due to long cables, possible presence of mercury, ...), and quantity of materials used? $\endgroup$ – Daniel Hatton Nov 3 '20 at 12:26
  • $\begingroup$ On the other, other hand, there were geostationary telecommunications satellites in the 1970s, so it might have been cheaper to work out the round trip time for a signal sent back and forth to such a satellite, position the speakers to make sure the required delay would be an integer number of times that round trip time, then send the signal back and forth the required number of times. $\endgroup$ – Daniel Hatton Nov 3 '20 at 12:38
  • $\begingroup$ @DanielHatton maybe nobody wanted to buy them so they weren't being made? $\endgroup$ – user253751 Nov 3 '20 at 18:09
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The band was the Grateful Dead and the invention necessary was an analog delay line that was inserted between the main audio mix signal line and the power amps that drove the speaker arrays (which were co-located with the speakers themselves). Delays on order of a tenth to a half a millisecond were needed and the amount of delay had to be adjustable to within a hundredth of a millisecond and locked to that setting with less than one percent of drift on timescales of a quarter of a second.

In the early 70's this would require all-custom design as no off-the-shelf components were available, and digital means of processing audio signals did not exist. I do not know what the delay systems cost.

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  • $\begingroup$ The problem this was solving must have been something other than matching the delay in direct arrival of sound from the stage, though, right? In a large stadium, the latter would have been of the order of hundreds of milliseconds. $\endgroup$ – Daniel Hatton Nov 3 '20 at 20:29
  • $\begingroup$ It's odd that they didn't use a tape delay. \$ delay = \frac {distance\ between\ head}{tape\ speed} \$. At 15"/s (studio speed) = 375 mm/s you get 3 ms/mm of gap between the record head and the replay head. $\endgroup$ – Transistor Nov 3 '20 at 22:07
  • $\begingroup$ @DanielHatton, it was to exactly match the stage sound with the sound out in the crowd. They were playing for crowds so big that the remote speakers were hundreds of feet away from the stage speakers. $\endgroup$ – niels nielsen Nov 3 '20 at 22:18
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    $\begingroup$ That was my point. 'Hundreds of feet away from the stage', the sound coming directly from the stage would be delayed by something of the order of $100\,\mathsf{ms}$, so an electronic delay line that only produced a delay 'on order of a tenth to a half a millisecond' wouldn't be partcularly helpful in matching it. $\endgroup$ – Daniel Hatton Nov 3 '20 at 22:25
  • $\begingroup$ Eventide DDL 1745 Digital Delay $\endgroup$ – StainlessSteelRat Nov 4 '20 at 19:16

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