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I know what a delay is, but what's a delay line?
What's the reason for calling it a "delay line"?

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  • $\begingroup$ My guess is that it relates to the historical use of electrical or acoustic propagation time down a physical object, such as a wire "line" or pipe "line", for time delaying signals by a deterministic and known amount. $\endgroup$ – hotpaw2 Dec 23 '15 at 19:47
  • $\begingroup$ I wonder if using the term "delay line" is a bit of a misnormer in digital implementations. en.wikipedia.org/wiki/Digital_delay_line, but is there a line in the digital implementation? $\endgroup$ – mavavilj Dec 23 '15 at 20:02
  • $\begingroup$ $z^{-N}$ ...... $\endgroup$ – robert bristow-johnson Dec 24 '15 at 6:18
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    $\begingroup$ hotpaw2: I agree. I once worked a commercial radio station and they had a rack-mounted device containing a stainless steel spring about 5" in length. They would apply a rotational torque to one end of the spring at audio frequencies. The vibration wave traveled down the spring and was sensed by a piezoelectric sensor at the far end. Adding the sensor output voltage to the original audio signal produced a desirable echo. They called that device "the delay line." $\endgroup$ – Richard Lyons Dec 25 '15 at 11:53
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The term delay line comes from (analog) telecommunications where timing between different signals needed to be adjusted by delaying one of them. For example, the chrominance and luminance channels in a cathode-ray-tube colour television.

The reason it's called a line is because that's all it really is: a wire. The website linked to above has the following diagram of one.

enter image description here

In the discrete (digital) domain, I've more usually heard delay lines called tapped delay lines. That's because each sample is a delay, and to form an FIR filter, we need access (to tap into) the signal at different delays. The line of $z^{-1}$ boxes in the diagram below is what I would call a tapped delay line.

enter image description here

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To substantiate hotpaw2's comment, I once worked a commercial radio station and they had a rack-mounted device containing a stainless steel spring about 5" in length. By way of electronics they would apply a back-n-forth rotational torque, at audio frequencies, to one end of the spring. The vibrating wave traveled down the spring and was sensed by a piezoelectric sensor at the far end. Adding the sensor output voltage to the original audio signal produced a desirable echo. The disc jockeys referred to that device "the delay line."

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