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The only place where I've encountered the term "impedance" is in relation to the hi-z and lo-z instrument cables that you use to connect eg. electric guitars to

The only place where I've encountered the term "impedance" is in relation to the hi-z and lo-z instrument cables that you use to connect eg. electric guitars to amplifiers. You'd sometimes see these hi-z to lo-z converters. Is this the same thing? The video's explanation would lead me to believe that impedance only applies to AC circuits. But instrument cables don't run AC, do they?

sparky 15y ago
Yes, the 'z' in hi-z and lo-z cables is the standard variable name for impedance (like 'v' for voltage or 'i' for current). 'AC' in this context means 'not DC', which is to say 'varies over time'. Instrument cables certainly qualify under that definition, though it is definitely confusing to call it 'alternating' (which we normally think implies 'goes back and forth between 2 states'), where perhaps 'varying' or 'time-varying' would be more appropriate.
tkok 15y ago
Audio signals are alternating. The voltage corresponds to the sound pressure picked up by the microphone, which typically looks more or less like a bunch of sinusoids.
- kwantam 15y ago
  It's not just typically a bunch of sinusoids: Fourier tells us that it is always the sum of a bunch of sinusoids. [1] http://en.wikipedia.org/wiki/Fourier_transform http://en.wikipedia.org/wiki/Fourier_transform
kwantam 15y ago
In the case you're talking about, impedance means "characteristic impedance", i.e., one parameter which (along with speed of light in the given medium and length) defines the behavior of a transmission line. [1] It is the same thing in the sense that mathematically both are a complex (i.e., Real plus Imaginary) relationship between voltage and current. However, a transmission line is a two-port network while a capacitor is a one-port network, so these two senses of impedance correspond to different physical phenomena. In a capacitor, the impedance defines the relationship between voltage and current at the port (i.e., between the two terminals of the cap; this is sometimes called the driving point impedance). For a transmission line, the characteristic impedance defines, among other things, the equivalent impedance seen at one end of the line when the line is either infinitely long or an integral number of wavelengths long and terminated at the other end by an impedance of the same value as the characteristic impedance. Instrument cables do indeed carry AC signals, inasmuch as the signal on the line corresponds to the frequency of the note(s) you're playing. AC is just "non-zero frequency"---any signal which is not a constant, unchanging (DC) value is considered AC. So, 50/60Hz line power is an AC signal, the 2.4GHz signals used by your 802.11 network card are AC, and middle C (261.6 Hz) is AC. [1] http://en.wikipedia.org/wiki/Characteristic_impedance http://en.wikipedia.org/wiki/Characteristic_impedance
- lcargill99 15y ago
  At audio frequencies, characteristic impedance is almost always irrelevant. hi-Z is a crystal mic, guitar pickup or crystal phono pickup source.lo-Z is a dynamic or condenser mic source. Obviously, a preamp may be used as an impedance converter to lower hi-Z to loZ. There's such a preamp in the condenser mic. The cables are hiZ or loZ only in that they're generally used with one source or the other. But that's by convention. loZ is usually a balanced XLR connection; hiZ is generally an unbalanced 1/4" connection. hiZ is generally at around 1 megohm. loZ is archetypally 600 ohm. Line is neither; it's 10,000 to 50 or 100k ohm.
HeyLaughingBoy 15y ago
Impedance applies to AC and DC circuits, but at DC the imaginary term is zero, so only resistance is present. However, the term "impedance" still has meaning. I think your question about AC on instrument cables stems from the popular use of the term, "AC" as in AC power. True, amplifier cables aren't carrying AC power but the signals are still considered AC since they are time-varying.
- mbell 15y ago
  In theory, sure. But in practice, true DC doesn't exist. All signals will have some components at frequencies other than DC and quite often in electronics design these components are the most important to consider, especially if your planning for your product to pass FCC part 15.

sparky 15y ago

Yes, the 'z' in hi-z and lo-z cables is the standard variable name for impedance (like 'v' for voltage or 'i' for current). 'AC' in this context means 'not DC', which is to say 'varies over time'. Instrument cables certainly qualify under that definition, though it is definitely confusing to call it 'alternating' (which we normally think implies 'goes back and forth between 2 states'), where perhaps 'varying' or 'time-varying' would be more appropriate.

tkok 15y ago

Audio signals are alternating. The voltage corresponds to the sound pressure picked up by the microphone, which typically looks more or less like a bunch of sinusoids.

kwantam 15y ago
It's not just typically a bunch of sinusoids: Fourier tells us that it is always the sum of a bunch of sinusoids. [1] http://en.wikipedia.org/wiki/Fourier_transform http://en.wikipedia.org/wiki/Fourier_transform

kwantam 15y ago

In the case you're talking about, impedance means "characteristic impedance", i.e., one parameter which (along with speed of light in the given medium and length) defines the behavior of a transmission line. [1] It is the same thing in the sense that mathematically both are a complex (i.e., Real plus Imaginary) relationship between voltage and current. However, a transmission line is a two-port network while a capacitor is a one-port network, so these two senses of impedance correspond to different physical phenomena. In a capacitor, the impedance defines the relationship between voltage and current at the port (i.e., between the two terminals of the cap; this is sometimes called the driving point impedance). For a transmission line, the characteristic impedance defines, among other things, the equivalent impedance seen at one end of the line when the line is either infinitely long or an integral number of wavelengths long and terminated at the other end by an impedance of the same value as the characteristic impedance. Instrument cables do indeed carry AC signals, inasmuch as the signal on the line corresponds to the frequency of the note(s) you're playing. AC is just "non-zero frequency"---any signal which is not a constant, unchanging (DC) value is considered AC. So, 50/60Hz line power is an AC signal, the 2.4GHz signals used by your 802.11 network card are AC, and middle C (261.6 Hz) is AC. [1] http://en.wikipedia.org/wiki/Characteristic_impedance http://en.wikipedia.org/wiki/Characteristic_impedance

lcargill99 15y ago
At audio frequencies, characteristic impedance is almost always irrelevant. hi-Z is a crystal mic, guitar pickup or crystal phono pickup source.lo-Z is a dynamic or condenser mic source. Obviously, a preamp may be used as an impedance converter to lower hi-Z to loZ. There's such a preamp in the condenser mic. The cables are hiZ or loZ only in that they're generally used with one source or the other. But that's by convention. loZ is usually a balanced XLR connection; hiZ is generally an unbalanced 1/4" connection. hiZ is generally at around 1 megohm. loZ is archetypally 600 ohm. Line is neither; it's 10,000 to 50 or 100k ohm.

HeyLaughingBoy 15y ago

Impedance applies to AC and DC circuits, but at DC the imaginary term is zero, so only resistance is present. However, the term "impedance" still has meaning. I think your question about AC on instrument cables stems from the popular use of the term, "AC" as in AC power. True, amplifier cables aren't carrying AC power but the signals are still considered AC since they are time-varying.

mbell 15y ago
In theory, sure. But in practice, true DC doesn't exist. All signals will have some components at frequencies other than DC and quite often in electronics design these components are the most important to consider, especially if your planning for your product to pass FCC part 15.