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Even though the audio output of the mobile phone (which we draw through 3.5 mm jack) outputs a signal that's a function of time, at a very short interval of time, this source acts as a dc source of constant magnitude.

During this interval, does this audio source act as a constant current source or a constant voltage source?

Also please mention RMS value (for sine wave) of this signal.

brhans
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Cinverse
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  • is this a question in a test? – jsotola Aug 27 '23 at 06:10
  • if the source is constant, then how would music be transmitted? – jsotola Aug 27 '23 at 06:12
  • @jsotola No, this is not for test, and I didn't mean that audio port provides constant signal. What I meant is, since the signal is a function of time, at every instant (or for a very small interval of time), we get a fairly constant amplitude of signal (constant voltage or current value). I'm asking for the behaviour of the source during this instant. – Cinverse Aug 27 '23 at 06:24
  • then say that in your question ... please use the edit button and update your question – jsotola Aug 27 '23 at 06:27
  • @jsotola okay, thanks – Cinverse Aug 27 '23 at 06:27

1 Answers1

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The 3.5mm output on a phone is an audio output, so the properties are generic to audio, and not that much relevant that the audio output is on a phone in general, but some details are different because it's a phone.

So, audio outputs are voltage signals, not current signals. But the voltage is still respective to load, because the outputs are not perfect voltage sources, and they have some non-zero output impedance, and they are driving loads with some input impedance.

Typical line output would have output impedance say less than 1k and it would be feedig a typical input with 10k or much higher input impedance. So voltage bridging is used, where the point is that most of the voltage that is sent is detected on the input, so there is not much waste of power.

But since it is a phone, it must be capable of driving low impedance headphones. The phone may be able to detect and switch the output properties between high impedance line input mode and headphone mode.

There is no specs what the RMS value is and you as the user have the ability to adjust the volume anyway which changes the RMS voltage.

The only quick reference is from Android specs, which require that you must have at least 150mV or more on the output when driving 32 ohm headphones.

So the RMS voltage will vary based on the load you put there and the volume setting, so expect a range of values and design the load you are going to present to the phone accordingly.

In general, there is no standard for what voltages are on audio outputs. Before anyone says that there are stanadars such as "consumer" and "professional" line level standards, they do not really define anything about the signal to be usable as they do not define the conditions of the measurements. Many manufacturers of audio gear may use an internal standard for consistency, which could be something along these lines: Playing back a specified digital signal such as a sine wave with specified frequency and digital amplitude, with user settable volume set to specified level, shall output some specified analog voltage into some specified load resistance, with some specified level of tolerance.

Justme
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  • Is this source (audio output of smartphone through 3.5mm jack) called a line out? https://en.m.wikipedia.org/wiki/Line_out_(signal) – Cinverse Aug 27 '23 at 06:56
  • @Cinverse I already explained it is a headphone output as most often you use headphones on your phone, but it can function as line output as well, and the phone may be able to detect and switch how it drives the output depending on what load is detected. But since it includes the mic interface, Android specs call it a Headset Jack. Line outputs are poor driving headphones and headphone outputs are poor driving line inputs. – Justme Aug 27 '23 at 07:00
  • okay, thanks for clarification – Cinverse Aug 27 '23 at 07:48