When I plug headphones into an ipod, I know that electricity of some kind flows through the wire into the headphones, producing sound. What I don't understand is, what exactly is done to the electricity to make an audible signal?
I'm pretty sure the ipod controls the volume by changing the amplitude. If you put a variable resistor between the ipod and the headphones, you can reduce the volume. How does the ipod control the pitch of the sound? Does it change the voltage, or the frequency (turning it rapidly on and off), or what?
Without going through all the interim steps, the basic concept is that the voltage coming out of the iPod (at any given volume control setting) is an 'analog' of the original vibrations of the source of the music.
Let's say that you're listening to somebody who was whistling into a microphone, and let's also say that the person was whistling a pitch that was a pure and simple sound (aka a sinusoid), and let's also say that the whistling was vibrating the air (and microphone diaphram) 422 times per second.
The voltage coming out of your iPod will be vibrating at the same rate, that is at 422 times per second. When this vibrating voltage is then applied to your headphones (because they're plugged into your iPod) then the headphone's voice coil and diaphram will be shaking back and forth 422 times per second.
If you turn down the volume control on the iPod, or add resistance between the iPod and headphone's wires, then the magnitude of the voltage vibrations at your headphones will be reduced, but the rate at which the voltage is vibrating is still 422 times per second.
The key thing to note is that the 'shape' or 'pattern' that the vibrations take will always be similar, or an analog, of the vibrations of the original whistler's microphone's diaphram. The relative size of these vibrations will vary depending on the setting of the volume control, but the shape of the vibrations will still be similar or analagous to the original whistle.
So, in a nutshell, the iPod does not directly control the frequency of what is fed to your headphones. It only is changing the voltage to the headphones in an attempt to maintain a similar looking pattern/shape/analog between that which has been recorded and the voltage to the headphones.
If you want a more detailed explanation of how the entire process works (i.e. analog-to-digital conversion, digital storage, digital-to-analog conversion) then you'll need to do a lot of reading.
Bob
--
== All google group posts are automatically deleted due to spam ==
Saying that the voltage in an audio signal is "going on and off" is true in a sophomoric sense, but seriously misrepresents the important parts of what is happening.
A signal at 422Hz would be going up 422 times per second, and going down
This is kinda what BobW said, only hopefully simpler:
What is done to the electricity to make an audible signal is to make a voltage that varies the same way as the sound pressure needs to vary at the output of the speaker.
If I speak, I create sound waves that vary the pressure of the air around the ambient air pressure value. I can capture that speech with a microphone, which turns the sound pressure signal into an analog voltage signal (look up the definition of the word analog, as in analogy -- the voltage in an analog of the sound, in that it carries the same information in a different form). Now I can record that analog voltage to disk, and I can stick it on an iPod.
Later, the iPod will play this sound by turning the numbers in the file back into a voltage analog of the sound, then amplifying the voltage, then handing that voltage drive to the earphones, which generate sound pressure that varies in a way that is like the original sound that I made that you can (hopefully) understand the speech.
To see what a "vibrating voltage" looks like, you can use an oscilloscope. It draws a graph on its screen that shows the instantaneous voltage versus time. The voltage can go positive or negative (called AC for Alternating Current). If you connect a microphone to the scope and whistle into it, you will see a nice sine wave that goes through about 1000 of these alternations per second.
You can download my Daqarta app to use your sound card and microphone for this. When it starts, click the Input button to start looking at the input. Click the thin button under Input to open a dialog that allows you to select Mic In (in case it defaults to Line In or something) and adjust the input level (set it to 0... negative numbers are lower levels.) You can magnify the view with PgUp.
I'll be glad to answer questions. See the Contact page on the site.
Best regards,
Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis
formatting link
Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!
If you measure the voltage coming from a new 9V battery, its voltage over time will remain virtually steady at roughly 9V. Of course, if you measure long enough (20 years or so) you'll see this voltage drop to almost 0V.
If you measure the pressure of the air in a room that has no fans running, no music playing, no persons sneezing, then that air pressure will remain virtually steady over time.
When someone makes a noise (e.g. whistling) in that room, the air pressure will slightly increase and then descrease in a repeating pattern at a rate exactly that of what that person's whistle are vibrating at.
If you measure the voltage at your iPod headphones while the iPod is playing Whistle.mp3, the voltage you measure will smoothly vibrate between, let's say, +0.31V and -0.31V peak four hundred and twenty two times per second (assuming that the original whistler is whistling a pitch that is at 422 Hertz (vibrations per second)).
As Bob Masta recommended, you should download his wonderful Daqarta application and set it for its oscilloscope function, hook up a microphone to your computer, and make some noise and see the results. Try whistling, too, because you'll see a nice steady vibration (aka oscillation) of the resulting waveform.
Bob
--
== All google group posts are automatically deleted due to spam ==
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.