How can sound be created by only using the amplitude of a wave form (such as sound-on-film)?


In this picture you can see the waveform of the audio track on film, but how in the world can you create sound from just a amplitude?:

Picture: [](
i understand how sound can be reproduced like on speakers and such, Does a image of a wave form ACTUALLY contain audio data?

In: 6

Remember the film is going by a set frame-rate, that’s the missing dimension. If you play it too slow or fast the pitch will shift, but since it’s made with an assumed framerate you don’t need more information encoded.

In the case of that sound-on-film it was down to using a light source modulated by those images to reproduce the sound.

Yes. Sound is local pressure, and you can only ever have one local pressure at a time. Sound can also have direction information, but we don’t usually encode that any more than just having two channels.

I wouldn’t exactly say that the image of a wave “contains” audio data. It might be more accurate to say that it “represents” audio data. But that’s just semantics.

When sound hits a microphone it creates a current/voltage signal. Accurately amplified and send to a speaker, the process is reversed. That signal is 2-dimensional: It has an amplitude that changes with time. We call that a waveform.

Anything that can accurately record that waveform can be used to re-create it. On old movie film, they used images like the one you posted. As it runs through the projector, it’s translated back to a voltage and can be amplified and sent to speakers for reproduction.

In modern digital systems, it’s converted into a series of numbers instead. One after another, very rapidly, they can also describe that waveform.

If you look at a speaker making sound ….all it’s doing is moving back and forth. That’s exactly what the waveform on the film is doing. So yes all you need is amplitude that varies over time to store sound information.

In terms of how it actually works…there is a photosensitive “eye” that can tell how wide the waves are on the film. It converts that width into a voltage signal that changes as the width of the waveform changes.

That voltage gets sent to an amplifier that creates a new, much more intense version of that voltage, strong enough to cause the cones in the speakers to move.

The word “image” just means something like “impression.” Like if I press a steel die into wax it creates an image of the die in the wax.

Same goes if I ‘press’ photons onto photo-film or a CCD in a digital camera. The ‘image’ file is just an impression of the energy that created it.

Same thing goes for audio, the data is just a series of pressure-frequency readings over time. You can take the time dimension and project it in space on the horizontal axis, and do the same vertically with the pressure dimension, and in this way you display the ‘image’ of the sound as a picture. You can also take the ‘image’ of the sound and play it back as audio through your speakers.

The picture you see on screen was created in this way, and although the file behind is isn’t considered a traditional ‘audio file,’ it still contains the data that the waveform displays. You could in theory recreate it by analyzing the graph and playing it from your speakers. Its just a much more roundabout way to store the data as a picture to be analyzed than it is to use a specialized audio storage format.

Converting to a visual file format could also cause data loss, however if there are parts of the waveform too small to be represented at the given resolution. This can be worked around by making the file’s resolution larger.