Analog sound wave (http://www.macinstruct.com/contest/mp3/)
Table of contents:
Introduction | Understanding Sound
| Analog vs. Digital Sound | Analog Sound
| Digital Sound | Digital vs. Analog |
Assignments | Additional References
| On to Lesson 2.2
According to our text, "Sound is the most sensual element of multimedia." It can provide the narration for a graphics or video, background music, or it can enhance special effects. Music, especially, causes people to feel emotion, either positive or negative. Hey, when we're watching a movie, we know who the bad guy is, just from the music--right?
Hi-fi, stereo, split-channel, digital, analog, CD-quality: these are some of the sound terms we have become familiar with in our technological age. We have now gotten used to the fact that computers can record, store, and play back any sounds, especially voices and music. Humans are inherently sound-oriented creatures. Again note those movie sounds tracks--even in the era of silent movies, theaters employed a live musician to play in the pit along with the movie. They knew that people respond to sounds. And just look at what movies include now--foley editors make their living playing with things that make sounds, and adding them to movie soundtracks.
So although sound does not have to be part of multimedia, I feel that judicious use of sound might just make or break your project. In this unit we learn about analog and digital sound, sound files for the Web, issues with audio on the Web, recording and integrating sounds into projects, and helper applications that you might choose to use for your project to make the most out of the sounds you add.
To understand the use of sound in multimedia applications, it's helpful to understand sound. When we speak, or when an instrument plays, vibrations called sound waves are created. These vibrations travel through the air at 750 mph (at sea level) to our ears, or to a microphone, or to any number of objects capable of transforming those sound waves into meaningful information. Sound waves have the characteristic of pressure, which is measured in decibels. Sound waves have a recurring pattern that is called an analog wave pattern. There's a sample analog wave pattern in the Analog section below. The wave pattern has two attributes, volume and frequency, that will need to be considered as you digitize sound for your multimedia project.
The height of each peak in the wave relates to its volume. That is, the higher the peak, the louder the sound. Volume is sometimes referred to as amplitude. The distance between the peaks is the frequency, also known as pitch. The greater the distance between the waves, the lower the sound. Frequency is measured in hertz (Hz). If a pattern recurs every second, then its frequency is 1 Hz; if 1000 times per secons, then its frequency is 1000 Hz or 1 KHz (kilohertz).
Acoustics is the physics of sound.
Before you begin this Unit, please read Chapter 9 in your text.
Analog sound is the sound you get from vinyl records or, these days, cassette tapes. Because analog sound has a smooth waveform, the sound can be very good (depending on the medium on which it is captured). Analog has a continuously variable signal. A sound wave traveling through the air is analog--it varies continuously in amplitude. Recorded analog sound results when a pressure wave hits a microphone. The microphone contains a vibrating membrane. The vibrating membrane pushes a wire through a magnetic field, which produces an electrical current. This current will vary in amplitude depending on the strength of the original sound wave.
Analog sound wave (http://www.macinstruct.com/contest/mp3/)
Thomas Edison is the creator of recorded analog sound. Edison was working on a machine that would transcribe telegraphic messages through indentations on paper tape, which could later be sent over the telegraph repeatedly. This development led Edison to speculate that a telephone message could also be recorded in a similar fashion. He experimented with a diaphragm which had an embossing point and was held against rapidly-moving paraffin paper. The speaking vibrations made indentations in the paper. Edison later changed the paper to a metal cylinder with tin foil wrapped around it. The machine had two diaphragm-and-needle units, one for recording, and one for playback. When one would speak into a mouthpiece, the sound vibrations would be indented onto the cylinder by the recording needle in a vertical (or hill and dale) groove pattern. (http://memory.loc.gov/ammem/edhtml/edcyldr.html) This is much the same way phonograph records were later created.
These days, the analog media used most frequently are cassette tapes.
Waveform audio is a representation of digital sound. Music, voice and sound can all be recorded as waveforms. To digitally record sound, samples of the sound are collected at precise intervals and stored as numeric data. In other words, digital sound is sampled sound.
Click here to hear sound (this 2 seconds of sound=25KB of storage) What this graphic illustrates best is the amplitude (height) of the sound waves as measured digitally, vs. an analog waveform. The frequency is too high to be shown on this graph, although in any sound editor you could actually display the wave itself.
The sampling rate refers to the number of times per second that the sound is measured during the recording process. The more samples per second, the more the digitized sound is like the original...and the larger the sound file grows. The height of each sample can be saved either as an 8-bit number for radio-quality recordings, or as a 16-bit number for hi-fi recordings. Commercial audio CDs are recorded at a sampling rate of 44.1 KHz, which means that a sample of the sound is taken 44,100 times per second. There are 16 bits per sample. To achieve stereo effects, you must take two of these 16-bit samples. Therefore, each sample requires 32 bits of storage space. When you sample stereo CD-quality music, you can store only 8 seconds of music on a 1.44 MB floppy disk.
Applications that do not require such high quality sound use much lower sampling rates. For example, voice is often recorded with a sampling rate of 11 KHz or 11,000 samples per second. This results in lower quality sound, but the file is only 1/4 the size of sound recorded at stereo CD quality.
We'll talk about audio file formats in Lesson 2.2.
There is no question but that digital sound is not of the same quality as a perfect analog recording. That means that, if conditions are perfect, Mozart will sound better on vinyl than on audio CD. Purists will tell you so, very loudly. However, conditions are rarely perfect, and a vinyl record begins to degrade the first time you play it, even on a good turntable with a new needle.
Digital versus analog behavior (back to top)
Analog sample (red line) with digital overlay (blue line) (from
Howstuffworks)
You can see from the graph above that the analog sound wave is smoother than the digital sound wave. If one were willing to "spend" the storage space, one could smooth the digital wave out much more. But is it worth it? Most people would say no, especially if they are playing the sound on a computer, which is, of course, a digital player anyway.
Bottom line: Why digital? (back to top)
When you have completed and emailed these assignments, please go on to Lesson 2.2.
Digitizing Sound and the MP3 Format
How Stuff Works - Analog Waves