# Sound and Movies

## Sound Waves

Sound is produced by the vibration of an object in air or liquid

Sound moves very fast through waves of pressure (in air, 340 m/sec = 750 miles/hour)

The sound wave for a pure tone is characterized by its frequency (pitch) and amplitude (loudness)

Frequency is measured in Hz or cycles per second. Humans can hear frequencies between 20 Hz and 20,000 Hz (20 KHz). Amplitude is measured in deciBels.

Sound waves for a short music clip:

## Analog vs. Digital Representations of Sound

In an analog representation of sound, some physical property, such as voltage, varies continuously in frequency and amplitude:

An analog signal can be sampled to create a digital representation of sound:

In a digital representation, the signal is conveyed by a sequence of discrete numbers:

0   5   3   3   -4   -6   -2   2   ...

Two important factors in the sampling process affect sound quality and the file size needed to store audio signals:

• sampling rate: number of samples per second (Hz),
usually between 11 - 44.1 KHz
• bit-resolution: number of bits per sample,
usually between 8 - 32 bits

The choice of sampling rate and bit-resolution depend in part on the type of audio information being represented:

• speech in native language: 11 KHz, 8 bits
• CD quality music: 44.1 KHz, 16 bits

How frequently should we sample? The Nyquist Sampling Theorem says:

Sample twice as often as the highest frequency you want to capture

When converting from a digital representation back to analog (e.g. listening through a speaker), the quality of the reconstructed signal (dashed curves below) depends on the original sampling rate:

High sampling rate and bit-resolution yield high quality sound, but require large file sizes!

The bit-rate is the number of bits needed to store 1 second of audio:

bit rate (bits per second) = bit-resolution x sampling rate

file size (in bits) = bit rate x recording time

How many bits in 1 second of monophonic CD music?

How many bits in 1 second of stereo CD music?

How many bits in one hour of stereo CD music?

Check out this comparison of a sound clip at various resolutions and sampling rates.

Will we need to have such large file sizes for sound clips on our web pages? No, thanks to audio compression techniques, used in file formats such as Quicktime, AVI, RealAudio and MP3.

Any time signals are transmitted, there will be some degradation of quality:

1. signals may fade with time and distance
2. signals may get combined with interference from other sources (static)
3. signals may be chopped up or lost

When we continue to transmit and transform signals, the effect is compounded.

However, digital transmission and storage of information allows the possibility for creating perfect (undegraded) copies of the information, because we only need to distinguish 1's from 0's, and because errors in transmission can be detected and corrected.

## Movies

A movie is a sequence of images displayed at a fast pace

The size of a movie file is many times the size of a single image.

Suppose we have a 3-second movie that displays 15 frames per second, and each image frame has a width of 160 pixels, height of 120 pixels, and bit-depth of 2 (only 4 colors!)

Size of each image:

(160 x 120 pixels) x 2 bits/pixel = 38400 bits ≈ 4.8 KB

Total file size:

4.8 KB/frame x 15 frames/sec x 3 sec = 216 KB

And this does not include any sound!

Fortunately movie files can be compressed! The bouncing ball movie is only 103KB in the Quicktime movie format.

## Add Movies and Sound to Web Pages

Sound and movies can be added to web pages using the `<embed>` tag:

```<embed src = "ghost.mov"
width = "160"
height = "136"
controller = "true"
loop = "false"
autoplay = "false">
</embed>
```

Some attributes for the `<embed>` tag:

• `PluginsPage` directs the browser to visit a website containing the appropriate plug-in to handle Quicktime movies
• `height` and `width` control the size of the movie window that appears on the page (16 of the 136 pixels are occupied by the controller, so the real movie is 160 by 120 pixels)
• `controller` is `true` or `false`, and controls the visibility of the controller bar below the movie window (without the controller bar, you can double-click and click on the window to start and stop the movie)
• `loop` is `true` or `false`, and controls whether the movie restarts from the beginning after it plays
• `autoplay` is `true` or `false`, and controls whether the movie starts playing as soon as a sufficient portion has been loaded

```<embed src = "http://www.youtube.com/v/v7Nmpp94buo"
type = "application/x-shockwave-flash"
width = "255"
height = "210">
</embed>
```

Use the `<embed>` tag to add sound:

```<embed src = "friends.mov"
PluginsPage = "http://quicktime.apple.com"
height = "16"
width = "200"
controller = "true"
loop = "false"
autoplay = "false">
</embed>
```

Remember Mission Impossible?

```<embed src = "mission.mp3"
PluginsPage = "http://quicktime.apple.com"
height = "16"
width = "200"
controller = "true"
loop = "false"
autoplay = "false">
</embed>
```