Sound is recorded by the decibel (volume) samples an object makes over time.

• Analog - Infinite resolution

• Digital - Fixed resolution

• Analog to Digital Conversion - Digitize

Types of sounds:

• Midi - device dependant (instrument sounds), small file size.

  • It contains a music score, like for a player piano.

  • It has a look table for an instrument's notes.

• Digital Audio - large file size, device independant

  • Digital Audio sounds is sampled.

  • 1kHz = 1000 Hz or 1000 samples per second.

  • Popular sample rates are 44.1 kHz (CD quality), 22 kHz, 11 KHz

  • Samples have a resolution typically of 8 or 16 bits per sample

  • As you can see at 1 or 2 bytes per sample, sound takes up lots of room.

  • If you record in stereo, then you need 2 channels for sound (left & right), so you do twice the samples ever sample taken.

  • Quantization is the rounding off of values to fit within the sample resolution.

Audio calculations for an uncompressed audio file:

sample rate x seconds length x (bit resolution/8) x channels of sound

10 seconds of 22 kHz audio in mono sound is:

22,000 x 10 x 8/8 x 1 = 220,000 bytes

Audio compression - generally lossy (you loose data)

• Reasons for compression

  • File size

  • Bandwidth

Computer monitors typically (unless low quality) do not interlace an image. TV displays odd and even lines of a frame. This was do to the fact the when early TVs drew the full frame at once, it would begin to fade from the phosphors on the screen. With even & odd scan lines, the fading wouldn't be as noticeable. These 2 sets are called fields. Odd & Even fields. Interlacing blends the two fields together. Since computer monitors are none interlaced, if you want your videos rendered on the computer for display on TV you need to do field rendering to digitally create interlacing of fields. Frame rate of 30 fps (actually 29.97 fps) is with 2 interlaced fields per frame. This gives a field rate of 60 (59.94) fields per second.

Most computers can't handle displaying full screen uncompressed video at 30 frames per second. A single frame of a 640 x 480 @ 24 bits in color depth takes up nearly 1 MB. At 30 fps, that's nearly 30 MB for 1 second of video. A 32x CD ROM, can only read 4.8 MB per second.

To see movie you must see rapid moving images on screen. This gives the illusion of moving objects. 30 fps (frames per second) is typical motion video (29.97 fps on the PC). However 15 fps is also adequate.

For uncompressed video, in 1 second of animation the formula for file
size is:

frames/second x image size x color depth (in bytes) = file size

So running 30 fps at (640 x 480) and 256 colors (8 bits = 1 byte)

30 x 307200 x 1 = 9,216,000 bytes or 9.216 MB for 1 second of animation

As you can see, video compression is greatly needed. A way of reducing the image file size is reducing the video size, color depth, and frame rate. 15 fps, at 320 x 240, at 256 colors is adequate.

Compression:

There are 2 types of compression.

• Lossless - preserves the exact image throughout the compression and decompression.

• Lossy - compression eliminates some of the date in the image provided greater compression ratios than lossless compression.

CODECs are used to enCODe / DECode a file. Without the same CODEC installed users can't play the files you compress.