-- techteach menu -- main menu --

File size, disk space, transmission


The size of a file matters, be it text, graphic, audio, video....../graphics/zebra_jpg_compression_rates.htm
The file takes space on the hard disk, and it takes time when you e-mail it or view it, or upload or download it to or from a website.
Many people nowadays use fast transfer but some people still use modem dial-on connection.
E-mail suppliers may detach big attachments.
The space on your web page may be limited by your server,

So,
Create small files -- see file_formats2.html
Zip your files, for instance, with Winzip -- see ../winzip/winzip.html
Compress your media files:
see graphics and zebra_jpg_compression_rates  for graphics
see Audio_on_your_web_page.html or Video_on_your_web_page.html  for audio or video.


Brief overview:

Text files: (based on the file file_formats2)
.txt -- text only,
no formatting, no graphics or other media
Created in or Saved as
Word, Notepad, Wordpad
Filesize:
5 K
.rtf -- formatted text, usable on any platform (PC, Mac, UNIX)
ditto
18 K
.doc -- formatted text
may include graphics, tables, other media
Word
26 K
.wps -- ditto
WordPerfect
9 K
.pdf -- format and layout, text with or without graphics
for viewing and printing
converted by
Acrobat
79 K


HTML documents -- for the Web: (based on the file file_formats)
Created in Netscape, or copied and pasted into Netscape
from text typed in Word
Filesize:
5 K
Created in Word and saved as .html
16 K


Images / still graphics:

Image size on the monitor/display depends on the display, its size and settings of your own and that of the user: e.g., a 640 x 480 image
VGA
640 x 480 pixels
see Control Panel, Display, tab Settings on your computer
 -- the user may see your image in a different size
full screen: Calculate
for different displays
SVGA
800 x 600 pixels

abt. 2/3 of the screen
SVGA
1024 x 768 picels

abt. 3/8 of the screen

Note:
Use your image editor to save your image in the desired size. Do not resize it in other software, or it will get corrupted.

Image file size:

This is different from image size(i.e., the size you can measure on your Display). Calculate image file size for say, an  640 c 480 image:  640 x 480 = 313.600.
But image file size also depends on image properties, such as bit depth reduction (e.g. Black and White for line drawings versus color with various options), and dpi setting (e.g., a low of 72 dpi for the web, and a much higher dpi for editing or printing).

So, the image file size should depend on your intention: for editing or printing or for the web.

The amount of bit-depth reduction is greater for graphics with few colors such as line drawings or illustrations. Photographic images require many colors so compression usually gives better results for transmission.  JPEG and GIF formats work well.  Select the compression rate you want. I find that 20% plus for photographic images produces an acceptable compromise between quality and file size.

Use uncompressed images for editing or for printing, but compressed images for e-mail or use on a web page.
Note: Image compression reduces file size and download time, but may also corrupt the image quality.

Image file sizes: (based on the file zebra_jpeg_compression_rates):

Format:
BMP
Image size
400 x 561
File size
657 KB
Compressed:
0 %
JPEG/JPG
144 KB 1 %
JPEG/JPG
16 KB 70 %


Audio:

Audio File Requirements in bytes per second:
number of samples (sampling frequency in Hz)
multiplied by Sample size (1=8-bit,2=16-bit; eg., 8-bit/sample divided by 8 bits per byte)
multiplied by Channels (1=mono, 2=stereo)

A good rule of thumb: Each minute of 16-bit stereo sound at 44.1 kHz requires about 10 Mbytes of disk space.
Thus, with an empty 200 Mbyte hard disk you can record a little less than 20 minutes of CD-quality stereo sound (eprecisely, 19 minutes, 20 seconds).

Example: I recorded 13 seconds of sound in the ITC corridor, 16 bit stereo 44,100 hz: This is a 2.5 M file, format .wav -- and it should  to be compressed. (For speech, you need only 8 bit mono but its file size would still be big and its sound quality would be so-so..)

Audio files compressed to other formats, such as rm or MP3, might be smaller, take less disk space and transmit faster.

Examples:

../../index.html sonic1ac.wav 155 KB
13 sec

sonic.rm (same as above)
65 KB
13 sec
greater bit depth
sonic1ac.mp3 (same as above)
225 KB
13 sec
../audio/heartsounds.html 00b10001.wav 62 KB 3 sec



Video:

640 x 480 pixels = 307,200 pixels per frame -- TV quality,
30 frames per second (fps).
307,200 x 24 bits x byte/8 bits = 921,600 bytes per frame.
921,600 x 30 frames per second = 27.648 MB -- 30 fps is desirable.

Reducing the frame size to 320x240 (one quarter size) is preferred for computer monitor display.

Reducing the frame rate to 10 frames per second may produce choppy quality and it leaves the data rate at 2.3 MByte/sec.
(This data rate is too high for old hard drives, old CD-ROM (2x drive ~ 200 Kbyte/sec sustained ) or modems (28.8 modem ~2.88 Kbyte/sec; 56.6 modem~5.66 Kbyte/sec).

Greater compression will knock this down further, but higher bit-depths make compressed video look better. Compromise!

video1.wmv
185 KB
12 sec
video2.mwv
87 KB
4 sec


Ursula Hoffmann, April 2005