Why Do you Need to Encode?
The technique of Streaming Media is made possible by reducing the data
that comprises each frame in video or sample in audio. For this discussion we
will concentrate on video, although similar techniques are also utilized
to prepare audio for streaming. Encoding is preparing/converting a file
for the purpose of streaming. To stream video, combinations of two
techniques are used:
- First, each frame is compared to the next frame. If no movement
is detected, then the unchanged pixel data is discarded. On playback,
the changed data is superimposed over the previous frame; the discarded
data in the new frame leaves a hole for the previous, unchanged image,
to show through.
- Next the remaining pixel data in the frame is compressed using a
"lossey" compression algorithm, CODEC, that mathematically
combines/discards pixels in the image that your eye does not perceive.
On playback, the CODEC mathematically decompresses the data and
restores the pixels. The target delivery rate and frames per second will
determine the amount of compression used; the images may be unchanged or
rendered in differing degrees of sharpness. The exact differences are
determined by the need to reduce the data required to render each frame
within the limit of the target delivery rate.
The most difficult task in the encoding process is balancing the fine
line between image/sound quality and the data required, the target delivery
rate or bandwidth, to reach a specific end user. The end user's
connection determines how much data they can receive on a continuous
Users can be divided into four target bandwidth groups:
- Slow Modem - 28.8k to 33.6k Modem connection Target Bandwidth: 22kbps
- Fast Modem - 56k, Flex56, K56 and v.90 Modem connection Target Bandwidth: 34-37kbps
- LAN - This is typically a user that is on a shared corporate connection Target Bandwidth: 50-100kbps
- Broadband - DSL, Cable Modem and dedicated T1 Target Bandwidth: 80-300kbps
Lower and higher target delivery rates can easily be supported but these
four will cover greater than 95% of the Internet audience. Each of these
segments can effectively receive Streaming Media by utilizing various
CODECs, audio/video sizes and settings.