NEC 128 manual Video Processing, Audio, Data Transmission Rate, for Example

Technical Description 3-5

Video Processing

Video Processing is a complex process to reduce the amount of image data that must be processed on any given picture frame. If processing is not used, videoconference performance is reduced by conditions such as complex picture backgrounds, poor camera lighting conditions, and object movement. This ensures a H.320 system processes image objects in the picture that actually move. This lowers significantly the amount of picture information that must be transmitted to the far end (for each frame) and provides a higher quality picture.

Audio

Three options for H.320 define the audio requirements used for a videoconference:

•G.711 - for narrowband transmission (at 48-56 Kbps)

•G.722 - for wideband transmission (at 48-56 Kbps)

•G.728 - for narrowband transmission (at 16 Kbps)

G.711 is telephone quality audio (narrow band - 3 KHz), while G.722 is used for stereo quality audio (wideband - 7 KHz). G.728 offers the highest quality audio providing a narrow band 16 Kbps rate. This is necessary for low bit rate transmissions.

A manufacturer’s videoconferencing system that supports all three audio requirements has the greatest capability and flexibility to provide the highest quality video at all data rates.

for Example:

In a 128 Kbps videoconference connection (typically BRI), the G.722 (48-56 Kbps rate) or G.728 (16 Kbps rate) options would be employed. If audio quality is more important than video quality (48 Kbps for the audio and 80 Kbps for video), the system could use G.722.

If video quality is more important than audio quality, the system employs G.728. The audio uses 16 Kbps leaving 112 Kbps for the video.

Data Transmission Rate

Data Transmission Rates impact the perceived quality of the video picture. The higher the data rate the better the video quality. At T1 speeds (1,536 Kbps) the video quality will be best. However, many connections will not use this rate because of cost. In today’s systems, some users connect at the 768 Kbps rate as a trade-off between high quality video and cost. Most use the 384 Kbps rate. The 128 Kbps rate is becoming more widely used with the increased availability of ISDN service.

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