ZyXEL Communications X2002, User Manual

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Chapter 13 Authority Group
X2002 User’s Guide
224
Video Codecs
Video codecs are used by video phones to compress the amount of information
sent between two devices. Video codecs encode video signals into digital signals
and decode the digital signals back into video signals. Although the X2002 does
not perform any video coding, it does support the passthrough of the following
video codecs.
G.726 This is an Adaptive Differential Pulse Code Modulation (ADPCM) waveform
codec that uses a lower bitrate than standard PCM conversion. G.726
operates at 16, 24, 32 or 40 kbps. Differential (or Delta) PCM is similar to
PCM, but encodes the audio signal based on the difference between one
sample and a prediction based on previous samples, rather than encoding
the sample’s actual quantized value. Many thousands of samples are taken
each second, and the differences between consecutive samples are usually
quite small, so this saves space and reduces the bandwidth necessary.
G.729 This is an Analysis-by-Synthesis (AbS) hybrid waveform codec. It uses a filter
based on information about how the human vocal tract produces sounds. The
codec analyzes the incoming voice signal and attempts to synthesize it using
its list of voice elements. It tests the synthesized signal against the original
and, if it is acceptable, transmits details of the voice elements it used to
make the synthesis. Because the codec at the receiving end has the same
list, it can exactly recreate the synthesized audio signal.
G.729 provides good sound quality and reduces the required bandwidth to
8kbps.
Table 68 Voice Codecs Supported (continued)
CODEC DESCRIPTION
Table 69 Video Codecs Supported
CODE
C
DESCRIPTION
H.261 This is an ITU (International Telecommunication Union) video coding standard.
H.261 was designed in 1990 and is considered the first practical video coding
standard. The data rate of the coding algorithm is able to operate between 40
kbps and 2 Mbps. H.261 was targeted primarily to work over circuit-switched
networks and has since been superseded by more efficient video coding
standards.
H.263 This video codec is based closely on the H.261 standard, but as a general rule
requires half the bandwidth to achieve the same quality video. H.263 is capable
of streaming video at bandwidths as low as 20 kbps to 24 kbps.
H.264 This video codec is video compression technology that falls under the MPEG-4
standard, also known as MPEG-4 Part 10. H.264 can provide excellent video
quality at bandwidth from 40 Kbps upwards.
MP4 MPEG-4 is an audio/video compression technology developed by the Motion
Picture Experts Group. You need to enable MPEG-4 if you want SIP phones to
make video calls via the X2002.