ZyXEL Communications X6004 User Manual page 228

Chapter 13 Authority Group
Table 68 Voice Codecs Supported (continued)
CODEC
G.726
G.729
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 X6004 does
not perform any video coding, it does support the passthrough of the following
video codecs.
Table 69 Video Codecs Supported
CODE
C
H.261
H.263
H.264
MP4
228
DESCRIPTION
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.
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.
DESCRIPTION
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networks and has since been superseded by more efficient video coding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.
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.
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.
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 X6004.
X6004 User's Guide