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RealPresence Collaboration Server (RMX) 1500/1800/2000/4000 Administrator’s Guide
Polycom®, Inc. 18
● The HD_THRESHOLD_BITRATE flag must be set in the System Configuration. The value of this flag
is the system minimum threshold bit rate for HD resolutions. The line rate selected in the conference
Profile must be the same as or higher than that specified by the HD_THRESHOLD_BITRATE flag.
The HD_THRESHOLD_BITRATE flag line rate value ranges from 384kbps to 4Mbs, default is
768kbps. For more information, see Modifying System Flags.
AVC Basic Conferencing Parameters
The main parameters that define the quality of an AVC-based video conference and its display are:
● Line (Bit) Rate - The transfer rate of video and audio streams. The higher the line (bit) rate, the better
the video quality. The MCU supports the following line rates:
CP Conferences: 64kbps to
4096kbps (RealPresence Collaboration Server 1500/2000/4000)
(RealPresence Collaboration Server 1800 supports up to 6144kpbs)
VSW Conferences: 64kbps to
6144kbps (RealPresence Collaboration Server 1500/2000/4000 only)
● Audio Algorithm - The audio compression algorithm determines the quality of the conference audio.
● Video protocol, video format, frame rate, annexes, and interlaced video mode - These
parameters define the quality of the video images. The Collaboration Server will send video at the
best possible resolution supported by endpoints regardless of the resolution received from the
endpoints.
When Sharpness is selected as the Video Quality setting in the Conference Profile, the
Collaboration Server will send 4CIF (H.263) at 15fps instead of CIF (H.264) at 30fps.
H.264 High Profile protocol provides better compression of video images in line rates lower than
384 Kbps and it will be automatically selected for the endpoint if it supports H.264 High Profile. If
the endpoint does not support H.264 High Profile, the Collaboration Server will try H.264 Base
Profile which provides good compression of video images in line rates lower than 384 Kbps (better
than H.263 and not as good as H.264 High Profile).
When working with Collaboration Servers at low bit rates (128, 256, or 384Kbps), HDX endpoints
will transmit SD15 resolution instead of 2CIF resolution.
When using a full screen (1x1) conference layout, the Collaboration Server transmits the same
resolution it receives from the endpoint.
● Video resolution:
H.261 CIF/QCIF – Supported in Continuous Presence (CP) conferences at resolutions of 288 x
352 pixels (CIF) and 144 x 176 pixels (QCIF). Both resolutions are supported at frame rates of up
to 30 frames per second.
H.263 4CIF - A high video resolution available to H.263 endpoints that do not support H.264. It is
only supported for conferences in which the video quality is set to sharpness and for lines rates
of 384kbps to 1920kbps.
Standard Definition (SD) - A high quality video protocol which uses the H.264 and H.264 High
Profile video algorithms. It enables compliant endpoints to connect to Continuous Presence
conferences at resolutions of 720 x 576 pixels for PAL systems and 720 x 480 pixels for NTSC
systems.
The HD_THRESHOLD_BIT RATE flag is responsible for negotiation only, It does not guarantee that
the endpoint will open an HD channel or transmit on an opened HD channel.