PolyCom's VSX 3000

Fine Tuning

To test the device's performance, technology editor Sean Doherty and I installed a pair of systems: one connected to a DSL broadband connection in my California office, the other in our Syracuse University Real-World Labs®. Setup was straightforward. I used the TV-style remote to navigate through the installation menus and enter data; alternately, you can configure the system using a Web browser or over telnet. By default, the system makes a DHCP request on boot.

Network configuration for a simple LAN environment was a breeze. However, Network Address Translation caused problems. The H.323 protocol used by the VSX 3000 isn't designed to work in a NAT environment--the packets it sends contain the sender's address in their payload. The VSX 3000 or a compatible router must replace the private address with a valid WAN IP. I set the IP manually, and the interface displayed my correct WAN IP once the network connection was established. When changing from PC display mode to videoconferencing mode, the image occasionally became corrupt, but cleared up after switching from one mode to the other a few times.

Not Picture-Perfect

Unfortunately, the device's camera left something to be desired. I encountered chromatic aberrations that were magnified by the device's block-based H.264 compression, which produced blotchy areas of color in the display.On the upside, the camera adapted well to various lighting. Under fluorescent lights, it could move away from the usual reddish hue and find the correct white point. Performance in dim conditions was also good, thanks to automatic backlight compensation.

Conferencing A to V

Call initiation was as simple as entering the target's IP address and pressing "call." The VSX 3000 did well under a range of test conditions, thanks to its ability to change codecs and data rates dynamically.

From the call-quality menu, you can set the desired outgoing call rate and receive rate. The system negotiates to its outgoing rate or the target's receive rate, whichever is lower. As a result, if a slower system receives a call from a faster one, the initial rate may exceed the slower system's outgoing preferences.Negotiated incoming/outgoing speeds synchronize, however, so even if one side is capable of high-speed transmissions, the connection will be limited to the slower rate. This affects only the max transmission speed, because the VSX 3000 modulates its bit rate and codec based on connection quality. Even if a connection is negotiated at 1,920 Kbps, the system will react to packet loss and lower the video quality, then reduce the audio quality. If more sophisticated compression is needed to take advantage of available bandwidth, the system can change encoders on the fly.

The new H.264 codec, used at rates of up to 768 Kbps, gave better "talking head" video than H.263. Also known as MPEG 4 Part 10 and AVC, H.264 offered performance on par with any encoder, but did a much better job of masking compression. This, in turn, let it transparently discard unimportant background detail.

The H.263 codec still kicked in on faster connections, enabling Polycom's Pro-Motion algorithm. Designed to transmit high-motion video over a fast connection, Pro-Motion retains both interlaced fields in a frame instead of discarding one. H.263 also can turn on at low bit rates, for error resiliency under high packet-loss situations.

Even under degraded conditions--during times of high network activity--the video stream was stable for normal conversation. Predictably, low bit rates ran into more problems with high motion, showing old frames bleeding through on some compression blocks and producing a mosaic effect over the foreground. After a few seconds, however, the image adjusted and gained more detail as the compressor learned about the scene.

Audio was by far the most remarkable component of the system. Even when video was distorted, there was no perceivable problem with the voice function. Low-bandwidth 32-Kbps audio streams retained a natural richness, with none of the tinny sounds sometimes associated with compression.

The integrated microphones easily picked up sound over a long conference table. In fact, during lulls in our test conference, the system captured conversations from the hallway and sounds coming through a window. Yet despite this sensitivity, signal noise was minimal to nonexistent at conversational volume levels.We experienced no problems with audio-video synchronization and sustained normal conversation despite periods of elevated latency.

Despite bandwidth limitations, Polycom's VSX 3000 performs remarkably well, especially considering it's small enough to fit on an office desk.

Michael Kuszczak-Bielecki is a network administrator and freelance technical writer in Hayward, Calif. Write to him at [email protected].