PictureTel recently joined forces with Zydacron to develop the forthcoming PictureTel 550, a desktop videoconferencing system for Windows NT that can operate in H.320 or H.323 mode. This system is based on the Zydacron Z350 codec board, which I tested in a beta version. I was impressed by the Z350's single PCI board packaging, 22-to-30-frames-

per second (fps) video performance and ability to run smoothly with Windows NT 4.0. It also touts new support for H.323 functionality.

Typically, most desktop videoconferencing systems of late have offered support only for Windows95/98, which lets application developers tweak performance by working with memory and hardware directly. Zydacron's support for Windows NT provides a more stable environment than Windows95/98, as NT enforces communication through a HAL (Hardware Abstraction Layer).

New H.323 Personality The Z350 supports H.323 protocols for operation over a TCP/IP LAN and/ or WAN. (The Intel PS500 and PictureTel LiveLAN are the only other systems we've seen that offer comparable picture quality for use over enterprise networks.)

Using H.323, the Z350 naturally communicates best with other Z350-based systems. But if you plan to use Zydacron H.323 devices in a multivendor setting, interoperability will become a critical issue. I tested the Z350 with a Pentium II 266-MHz NT Workstation 4.0. As part of my test bed, I ran the Z350 against a PS500 in a Pentium II 233-MHz Windows95 PC across a 10-Mbps Ethernet network segment.

During testing, the Z350 stayed legally within the H.323 rules when it accepted or placed calls to the PS5000. However, I discovered some poor automatic selections. On the audio side, at least three algorithms--G.711, G.723, G.728--are available. A PS5000 always offers G.723 as first choice. This is preferred because G.723 consumes the least amount of bandwidth for audio. However, I observed the Z350 counterproposing G.728 or G.711. Zydacron claims that the final Z350 will be able to use G.723, but the beta software I used didn't select this option.

Another problem in H.323 implementation arose during bit-rate negotiations. To ensure an easy selection of call sizes for calls to H.320 systems through LAN-to-WAN gateways, the Z350 required that I choose from 384, 256, 192, 128 or 64 Kbps.

When the Z350 offered 384 Kbps, the PS500 used in my test bed did not adjust accordingly. As a result, both systems locked in at 128 Kbps. Zydacron said it was looking into this problem.

Where To Put the Codecs? Because audio and video compression are computational-intensive tasks, DVC manufacturers institute various strategies to differentiate themselves and their products, balancing videoconferencing performance and system price points. Zydacron builds digital signal processors for encode/decode (codec) directly onto its Z350 board. These processors convert NTSC or PAL video input from two software-selectable input channels. In this way, a relatively inexpensive 166-MHz PC can host the Z350, while running video at 22 to 30 fps.

Unfortunately, the board is expensive--it's priced at almost $1,600. Other H.323 DVC-based vendors offer products at lower prices by implementing most codec functions in software. But you'll have to supplement them with a desktop PC that has more horsepower.

Zydacron software supports far-end camera control and picture-in-picture (PIP) self view--features usually found only in more expensive videoconference room systems. If you need to join a boardroom conference from your desktop, for example, you can start the Z350 in H.320 mode and set it up to use a companion inverse-multiplexing ISA board that supports ISDN operation at speeds up to 384 Kbps.

When used in the desktop-to-desktop setting, Zydacron software offers handy GUI features that include drag-and-drop file transfer initiation and a customizable toolbar. Microsoft NetMeeting 2.1 is bundled with the software to support application sharing and whiteboard collaboration.

Dave Brown is an independent videoconferencing consultant. Send your comments on this article to him at Dave@dbec.com.