Testing the Vitec MGW Ace and MGW D265 for HEVC Encode/Decode
Like the Ace, the MGW D265 offers both HD-SDI and SD-SDI outputs via a single BNC connector, as well as an HDMI output with embedded audio.
The MGW D265 isn’t limited to receiving HEVC streams from the Ace, and its ability to receive and decode H.264 streams means that it can be used as part of an IPTV solution.
Unlike the Ace, the D265 has no front panel connectors. It also has no rear panel USB or SD card slot, meaning that the decoder itself cannot record the stream it is receiving. This is both a practical and security measure; it’s practical in that—as we note below, when covering error correction—not all the encoded data makes it to the decoder, and secure in the fact that stream recording can only occur at an encoder rather than remotely.
The Ace and D265 share similar physical dimensions, at roughly 2.5" high and just less than 8" wide. The units are also only about 8" deep, meaning they can be used in tight confines that a server-based encoder can’t be used. In addition, the wide-voltage DC connector can handle from 20 volts to 50 volts DC, making it ideal for field use.
Correcting the Imperfect Stream
What about receiving streams on an imperfect network? It turns out the D265 can handle that too, with the included Pro-MPEG and Zixi technologies offering unidirectional or bidirectional error correction protocols.
The intent of both Pro-MPEG and Zixi forward error correction (FEC) is to offer a way for acquisition from the Ace in less than ideal network situations. Both of the FEC offerings allow—within particular parameters—for artifact-free delivery over public networks.
“ProMPEG is an industry standard that requires upstream connection only as it includes redundant info,” says Garten. “But it has many disadvantages including no more than 5 percent packet loss protection and bandwidth overhead even when network is clean.”
Garten says ProMPEG has an established track record, and many studios use it for their inbound streams (IRDs) so support of ProMPEG was a business decision.
Zixi, on the other hand, is a full duplex protocol with an ability to protect up to 30 percent of the stream.
“In addition, it has a very efficient approach that doesn’t send redundant data if there is no loss or need for it to be sent,” says Garten.
Zixi also works over transport stream (TS) which fits into broadcasters workflows. In addition, it has strong security and encryption, versus the ProMPEG “in the clear” approach.
Vitec supports both ProMPEG and Zixi at no additional charge, baking it in to the ACE and D265 products.
Controlling the Encoding Session
MGW Ace and MGW D265 both use a web interface to access encoding and status settings. For the Ace, the interface is accessed by connecting to a portion of the network that either of the two Gigabit Ethernet ports are on. Encoding sessions can be scheduled, using either custom settings or presets, and the schedule can also be overridden by a manual start-stop process. Like most solutions for real-time encoding, parameters or presets cannot be changed or created during an encoding session.
As standalone devices, both the MGW Ace encoder and the MEG D265 decoder have strong performance capabilities. They can handle multiple input and output types, they offer both H.264 and HEVC encoding and decoding, and they have the ability to work with other standards-compliant encoders and decoders.
Together, though, the combination of an HEVC hardware encoder and decoder, coupled with two forward error correction options—the Pro-MPEG and Zixi technologies—offers a very compelling solution. In our demos for the review, it was clear that the HEVC encodes, even at lower bitrates, offered at least a visible difference over the real-time H.264 encodes of the same content. Yet the versatility of the Ace and D265, including their ability to encode or decode at H.264 data rates, means that customers can purchase these two units today for H.264 needs, with the understanding that they’ll be able to use the units for H.265 needs tomorrow.
Vitec chose to stick with a single HEVC codec, but tweaked the parameters at lower bitrates. It was able to do so by sticking with 1080p rather than jumping on to the 4K bandwagon prematurely. We have been impressed with what we’ve seen with MGW Ace and MGW D265 so far, and look forward to web control enhancements and additional BNC and Ethernet port availability as Vitec continues to roll out firmware updates.
This article appears in the November/December 2015 issue of Streaming Media magazine as “Review: Vitec MGW Ace and MGW D265.”
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