Compression involves reducing the amount of data required to display a stream of digital video (HDMI/DVI-D) frames. It is a tradeoff between image acuity and bandwidth. A stream of uncompressed HDMI video, for example, requires up to 10.2Gbps of bandwidth - greater than the combined capacity of 100 100Mbps Ethernet networks. The disk storage capacity of uncompressed video is equally eye-opening.
For these reasons, compression techniques are widely used to enable cost-effective transmission and storage of digital video. Video encoders compress a video stream down to a video bit rate that is compatible with commodity transmission and storage media. The following chart illustrates standard bit rates in Mbps for various video offerings.
For these reasons, compression techniques are widely used to enable cost-effective transmission and storage of digital video. Video encoders compress a video stream down to a video bit rate that is compatible with commodity transmission and storage media. The following chart illustrates standard bit rates in Mbps for various video offerings.
At 40Mbps, the ultimate in video quality, Blu-ray, is highly compressed when compared to 10.2Gbps.
What implications does this have on video extension?
There are two types of digital video extenders on the market. Some encode digital video and audio data for transmission over commodity IP networks while others transfer digital signals in an uncompressed manner over point-to-point dedicated cabling. So, which is better, and why?
As always, the answer depends upon your needs. From the chart above, we can deduce that compression is not a critical factor unless you’re involved in video post production. As long as the video bit rate is 20Mbps (HDTV) or greater, the quality should be adequate for most normal needs. Bit rates of 50 (more than Blu-ray) or greater are quite acceptable for most critical viewing needs. If compression is not the primary deciding factor, then what is?
Consider factors such as the following when making your decision:
• Is your customer able to leverage an existing IP network infrastructure?
• IP networks can send data around the globe, whereas point-to-point extenders offer quite respectable but limited transmission distances.
• Is HDCP required for display of protected content? Few, if any, non-IP extension products are able to support one-to-many extension of protected content.
• Point-to-point extenders cascade from one receiver to another in serial manner. Thus, break in the cable will affect all downstream displays. IP networks support a host of matrix and redundancy options.
• Ethernet networks are designed to span across buildings with dissimilar ground planes. Thus, extender burnout is not an issue.
• Other features to look into involve bi-directional serial and IR data transfers, analog audio for DVI-D connections, and central management features.
The above list is intended to introduce topics for discussion in the weeks to come rather than an exhaustive treatment of this topic. Stay tuned for a deeper dive into these emerging video extension techniques.
What implications does this have on video extension?
There are two types of digital video extenders on the market. Some encode digital video and audio data for transmission over commodity IP networks while others transfer digital signals in an uncompressed manner over point-to-point dedicated cabling. So, which is better, and why?
As always, the answer depends upon your needs. From the chart above, we can deduce that compression is not a critical factor unless you’re involved in video post production. As long as the video bit rate is 20Mbps (HDTV) or greater, the quality should be adequate for most normal needs. Bit rates of 50 (more than Blu-ray) or greater are quite acceptable for most critical viewing needs. If compression is not the primary deciding factor, then what is?
Consider factors such as the following when making your decision:
• Is your customer able to leverage an existing IP network infrastructure?
• IP networks can send data around the globe, whereas point-to-point extenders offer quite respectable but limited transmission distances.
• Is HDCP required for display of protected content? Few, if any, non-IP extension products are able to support one-to-many extension of protected content.
• Point-to-point extenders cascade from one receiver to another in serial manner. Thus, break in the cable will affect all downstream displays. IP networks support a host of matrix and redundancy options.
• Ethernet networks are designed to span across buildings with dissimilar ground planes. Thus, extender burnout is not an issue.
• Other features to look into involve bi-directional serial and IR data transfers, analog audio for DVI-D connections, and central management features.
The above list is intended to introduce topics for discussion in the weeks to come rather than an exhaustive treatment of this topic. Stay tuned for a deeper dive into these emerging video extension techniques.
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What bitrate does Avocent use on it's IP products? I am considering the ECMS4000 unit for a dual monitor solution. I am thinking of running a dedicated network just for the extenders but hope not to use a switch until needed so really just Cat-6 cable from transmitter to receiver. Thanks.
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Our ECMS2000 and ECMS4000 products rely upon an Avocent proprietary video compression algorithm that is geared toward needs of computer graphics rather than motion video although it can handle either. Computer graphics refers to renderings that are somewhat static such as flight information displays, web pages, and informational signage. These applications involve video that changes very little from frame-to-frame. Thus, the hardware video encoder engine in these products is based upon frame-to-frame compression. Only changes are sent. Here are some data points that affect bit rate:
ReplyDelete• A still image will cause an initial burst of data followed by nothing but occasional TCP keep-alive packets.
• Analog video is somewhat noisy and results in some amount of change even for still images.
• Digital video is rock solid and is not subject to noise.
• The product will throttle back it’s bit rate (compress) harder, if bandwidth is not available due to LAN contention of low-bandwidth links.
• The product offers a few levels of manual bit rate control that you can manually select.
• Bit rate is affected by video resolution. More pixels can result in more change. A full-motion full-screen video can consume up to 500Mbps at 60fps.
• Frame rate affects video bit rate. The ECMS4000 product supports frame rates as follows:
o Up to 60 fps up to 1440x900.
o Dual channel 1600x1200 up to 45 fps
o Dual channel 1920x1200 up to 39 fps
• Refresh rate will remain at 60 regardless of frame rate.
As you see, bit rate is dependent upon a number of factors. Nonetheless, if a dedicated gigabit switch or direct-connect cable is used, you should experience flawless video.
Our MPX products rely upon a wavelet compression engine that is much more constant in terms of bit rate. With that product, you can select a bit rate and it will send up to the desired bit rate, such as 50Mbps, if the image requires the full bandwidth. The MPX products are geared toward motion video and do not attempt to compare one frame to next. Thus, they send continual data even for still images but less bandwidth for full motion video than the ECMS products.
I hope this helps. Feel free to contact us directly if you have more questions.