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Real-time media networks

Oct 19, 2007 1:39 PM

Transcoding compressed audio and video, also sometimes referred to as format conversion, for multichannel delivery can be divided into two broad classes: real-time, on-the-fly (such as a simulcast to the Web) or delayed redistribution, where content is converted, stored and distributed. Before addressing real-time transcoding for multichannel distribution, there must be an understanding of networks and the factors that influence their performance.

Based on the premise that a transcoder/format converter processes data delivered over an IT network on a video frame basis, the performance of the media network can be viewed with respect to events that occur in the duration of a video frame. An analysis of the numerical performance characteristics illustrates what factors should be considered when designing networks to support parallel production.

If the media distribution network can’t deliver content at the required data rate, then the transcoded content will not be artifact free. As media is concentrated in a centralized storage architecture, multiple transcoding and playout devices will be concurrently accessing content over a network that will be simulcasting content over various channels. Network routing devices will be pressed to meet the demand of video system, and packet jitter and latency may exceed video frame refresh rates. A look at system performance metrics can aid in system design.

The meaning of frame accuracy

Frame accuracy is a term that gets used a lot when talking about video over IT networks and format conversion. With many broadcast technical processes occurring on a frame basis, this is one of the fundamental issues of importance in the marriage of broadcast engineering and IT.

Frame rate events occur in milliseconds. The durations are:

60Hz = 0.01666… seconds or 16.7ms
30Hz = 0.03333… seconds or 33.3ms
24Hz = 0.04166… seconds or 41.7ms

Consider how many bits occupy a 60Hz, 30Hz or 24Hz duration of an uncompressed SDI signal. As Table 1 shows, SD at 270Mb/s pumps a 4.5Mb (562KB) frame over an IT network every 1/60 of a second. For HD at 1.485Gb/s, 24.75Mb (3.1MB) must be transferred every 1/60 of a second.

	60fps	30fps	24fps
270Mb/s	4.5Mb (562.5KB)	9.0Mb (112.5KB)	11.25Mb (1.406MB)
1.485Gb/s	24.75Mb/s (3.094MB)	49.5Mb/s (6.188MB)	61.88Mb (7.734MB)

Table 1. Uncompressed video data rate vs. frame rate

For uncompressed SD and HD, these are the constant data rates that must be fed to a device for real-time processes, such as playout or on-the-fly format conversion (transcoding or transrating). Now, consider the capability of 100Mb/s, 1Gb/s and 4Gb/s data transfer bandwidths as listed in Table 2 with respect to a video frame. The 4.5Mb per frame requirement of 270 Mb/s SDI can be met by 1Gb/s networks, while 4Gb/s is required to carry HD. This may be obvious, but real-world systems don’t always meet ideal performance specs.

	60fps	30fps	24fps
100Mb/s	1.667Mb (208.3KB)	3.333Mb (416.7KB)	11.25Mb (1.406MB)
1Gb/s	16.67Mb (2.083MB)	33.3Mb (4.167MB)	41.67Mb (5.208MB)
4Gb/s	66.67Mb (8.333MB)	133.33Mb (16.67MB)	166.67Mb (20.83MB)

Table 2. Network and Fibre Channel data rates vs. frame rate

Next, consider Ethernet packets. Two common sizes, 1500B and 9000B jumbo packets, transfer a given number of packets as described in Table 3 with respect to 100Mb/s and 1Gb/s network bandwidth.

	100Mb/s	1Gb/s
1500B/packet	8333 packets/s	83333 packets/s
9000B/packet	1388 packets/s	13888 packets/s

Table 3. Ethernet and jumbo Ethernet frames per second on 100Mb/s and 1Gb/s networks

An Ethernet packet of 1500B, or 12000b, on a 100Mb/s network occupies 120 microseconds. A 100Mb/s HD video file loads a network with a data burst of 1,666,667b every 16.7ms, which equates to 208,333B. Therefore, 138 Ethernet packets are transferred in one 60Hz video frame, or 1388 packets over a GigE network. Table 4 lists the number of packets for each frame rate for both packet sizes.

Each of these packets has an overhead (header and footer) associated with it, which reduces bandwidth. Jumbo frames of 9000B are used to cut down on these non-payload bytes. A 9000B jumbo frame is 72000b. On a GigE network, a packet occupies 72 microseconds; 13888 jumbo frames are transmitted in one second, or 231 in a video frame.