IPTV content delivery issues
Nov 1, 2008 12:00 PM, By Patrick Waddell
Delivering Web video isn’t as easy as 1-2-3. Content must be unique and high quality.
IP infrastructures were originally designed for the delivery of computer data that could be retransmitted (TCP) as well as for voice traffic. In data applications, and to a lesser extent voice, a dropped packet or two is not perceived as a problem. However, when an IP network delivers pictures and synchronized sound, if there is a sudden packet loss or packet reordering, there will be a noticeable service interruption at the set-top box (STB). Operators need to constantly be aware that trying to add the transport of real-time A/V content onto a network already loaded with a lot of computer data or voice data may result in a less than optimal viewing experience for their IPTV customers. Traffic grooming becomes an important aspect of maintaining high quality.
Delivery standards
Most IPTV systems today use AVC video with either Dolby Digital or HE AAC stereo (and multichannel) audio. The A/V streams are transported using either MPEG-2 TS over UDP, or increasingly MPEG-2 TS over RTP along with FEC (standardized in SMPTE 2022, which was based on the Pro-MPEG COP3 FEC). While FEC adds some overhead, it permits perfect delivery of the A/V in situations where there was packet reordering or burst noise.
In addition to their own network considerations, IPTV operators (as relatively new entrants into the content delivery arena) may not have a close relationship with their content providers, and the incoming signals may not always be of the highest quality. By their intrinsic nature, IPTV systems are usually at the end of a potentially long chain of compressed feeds, and operators have to ensure that incoming video and audio quality are not compromised. For example, incoming A/V synchronization may not be good or may vary over time. The IPTV operator who foresees this issue might install A/V retiming equipment between incoming signal reception gear and compression gear to permit proper A/V sync delivery to its customers.
Lip sync
A/V synchronization, more commonly known as lip sync, is a nontrivial issue that has affected the entire broadcast production and delivery system since digital equipment arrived in the 1970s. Both equipment manufacturers and users were initially slow to realize it was a problem, which means that it is still an issue today, made more noticeable by the increased video quality of HDTV. Many production systems do not provide for keeping or restoring A/V sync, which causes the problem of crossing system boundaries. Further insight reveals that it impacts multiple standards development organizations and is codec agnostic. This has slowed systemic response to the issue, although progress is being made.
Figure 2. Management of upstream processing can control A/V delay.
Click to enlarge
Program production requirements that need to be considered include whether or not the video production switchers change video delay depending on the effects selected by operators. Newer switchers maintain a uniform delay, but there are many older switchers still in use that do not. For live events, wireless camera systems often have much more video delay than wired cameras. Media servers almost uniformly separate the video and audio for storage and recombine on playback. Few of them provide a reliable method of ensuring correct synchronization at that point. Even though IPTV operators may not have any control over these factors, they need to be aware of them and be prepared to possibly correct for them in their own facility. (See Figure 2.)
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