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.
Also, IPTV operators need to ensure that incoming signals maintain their A/V timing. They should understand also that there is neither vertical blanking information nor a vertical ancillary channel in MPEG systems, and the MPEG signal is no longer in the time domain. Some program providers routinely transmit A/V timing test signals at off hours, but many do not.
Measuring A/V offsets
In-service measurements are difficult to carry out, and further standards are not in place to help. For example, in relationship to the image raster, it makes a big difference if the video is part of the measurement. Because a frame takes 40ms for 25Hz systems and 33ms for 30Hz systems, two measurement systems, one measuring at the top of the raster and another in the middle of the raster, could be by definition 20ms or 16.5ms apart. Also, without suitable standards for expression of these metrics, users cannot reliably compare device specifications, which can have an effect on ultimate system design quality.
Figure 3. Older lip sync standards may not meet the expectations of viewers with large flat-screen displays and surround sound.
Click to enlarge
Existing standards, like ITU-R BT.1359, are now considered loose: -185ms to 90ms. But they were developed for SD images on a CRT display. (See Figure 3.)
Beyond measurement issues, there are basic unintended consequences related to the MPEG Committee's approach of only standardizing the bit stream and not requiring a consistent use of the clocks provided by that bit stream. Because of this lack of specification, practical implementations have synchronized audio and video in diverse ways. Buffer handling varies widely, and overflows and underflows caused by transmission errors are treated differently by decoders. Further, the video and audio decoders are often in physically different chips and may rarely communicate. This means that clock samples may only be shared at startup or at channel change.
System management
It's not just about technology issues; system management is also vital. New entrants to the IPTV market need to realize why their competitors have significant management and operations staff. Some IPTV operators don't employ any operations staff other than a call center to handle complaints. Others believe that system monitoring only consists of having a large plasma screen and an STB in an office and just watching what is transmitted, rather than installing the necessary test equipment such as real-time MPEG and IP analysis and monitoring equipment. The leading IPTV players have addressed these needs both in terms of management and technology.
Another area that exemplifies the difference in operator insight is encoding. Smart operators realize that they simply get what they pay for, and those who have bought the cheapest encoders will then wonder why the end picture looks so bad. Cheap, single-pass encoders will not satisfy a broadcast audience; two-pass encoding is a must. Capable, adaptive noise reduction filters are essential to delivery of high video quality, as operators do not typically have time to dynamically tune for content. All signals carry noise, even HD, and the bottom line is that noise in the picture will take bits away from clean picture coding and detract from the overall quality of the picture.
Transcoding
IP systems need to be able to serve different-sized pictures such as full-screen images as well as thumbnails for the program guide, so operators also need to look at transcoding, rescaling and reformatting of their content for each application. Transcoding is considered by some as a low-cost requantization device where video quality may be secondary, and as such it has developed a poor industry reputation.
Furthermore, classic compressed-domain transcoding between MPEG-2 and AVC has proven disappointing in terms of picture quality and bit rate savings. This is attributable to the differences between MPEG-2 and AVC compression technologies. Currently, the best method of achieving high quality is to decode to baseband video, resize or rescale in baseband and compress the result with the new encoder.
The state-of-the-art integrated decode and re-encode solutions will deliver the desired bit rate savings at high picture quality. Integrated solutions may also permit reuse of picture coding metrics and do not require additional devices occupying rack space, meaning that installations use up less space and ultimately less power.
And that in the end is what it is all about. Some firms have looked at the robust growth of IPTV and calculated that all they need in order to take a slice of revenue is to build a headend, turn it on, walk away and collect money. This could not be more untrue. Delivering a successful deployment involves investing appropriately in system design as well as in network design. It also requires the necessary management and operational staff to make sure that the viewers receive a high-quality service capable of competing against the satellite and cable companies.
Patrick Waddell is manager, standards and regulatory, for Harmonic.
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