What is in this article?:
- Next-generation broadcast: more bandwidth for the future
- Audio compression
The development of new broadcast technologies that offer higher efficiencies and/or performance is an ongoing quest, with researchers continuing to push on methods to achieve a higher data throughput. The result is that the hardware and software providing media delivery will continue to evolve. On the broadcast side, modulation and compression are key components of a next-generation broadcast system.
Digital modulation customarily takes the form of transmitting symbols that are mapped from groups of bits. These symbols can be sent using one of a variety of modulation schemes, including VSB, QAM and COFDM. Because reception is usually limited by channel noise, interference and distortion, there is a certain carrier-to-noise (C/N) ratio that is required for error-free reception. For this reason, the symbol rate is usually limited in a channel, regardless of the number of different symbols (or constellation points) that can be transmitted.
But there are ways of pushing this limit, either by clever channel coding, or by changing the necessary transmit power. New error correction schemes are constantly being developed — pushing the envelope to its theoretical limit and making higher-order constellations like 256QAM possible. But another way to maximize channel usage is by adopting a “cellular” approach, where multiple transmitters would blanket an area, each with a lower power than a single one, but able to get a stronger signal to receivers — yielding a higher C/N ratio and thus increasing the potential throughput.
Adaptive transmission technologies are also gaining interest in the research community. The idea is to get feedback of instantaneous channel-state information from a number of receivers, and use this to dynamically modify characteristics of the transmission system — including output power; transmission architecture (i.e., number and individual power of active transmitters); and even modulation parameters. Any one of these systems would represent a radical change in broadcasting infrastructure and federal regulations, but could potentially generate a sizeable increase in available bandwidth.
As you read this, the latest MPEG compression specification, High-Efficiency Video Coding (HEVC), was due to be released as a final draft international standard, called MPEG-H Part 2 and ITU-T H.HEVC (and also referred to as H.265). HEVC promises to increase coding efficiency by up to 50 percent compared with AVC/H.264.
The HEVC specification is organized into profiles, tiers and levels, allowing equipment of varying complexity and cost to fit into well-defined architectures. As with its MPEG predecessors, an HEVC profile is a subset of the entire bit-stream syntax, and a level is a specified set of constraints imposed on values of the syntax elements in the bit stream. The constraints may be limits on values, or arithmetic combinations of values (e.g., number of pixels per frame multiplied by number of frames per second). Currently, three HEVC profiles have been established: Main, Main 10 (with up to 10 bits per color), and Main Still Picture.
HEVC introduces the concept of tiers within each profile, which carry their own constraints; tiers were established to deal with applications that differ in maximum bit rate. A level specified for a lower tier is more constrained than a level specified for a higher tier, and levels are similarly constrained within the tiers.