HD Lite - Background

Background

In a simplified view of US digital-TV distribution, the cable/sat operator receives a programing feed (producer) from a network station, repackages it for carriage on a data-network or distribution channel (satellite, digital-cable) of known parameters, then re-transmits the modified bitstream to the customer-site (viewer.) The viewer's set-top-box decoder decompresses the delivered bitstream, and displays the program on-screen.

HD Lite refers to the TV-program received by the viewer, which has been somehow compromised (reduced) in fidelity. In internet vernacular, HD-Lite generally refers to programming delivered by commercial (subscription-based) providers such as DirecTV, Dish Network, and the major cable-TV operators. This is likely due to the customer's (heightened) expectation of a base quality level of service, that a commercial operator should provide picture-quality equal to or better than public over-the-air (free) ATSC-broadcast programming.

HD Lite can be achieved by any combination of several techniques. Rate-shaping dynamically adjusts allocated bit rate for each of a set of TV-channels, based on an allocation-policy (which can come from realtime video-analysis or an operator-specified program weighting.) Rate shaping allows a set of channels to be transmitted with less bandwidth, based on the statistical observation that not all channels display the same level of motion-activity at a given instant of time (or the period of observation.) Downsampling reduces the spatial (horizontal and/or vertical) resolution of the TV-program, reducing the TV-signal's pixel-rate, and therefore its bandwidth requirements. Thus far, customers have reported downsampling on "1080i" signals only; 1920x1080i can be downsampled to 1440x1080i or 1280x1080i, with a corresponding reduction in transmission bandwidth. In contrast, over-the-air (ATSC) broadcasts of 1080i are fixed at 1920x1080. Temporal (frame-rate) reduction has not been attempted yet, as it unacceptably changes the character of motion video sequences.

Any form of rate-shaping or downsampling is inherently intrusive, in that the source bitstream is altered significantly, often due to a full re-compression process. The re-compression process is the point of contention raised by critics: "HD-Lite" programming is perceptibly worse than the original HDTV broadcast, to the point where the degradation is discernible absent a direct (A/B) comparison against the original source. Distortion (caused by the operator) is characterized by reduced sharpness, reduced detail, excessive compression artifacts (mosquito noise and blocking), and in some cases, alteration of the color-palette. The reduced video quality is assumed to be introduced by the sat/cable operator's handling of the source video (recompression.)

It is important to note that digital video compression is a complex field of study. Downsampling and bitrate-reduction are often deployed together, to prevent the pixel/bitrate ratio from falling below acceptable levels.

Some material shown on 1080i high-definition channels in the US originates from material shot on older cameras that was only capable of 1440 samples per scanline, yet this material is generally quite acceptable to most viewers and is considered high-definition. Focusing on resolution alone can be misleading. For example, a signal transmitted in its original 1920 x 1080 format, even if only having 1440 unique samples per scanline, will likely appear superior to a highly recompressed signal shown at 1440 x 1080 with a lower bitrate than the standard 19.2 Mbit/s.

In the US television programming market, cable and DBS/satellite operators compete against each other to deliver HDTV programming. An HDTV program requires much higher datarates (3-4x) than a standard-definition program. This places a huge burden on a service-operator, which must deliver a variety of programming (many channels), including increasing amounts of HDTV programming, over a resource-constrained distribution medium. Re-compression by cable/satellite operators is a technical necessity for carriage of diverse TV-programming over limited bandwidth capacity of the respective providers.

What remains to be seen, is whether the cable/sat service providers leverage re-compression of TV-programs only as a short-term arrangement until greater capacity can be brought online, or a permanent fixture in their distribution and business model.