سود و زیان کدگذاری مقیاس پذیر ویدئو برای جریان اینترنت

Benefits and costs of scalable hybrid video coding techniques are analyzed with respect to internet streaming. Temporal, spatial, amplitude scalability, and combinations as described in MPEG-4 are considered. Benefits are a reduction of the server storage capacity, a reduction of the netload for multicast delivery and a graceful degradation in case of transmission errors. Costs are an increasing netload for unicast delivery and an increasing computational expense in the decoder. The result of an evaluation shows that temporal scalability has minimum costs among all analyzed techniques. It increases the netload for unicast only marginally with no additional computational expense in the decoder. Temporal scalability provides a reduction of the server storage capacity and netload for multicast by about 30% and two steps of graceful degradation. All other known standardized and nonstandardized techniques of spatial and amplitude scalability are associated with costs that appear too high to be attractive for internet streaming. Therefore, only temporal scalability is used at the present. Some of the scalable video coding techniques may become of interest for other applications where the investigated costs are less relevant.

The transmission direction can be either unidirectional or bidirectional. The variety of individual clients causes the challenge to provide bitstreams of different data rates simultaneously for the same content because of different connections to the network and different processing speeds of these clients. Due to the heterogeneity of the network, beside various different available channel capacities, also different transmission error behaviors have to be handled, as well as fast variations of the available channel capacity. Scalable video coding allows the decoding of only parts of the whole bitstream. Scalable encoded data contains one so-called base layer bitstream and one or more so-called enhancement layer bitstreams. A video of low resolution can be received by decoding just the base layer bitstream. The resolution of this video is reduced in amplitude, spatial, or temporal dimension. The enhancement layer bitstreams contain all additional information that is necessary for the decoding of the higher resolutions. Dependant on the transmission mode, scalable video coding offers different benefits and different costs, which are analyzed in detail in the following section. All standardized scalable video coding techniques are based on hybrid coding. Amplitude scalability is standardized in the MPEG-2 SNR scalable profile [1]. This technique uses a two step quantization of the DCT coefficients. Coarse quantized DCT coefficients are transmitted in the base layer. Finer requantized quantization errors of these coefficients are transmitted in one single enhancement layer. In H.263 [5], the amplitude scalability is achieved by enhancement layers using hybrid coding techniques. The coding efficiency of these techniques is low [2] and [6]. MPEG FGS (Fine Granularity Scalability), a technique standardized in MPEG-4 [4], achieves a higher coding efficiency due to the use of bitplane coding in the enhancement layer [7]. In MPEG-2 [1], MPEG-4 [8], and H.263 [5], techniques for spatial scalability are standardized. Two or several spatial resolutions can be provided by using pyramid coding. Temporal scalability is standardized in MPEG-2 [1], MPEG-4 [8], and H.263 [5]. It is efficiently realized by using B-Frames in the enhancement layer. They allow a hierarchical prediction order and can be omitted without any drift effect on the prediction loop. In this paper, the benefits and costs of scalable video coding are analyzed for different transmission modes. They are evaluated for the scalability techniques as described in MPEG-4: • AS: MPEG FGS, standardized in MPEG-4 [4] • SS: MPEG SSP, standardized in MPEG-4 [8] • TS: Temporal scalability using B-Frames in the enhancement layer, standardized in MPEG-4 [8].

Benefits and costs of scalable video coding are analyzed with respect to internet streaming. Benefits are a reduction of the server storage capacity, a reduction of the netload for multicast transmission and graceful degradation in the case of transmission errors. Costs are an increasing computational expense in the decoder and an increasing netload for unicast transmission. These benefits and costs are evaluated for different standardized scalability techniques which are based on hybrid coding and for combinations of these techniques. The result is that presently TS has minimum costs among all analyzed techniques. The server storage capacity and the netload for multicast delivery can be reduced by 34% compared to nonscalable coding. It increases the netload for unicast by just 2% with no additional computational expense in the decoder. Changes of the available channel capacity of about 35% can be balanced. But only two steps of graceful degradation are enabled by this technique. All other known standardized and nonstandardized techniques are associated with costs that are too high to be attractive for streaming applications. Thus, TS is the main technique being used at the present. Beside scalable hybrid video coding techniques also scalable 3D-Wavelet coding techniques are developed. These techniques are associated with an increase of the netload for unicast of about 10–15% but they require an increase of the computational expense in the decoder of about 100%. These high costs are problematic for mobile receivers but might be acceptable for special applications. To encourage the development of scalable coding techniques with lower costs the ISO has recently launched a call for proposals [24].