Three Screen Television: The Home Gateway
Prof. Nikil Jayant, Eun-Seok Ryu, Hye-Soo Kim
Background and Context. Video Service Providers are looking to quantify the advantages of a
home gateway server for managing content delivery for three screen television, a scenario with
large-screen TV, computer and handheld, with the user expecting satisfactory Quality of
Experience (QoE) in each of these modalities, and in transitions across them. Focus in our work
is on information efficiencies in the pathways to the home and within the home. The following
paragraphs summarize three thrusts in the Gateway research: Scalable Video Coding (SVC),
SVC Layer Switching and Smart Packet Dropping. Figure 1 depicts the gateway, the threescreen clients and in-home wireless paths that are assumed to be bidirectional.
Scalable Video Coding (SVC): We have developed a real-time decoder/player for a three-layer
H.264-SVC codec and quantified information rates related to the standard H.264-AVC. By
embedding lower spatial richness layers into the multilayer arrangement, SVC provides a total
bit rate (for HD, SD and LD) that is significantly lower than the sum of individual HD,SD and LD
rates as in H.264-AVC. Figure 2 demonstrates typical high-quality examples of HD, SD and LD
pictures and a typical quality-bit rate profile, with quality measured as signal-to-noise ratio
(desirably 40dB or higher). Figure 3 shows the significant overall bit rate gains over AVC at
three quality levels. These economies translate directly to lower information flow per program in
the last mile reaching the home – or equivalently – the number of program channels that the
provider can offer for a given capability in the last mile. SVC provides additional flexibility in inhome information flows, as in the technologies described in the next two thrust.
SVC Layer Switching to avoid compression artifacts: We have developed a reliable method for
automatic switching between the three spatial layers in the SVC codec. The switching is
performed in the gateway server as it responds to feedback of channel information from the
client. Adaptations to channel conditions can be infrequent, periodic or continuous. The situation
of greatest interest is in the mobile TV mode that relates to the second and third clients (PC and
handheld). As a user moves from an area of higher (or lower) channel quality, the gateway
switches its service to a higher (or lower) level of spatial resolution, while maintaining an artifactfree display (say SNR > 40 dB) as in Figure 2. Figure 4 depicts the system that uses RSSIbased feedback of current channel state (available bandwidth for TV flow) to effect the adaptive
action of the three-screen gateway.
Smart Packet Dropping to avoid network artifacts: While SVC and Layer Switching provide most
of the flexibility needed for seamless continuity in user experience, packet losses are
unavoidable in real wireless networks, especially in the context of multiple wireless networks
and services in the home (causing bursts of interference and low bandwidth). To address this in
multiple contexts -- including the context of downward transitions to less demanding SVC layers
(or to extend the quality curve when a client is already at the least demanding spatial layer) –
we have developed a smart algorithm for packet dropping. Based on loss anticipation and
feedback, the gateway server drops the least important video packets, minimizes future network
congestion and thus avoids losing important packets that, if lost, cause serious network streaks
in the video. The gains due to smart packet dropping are illustrated in Figure 5.
Future work: Fortification of design with elastic FEC, demonstration of gateway in GTBI-ResLab.
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