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New expectations set for mobile video service

Last year, the Open Mobile Video Coalition (OMVC) released a specification, the ATSC Mobile DTV Receiver Profile Guidelines. The spec, issued by the OMVC Technical Advisory Group (OTAG), recommends two profiles for mobile receivers: a required Base Profile and an optional Enhanced Profile. (Let’s call it an “E-Profile”). So consumers can sort it all out, it’s conceivable that support for the Enhanced Profile could lead to a class of devices with a user-recognizable label. Although the OTAG spec concerns implementation of the ATSC A/153 standard, many of the embodied concepts apply to other systems throughout the world, such as DVB-MHP (Multimedia Home Platform).

Video and audio, while customizable, will likely converge to common formats. H.264 (Base Profile Level 1.3 and Main Profile up to Level 3.1) is recommended by OTAG for both mobile profiles, at supported resolutions. Although Scalable Video Coding (SVC) is a broadcast option, OTAG has not entered it into a mobile profile, citing a lack of marketplace maturity for SVC-compatible consumer products. However, DVR-pause is required, and impulse recording and scheduled DVR are “desirable” in the E-Profile. These step-ups will require additional storage memory. As an example, a combined data rate of 600kb/s for audio and video (reasonable for 416 x 240 resolution) would require 4.5MB for each minute of stored content — not too demanding given today’s flash memory prices.

The Scalable Full-channel Mobile Mode (SFCMM) is an optional transmission mode that scales mobile payload capacity up to the total available from the channel; such a device, of course, would not support legacy mobile broadcasts. SFCMM is deemed “desirable” in the E-Profile, as is a second tuner. HE-AACv2 (w/SBR) (High-efficiency Advanced Audio Coding with Spectral Band Replication) is required for all devices, as constrained in A/153 Part 8, with the average loudness of the audio at -14dB LKFS.

Interaction (return) channel

In both profiles, an interaction (or return) channel by one or more out-of-band sources (Internet, Wi-Fi, cellular, USB, Bluetooth) is required from all devices at least once per week. Interactivity, both active and passive, is a key differentiator that can set mobile apart from fixed-device broadcasting; users naturally want to interact with an audio/video device they hold in their hand, and this factor should pave the way for applications that leverage social and informational enhancements to linear TV viewing.

Many of these interaction-channel media have persistence and connectivity challenges, which have led to various service-provider solutions. Under the DVB-MHP banner, interactivity and other enhancements have been defined using an Interactive-broadcast profile or an Internet-access profile, both being over and above the Enhanced-broadcast profile.

Figure 1. As part of the requirement to provide their mobile viewers with connectivity, broadcasters can deliver electronic service guides in band through the OTA terrestrial channel, or over a separate interaction channel.

Electronic service guides (ESG) can be delivered in-band (i.e., through the OTA terrestrial channel) and out-of-band, over a separate interaction channel, as shown in Figure 1. An in-band basic ESG is required in both OTAG profiles, while an enhanced ESG over an interaction channel is required in the E-Profile. Aggregation of multiple-ESG information collected from multiple providers is required by OTAG on all devices. File-based delivery is required on E-Profile devices to support the mobile aspects of the non-real-time (NRT) standard that allows in-band pre-caching and downloading of content.

The rich media environment (RME) is required for supporting interactive components in the E-Profile. Defined by the Open Mobile Alliance (OMA), RME provides a rich graphical and media experience by integrating Scalable Vector Graphics (SVG), for graphical object creation, and ECMAScript, for script support, into a media stream.

Conditional access and service/audience measurement require an interaction channel. The A/153 Mobile DTV standard incorporates a conditional-access system (CAS) that is based on the OMA-DRM standard. Conditional access and service protection require that encryption key exchange take place over an interaction channel, due to the capacity limits of OTA content delivery. Content security, while not explicitly specified, is envisioned using an exchange of short-term keys over the broadcast channel, and long-term keys over the out-of-band return channel.

Simple devices without an interaction channel will, therefore, risk missing out on some services in the broadcast stream. Of course, services and content can always be sent in the clear, with no encryption.

There are several items in the E-Profile that were not completely set in the standardization process when the OTAG document was finalized. Hence, we might expect additional elements to be defined, in the area of conditional access, in the coming months. For the time being, the OTAG recommends that service providers (i.e., broadcasters) supply an optional conditional-access application to be downloaded and installed in receivers that support the E-Profile.

All devices are expected to provide a facility for collecting and reporting on content consumption. In order to support audience data gathering and reporting, user data collected on all devices should be uploaded to a service agency at least once per week, or the same time as a long-term key is delivered.

Geo-location is required on all devices if GPS or other positioning data is available from the device. Wake-up functions are required in the E-Profile, such as to provide emergency alerts. The Cell Information Table is required on all devices, providing carrier frequency information on selected available transmitters, so that cell-like handoffs would allow near-seamless roaming from one transmitter footprint to another.

As defined in ATSC A/153, mobile content data is organized into groups called Parades, each of which can carry different kinds of services: Data groups within a Parade all have the same Forward Error Correction (FEC) parameters, and a Parade can carry simultaneously up to two Ensembles, which are logical pipes for IP datagrams. Although it is usually unnecessary for a receiver to simultaneously decode all of the services present in a stream, decoding parallel Parades is required on E-Profile devices. Expected uses for this functionality include PIP, or transmitting CAS Entitlement Management Messages in Parades separate from the A/V content.

Starting point

Receiver profiles, although demonstrating a path to viable business models, do not constitute a “standard.” Readers must keep in mind that the Mobile DTV Receiver Profiles specification was published for illustrative purposes only, and does not carry the weight of a regulation or standard. Nonetheless, the spec was developed cooperatively by broadcasters and consumer-electronics manufacturers, and thus represents a snapshot of their combined expectations for services and products.

Aldo Cugnini is a consultant in the digital television industry and a partner in a mobile services company.