The extra bits in IBOC
Enhancements over the existing analog service have been part of the promise of completing the digital transition in radio. An improvement in audio quality is cited most often to consumers because this change is easy to demonstrate. More recently, Supplemental Program Service and surround sound have been added to the audio improvement list. Throughout the transition, the ability to transmit data has been listed as an enhancement as well. This change leaves more to the imagination of the broadcaster, which makes it harder to evaluate and fully understand the benefit. Now that consumers have come to recognize and value the enhanced displays of satellite radio, Internet radio and RBDS, IBOC is poised to provide this ability and more.
Data has always been a part of the IBOC plan, but it is just being recognized for its current and future capabilities. In the HD Radio system, there are three parts of the data stream: Main Program Service (MPS), Station Information Service (SIS) and Advanced Application Service (AAS). These are all part of the IBOC service interface, which dictates how data is transmitted. IBOC is a pipe that transmits data, with a large chunk of the data being audio.
The Main Program Service provides the data functions that consumers are already accepting from the familiar media offerings. The MPS Program Service Data (PSD) provides a description of the current audio segment such as artist name and song title.
The MPS uses an information format that is described by the ID3 standard. This format includes fields called title, artist, album, genre, comment and commercial. Figure 1 shows the type of information that can be used in these fields depending on the type of audio being transmitted. Most if not all of this information is already stored in a station's automation system. The leading automation systems provide a utility to format or map the fields to be delivered with the audio. This data can be provided via an Ethernet, serial or other data connection. For HD Radio, this stream is an easy payload that requires less than 1.516kb/s to deliver.
The data is formatted through an MPS Program Service Data-enabled Program Server. The MPS PSD data requires a relatively slow data rate of 1.2kb/s, which must be transmitted along side the audio to the transmitter. The data can be serial or Ethernet.
Today, the Station Information Service is a common information channel for all modes of IBOC. It provides the station ID in a short and long form (call letters and station slogan or moniker). Planned enhancements to this data string will be used by a receiver to immediately identify available services of the station. It will instruct a receiver that the station offers traffic information, programming in surround sound, general location information and other data. It includes provisions for future expandability, such as an enhanced electronic program guide (EPG) and a station message that can be changed depending on the daypart.
Looking to the future, the Advanced Application Services provide the ability to deliver new features. On-demand audio, reading services, traffic information and telematics are some of the capabilities of AAS.
|Artist||Artist||Host||Author or Sponsor|
|Album||Album Name||Show Name||Sponsor Name|
|Genre||Jazz, Rock, etc.||Speech||Speech|
|Comment||Contact Info||Call-in #||Information|
|Commercial||Product Sale Info|
Figure 1. The types of information that could be placed into the MPS data fields.
Adding a data stream
To describe how data is inserted into an HD Radio transmission, we first need to understand how the HD Radio signal is created. In the first generation of HD Radio exciters, all the functions were in one unit, so audio and data were fed into that one device. The drawback to this design is that all the elements must be delivered to the IBOC exciter. Creating a linear path for the stereo audio and data could require a path capable of delivering more than 1.44Mb/s. It also required all the transmission equipment to reside at the transmitter site.
To make more efficient use of the spectrum needed to send audio to the transmitter, the second generation of HD Radio exciter was developed. This separated the functions of encoding audio and data from the modulation. It also reduces bandwidth requirement for an STL from 1.44Mb/s to about 356kb/s.
In this split operation mode, three devices comprise the HD Radio chain.
The Exporter handles the encoding of the primary Main Program Service (MPS) audio and program service data (PSD). This device currently operates on Linux. The Exporter is intended to be placed at the studio.
The Importer serves two basic functions: to manage the data, bandwidth and quality of service (QOS) of the signal, and to act as an insertion point for the Supplemental Program Service (SPS) data and audio. The SPS channels can be inserted as AES-3 at 44.1kHz or as IP audio. The Importer can be connected to a first-generation HD Radio exciter or an Exporter. However, the data interface between the Importer and the Exporter or first-generation exciter must be bidirectional Ethernet. If an Exporter is not used, the MPS audio data can be sourced directly to the Importer.
The Importer is also called the Ensemble Operations Center (EOC). EOC is the general term to describe a data management and data generation system, of which the Importer is a specific implementation. The Importer operates on Windows Server and XP. The unit has several applications running on it to encode the SPS audio and generate the PSD for the SPS.
The Exgine is a DSP implementation of the modulation for HD Radio. It handles the OFDM transfer of the data. Several manufacturers are looking at updating their current analog transmission exciters and adding an Exgine card to handle the analog and OFDM modulation. The Exporter connects to the Exgine via a User Datagram Protocol (UDP) Ethernet connection.
The functions of the Importer and Exporter currently run on separate hardware and software systems, but it is conceivable that the Exporter functions could be reduced to a card that becomes part of a station's audio processor or STL, or a function of the automation system.
A single Exporter can be used to distribute a signal to many Exgine-based HD Radio exciters. Each Exgine can then insert its own Station Information Service (SIS) data. This was successfully tested at the end of 2004 by WUMB in Boston and National Public Radio Satellite Services. In this test, a single Exporter encoded information that was received via a 200kb/s SCPC satellite channel at several locations.
Ready to go
The data capability of HD Radio is designed to provide at least as much information to the listener as RBDS. With listeners already expecting enhanced displays on media services, HD Radio can easily make the move to provide this and more.
Jeff Detweiler, Senior Broadcast Technology Manager at Ibiquity, provided information for this article.
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