There are four underlying transport protocols used to encode the data received from a data client prior to application into the RLS transmission stream. When transferring synchronized images, the difference between the packet delivery methods can dictate how far in advance of the associated audio the image must be sent to ensure it is available for display when the associated audio arrives:

• Standard packet encoding encapsulates a predetermined set of bytes received from the client and is best suited for non-real-time applications and file transfers where the file sizes are known and packet encapsulation is fixed by the originating application.

• Flex packet encoding lifts the packet size restriction and allows packet sizes independent of allocated bandwidth and is best suited for non-real time applications.

• Byte streaming protocol simply accepts bytes from the client as they arrive. As the bytes flow into the RLS they are buffered and the transmission system decides how best to break them up into frames based on the allocated bandwidth. Byte streaming is the most efficient method of data transfer and should be considered for custom application development that can provide message management.

• Large Object Transfer (LOT) is an Application Programming Interface (API) that allows a station or service provider to transfer large data objects of any type through the HD Radio transmission system to a receiver equipped with a LOT decoder. Album art applications use the LOT protocol.

A consideration in transmitting audio associated data, whether PSD or album art, via HD Radio is that there is no inherent provision for synchronization of audio with the data services. This makes synchronization of audio and graphics quite unpredictable without some tools and methods of synchronization.

To address this, system latencies are calculated to determine when the data client application needs to start transmission for on-time arrival. The actual presentation time of each image is controlled by including a custom ID3 "trigger" tag that is transported along with the other Program Service Data (PSD) information. Upon receiving this trigger tag, the receiver executes the appropriate command to provide display synchronization with the audio content.

Receivers

Special application software is needed on the receiver side when presenting multimedia data to the user. This software is used to decode and present a specific type of data. Standards are defined as to how the data is structured and transported, and how it is to be decoded and presented by applications within the host processor. These specific applications reside on the receiver's host processor.

Figure 2. The two components of an HD Radio receiver are the baseband processor and the host porocessor.

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There are two primary components in an HD Radio receiver: The baseband processor and the host processor. RF signals are received by a fairly conventional tuner which provides a 10.7MHz IF signal to the IF processor, where it is demodulated into digital I and Q baseband signals. The digital I/Q are de-multiplexed into audio and data streams, and audio is sent to the audio decoder for conversion to an audio output for amplification. All data is sent separately to the RLS decoder, where it is reformatted back into the original structure and sent to the host microprocessor. Here, the appropriate application prepares the data for presentation.

The host microprocessor contains the application software to initiate the necessary interaction with the various applications. The LOT decoder (if present) reformats any LOT data received and sends it to the RLS decoder prior to processing.

While displays and capabilities vary, the necessary hardware for these advanced data multimedia applications already exists in most of the current generation HD Radio receivers. It is up to the receiver manufacturers to develop the host micro applications to make these new features available. This is where most of the development work needs to take place to provide media-rich content and data services over digital radio broadcasts.

Anderson is manager, radio transmission strategic market and product development, Harris, Mason, OH.