Understanding HD Radio: The program audio chain - Part 3

Program Service Data
Program Service Data (PSD) may be transmitted with each Program Service.⁴ As described above, PSD is a subset of the family of data that is considered Program Associated Data, because PSD is one way of transporting information about an audio program service. In NRSC-5, PSD is strictly a text-transmission protocol conveying information such as Artist and Title.

Each Program Service (MPS, SPS-2, etc.) conveys its own PSD within its Program Service PDUs. Like Opportunistic Data, PSD opportunistically relies on the likelihood that there will be space for it in the Program Service PDU. Source audio that is more complex to encode will leave less room for PSD and Opportunistic Data.

Table 4.8 provides an estimate of the typical rates of transmission of PSD. It is based on the assumption that only MPS is being transmitted at the full capacity of the IBOC signal. When multiple services are transported on the same Logical Channel, each service is operating at a fraction of the total available rate of transfer. If this were the case, the typical PSD rates shown in Table 4.8 could diminish for each Program Service.

*Hybrid FM operation conveys the full fidelity of the audio in one stream, called "Core."

Note: The Estimated Typical PSD Bytes columns show approximate rates of an MPSD stream if the entire Logical Channel were employed for MPS.

Table 4.8 Typical Program Service Data Capacity

PSD Protocol and Transport
Program Service Data is prepared for transmission in two steps. First, NRSC-5 Reference Document (7), Program Service Data, describes the manner in which the content of PSD transmissions is formatted. Reference Document (9), Program Service Data Transport, describes the rules for encapsulating blocks of PSD into packets and transmitting them as a stream.

PSD ID3 Tags
PSD consists of various types of text information that describe program content. These pieces of information can be transported together in the form of "Tags." The Tags are based on the ID3 format, which is discussed further below. The transported information in a Tag is classified among several types: Title, Artist, Album, Genre, Comment, and Commercial. The types are called Attributes. Each Attribute is given a Frame ID, which is an alpha-numeric code that is transported with each piece of information to inform the receiver what the Attribute of the information is.

Each Frame ID is associated with a frame structure. In the cases of Title, Artist Album and Genre, the structure is as simple as can be. For example, a Title frame is labeled with a "TIT2" Frame ID and contains one field with the title of the song being played.

The Comment and Commercial frames are more complex. The Comment frame, identified by the Frame ID "COMM," has a field for a short description of the comment and a field for the comment itself. The Comment can be anything. It could be used for contact information, with "contact" in the description field and the comment could contain an address, phone number and web address. It could contain a headline in the description and news or information in the comment. The ways to use this field are up to the imagination of the broadcaster, with one caveat. The field is only useful if receiver manufacturers find the field is consistently used in a manner that they can design receivers to utilize.

Higher in complexity is the Commercial frame. It is structured rather like a classified advertisement with fields available for a Price, "Valid Until" expiration field, Seller Name, Contact URL (website), "Received As" method by which the goods are delivered, and Description of the advertised item.