From the Top Down: Program Services
This chapter covers the process of assembling the program audio and program related text into a monolithic package to hand off to lower layers of the NRSC-5 protocol. It begins with a discussion of audio codecs, despite the lack of a specified codec in NRSC-5. This will provide the  necessary background in the role of the codec to create context for the discussion of the program service protocols.
This chapter discusses the Audio Transport Protocol, as well as two protocols that supply program related data to the audio transport—the Program Service Data protocol and the Program Service Data Transport protocol. These are shown in Table 4.1.
Program audio is melded with program related text to create each of the program services. The Main Program Service (MPS), and on FM IBOC, the optional Supplemental Program Service(s) (SPS), are assembled by the Audio Transport. NRSC-5-A Reference Document (4), Audio Transport describes the process of preparing the program services for Layer 2.
Table 4.1 Protocols Addressed in Chapter 4
If any supplemental program streams are being transmitted, the Audio Transport processes them in the same fashion as the main program. The reference document titled Program Service Data applies to Main and to Supplemental Program Service Data. The data related to each program audio channel is transported with that program's audio.
Figure 4.1 Audio Services Transport Funnel
Audio Codec
The term "codec" is a contraction of the term "coder/decoder," which describes the function of encoding data for transmission or storage, and decoding it upon reception or playback. NRSC-5 specifies no audio codec for main and supplemental audio services. All of the HD Radio-branded transmission and reception products utilize the proprietary HDC codec by iBiquity. HDC is a trademark of iBiquity Digital Corporation.
People involved in the technical side of the electronic media are likely to be familiar with the characteristics and purposes of so-called lossy coding/decoding schemes. HDC is one among an evolving marketplace of lossy codecs designed to optimize the use of transmission bandwidth and storage capacity by "losing" information about the audio that is considered inaudible or tolerable in its absence. As a result of lossy coding, the number of bytes of data required to transmit or store a given amount of audio is substantially less than that with the uncompressed audio data or a nonlossy form of data compression.
A codec selected for operation on IBOC broadcasting has upper limits on the bandwidth available to it. Nominally the hybrid FM IBOC channel has capacity for up to 96 kilobits per second (kbps) of Main Program Audio. The limit for AM is a little more complicated, due to the use of two layers of audio coding, referred to in the standard as "streams." The streams are hierarchical, bearing the names "core" and "enhanced." (Core audio is not related to the term "core" as it applies to the initial version of the NRSC-5 standard, discussed in the previous chapter).
Hybrid AM IBOC is set up with about 20 kbps core audio and 16 kbps enhanced audio. In the Audio Transport reference document (6), iBiquity calls this a "multistream codec" that is used to "provide robust coverage and fast tuning times." Core audio data is transmitted with a greater degree of error protection, making it more "robust" than the enhanced audio stream. It may also be transmitted in smaller blocks of data to provide more rapid acquisition. Core audio stands on its own, providing enough information to reproduce an audio output at the decoder. Enhanced audio, on the other hand, is only useful in the presence of core audio. As the name suggests, enhanced audio data, when it is available, provides an enhancement to the perceived quality of the core audio.
IBOC Frames
IBOC continuously transmits a series of "frames" of data. Each frame has a duration of approximately 1.49 seconds. This is true for both AM and FM IBOC signals. The frame is the basic building block of the IBOC transport. The characteristics of the framing of IBOC signals are discussed in Chapter 6.
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