Understanding HD Radio: The program audio chain - Part 2

Audio Transport
NRSC-5 Reference Document 4, Audio Transport, briefly mentions the role of the Audio Encoder and the Audio [Encoder] Transport in its sections 4.2 and 4.3 to provide a top-level understanding of these coder functions residing outside NRSC-5.

The 32 audio packets per frame, per audio stream (core and enhanced) are delivered to the Audio Transport for preparing program service PDUs. Because of the variability of the packet size, upper and lower limits are established for the actual packet count in a frame. The reference document gives an example of a setting in which as many as 40 and as few as 24 may be delivered to the Audio Transport per stream per frame, as long as the average is 32.

The Audio Transport handles each stream independently, producing PDUs for each stream. Core and enhanced PDUs are transmitted on independent "Logical Channels," which are handled in Layers 2 and 1. Because of their variable size, the number of packets per PDU is variable. However, to maintain data flow like clockwork, the number of PDUs per frame is fixed for the particular logical carrying an audio stream.

In the simplest case, hybrid FM IBOC MPS consists of only one audio stream, with no segregation into a core and enhanced pair. One Main Program Service PDU is sent per frame, with the 32-packet average per PDU.

Core and enhanced audio streams are always transmitted in the other IBOC transmission modes: AM hybrid mode, AM all-digital mode, and FM all-digital mode. In these dual-stream modes, core audio packets are grouped 4 (average) per PDU, with 8 PDUs per frame (fixed). This provides the receiver with a faster-arriving, smaller PDU to disassemble, to more rapidly recover the core audio. Enhanced packets are always transmitted 32 per PDU (average), one PDU per frame (fixed).

Notes: The Effective Bit Rate of Payload column shows the nominal maximum rate of an MPS stream if the entire Logical Channel were employed for MPS. This table does not address all possible service modes of FM IBOC signals. Also, FM All-digital modes have additional RF channel capacity in the center of the channel that can provide full redundancy to the figures shown for MP5 and MP6 modes; this increases the robustness of the All-digital signal.

*The Audio Transport specification refers to the single audio stream of hybrid FM IBOC as "core," which has full fidelity, with no companion enhanced stream.

In addition to preparing for transport the MPS core and MPS enhanced audio streams, the Audio Transport prepares SPS audio in the same manner. Once the Audio Transport has received a batch of MPS audio coder packets and SPS audio coder packets, intended for the next frame, it groups each audio stream (core or enhanced) of each service (MPS or SPS) into its own PDUs. These PDUs of each service/stream are then grouped into the larger Audio Transport PDUs by the Logical Channels to which they belong. An SPS PDU can be transported alongside an MPS PDU in the same Logical Channel, but a core and an enhanced PDU of the same service cannot. (See Figure 4.6 for an example.)

The system operator allocates bandwidth for each MPS and SPS, the total of which cannot exceed the capacity of the Logical Channels to which they are assigned. The Audio Coder must be aware of this setting in order to pass 32 correctly sized audio packets for each service/stream to the Audio Transport. The Audio Transport assembles the packets into the PDUs, and must be aware of the bandwidth settings for each service and for each Logical Channel. It passes an Audio Transport PDU, consisting of MPS and SPS PDUs, down to Layer 2.

The Audio Transport performs an additional function for the MPS before sending MPS PDUs to Layer 2. Since the MPS audio is backed up by the analog audio, the Audio Transport must synchronize analog audio and Main Program Service audio. Supplemental services do not have an analog backup in NRSC-5.