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IBOC RADIO APPLICATION NOTE

Connecting Audio Processing, Importer, Exporter and Exciter

The new Exgine architecture for HD Radio has raised many questions on hookup and variations to the standard configuration. Flexstar is the Harris implementation of Exgine, which utilizes an Importer and Exporter at the studio, and an HD exciter at the transmitter site. Fig.1 shows a signal flow of how all the components would normally be connected, and Fig 2 shows a zoomed view of the studio end with Ethernet connection detail. Your installation may vary from this configuration, but a good understanding of this preferred configuration will aid in the understanding of its variations. Note first that because all data input and output must pass through a single Ethernet connector on the Importer and Exporter, a user-supplied Ethernet switch (not hub) will be required for interconnect. Importer and Exporter must communicate bi-directionally with each other through a reliable connection, so co-location of these two boxes is highly recommended. Originally specified as a UDP connection only, iBiquity recently implemented TCP/IP as a choice protocol between Importer and Exporter to improve reliability of the connection. The Exporter output, on the other hand, remains a UDP connection in order to traverse 950 MHz STL systems, which are typically half-duplex. If provisioned at 300 kbps initially, this stream will accommodate additional services later on without the need to allocate more STL bandwidth. The second point of note is that the Exporter provides a 44.1 kHz clock sync signal to the source input feeding the Importer AES input. By clocking the AES stream in this manner, over-run and under-run of the data buffer in the Exporter can be prevented. If the input to the Importer came from a free-running (non-synched) source, the buffer would "accumulate" the sampling error between the two clocks over time. Although the buffer is large enough to accumulate these errors over days or weeks, the result of an over-run (filling) or under-run (emptying) of the buffer is an eventual on air audio glitch (dropout). The best place to inject the sync is at the nearest upstream rate-converter equipped device. Do not inject the sync at the "clock in" port of the Importer. Many, but not all, digital audio processors have rate converter and external sync capability at their inputs. In this case, an inexpensive external rate converter, such as the Behringer Ultramatch Pro SRC-2496 can provide the necessary sync with the reference from the Exporter. For main host channel (FM) audio processing, a diversity delay (with an internal relay bypass) is provided in the Exporter as a "loop thru". If a dual processor (single input-

dual output) is employed, the studio's AES stream must pass through that processor first before being split for the FM diversity delay path and the digital MPS path. See Fig 3. It may be advantageous to externally bypass the diversity delay provided in the Exporter if the processor is equipped with its own diversity delay. Not only does this provide the host channel with immunity to failures within the Exporter, but it also enables the user to utilize the composite output of the processor. See Fig. 4. Transmitter Site Referring to Fig. 5, note that at the transmitter site the HD Radio signal is a pure data stream (300 kbps). It interfaces with the Exciter through a single RJ-45 connector. On the host channel audio side, however, the customer may continue to employ whatever audio format was previously used--Analog, AES, composite, etc. Importer with Dexstar When employing Advanced Application Services (AAS), including Supplemental Program Services (SPS) on pre-Exgine architecture, such as Dexstar, alternate configurations must be employed. Because earlier generation equipment did not include an Exporter (those functions were performed in the Dexstar), placement of the Importer at the TRANSMITTER site is first considered. Doing so fulfills the requirement that a reliable bi-directional TCP/IP connection be established between the Importer and the exporter functions (in this case, in the Dexstar). It also allows the Dexstar to send the required 44.1 kHz AES sync to the audio processor for the Supplemental Channel, as described above. See Fig 6. For stations with existing LAN/WAN connectivity at the transmitter site and the appropriate firewalls, the Importer can be located at the studio, and utilize this network connection to communicate (bi-directionally) with the Dexstar. However, there is no means to sync the incoming AES stream to the Importer from the Exciter's 44.1 kHz clock, so risk of buffer over-run or under-run must be considered, or the occasional audio glitch tolerated. For this reason, this configuration is considered an "interim" solution. See Fig. 7. In conclusion, the advantages of deploying the full Flexstar solution, including Exporter, will become evident as the number of Advanced Application Services grows. It is recommended that users plan their system configurations to be able to take advantage of new services without the need to allocate additional STL bandwidth for discrete signal delivery to the transmitter site.

MPS Audio AES3 32kHz to 96kHz

FM +HD Audio Processor Dual Output

AES3 AES3 ONLY AES3

Harris FlexStar HDE-100 Exporter

HOST CHANNEL AUDIO

Dedicated Ethernet (~300kbps Unidirectional UDP)

ORBAN 8500 or OMNIA 6-EXi

MPS Audio AES3 32kHz to 96kHz

FM +HD Audio Processor Dual Output- w/ delay

AES3, or Composite

HOST CHANNEL AUDIO

AES3

Harris FlexStar HDE-100 Exporter

Dedicated Ethernet (~300kbps Unidirectional UDP)

Fig. 4 Dual Processor Implementation using Diversity Delay in Processor

44.1 kHz clock

MPS Audio AES3 32kHz to 96kHz

Harris ePAL

Harris Dexstar Exciter

MPS PAD (400bps)

STL

SPS AES3 Audio

SPS Audio Processor

AES3

Harris Neustar SPS Processor

SPS PAD (400bps)

Harris FlexStar HDI-100 Importer

AAS Data

Fig. 6 Importer at Transmitter Site for Dexstar Installations

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Microsoft Word - Flexstar-AppNote#1_RevA1.doc