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iBiquity's HD Radio system employs a process known as blending with time-diversity to ensure fast tuning and graceful degradation of the received digital audio back to analog when the HD Radio signal is lost. Diversity delay is the amount of program audio delay that is required to be inserted into the analog broadcast's signal in order to match the inherent latency of the HD Radio digital audio - typically about 8 seconds. Proper diversity delay synchronization and level alignment between the HD Radio Main Program Service (MPS) and the legacy analog audio program are critical to the listener's experience and ultimately the successful implementation of any HD Radio installation.
In order to avoid annoying artifacts during blends, the audio alignment between analog and digital should be kept within 1ms or about 50 audio samples. While many tools exist to assist the engineer in obtaining optimal initial alignment, Ethernet timing, changes in audio processing and synchronization issues can cause continual drift or sudden shifts in the otherwise perfectly aligned installation. It becomes a constant battle to detect and correct slight misalignments before they become noticeable to listeners.
The M4DDM is a turnkey, single-box solution designed specifically to solve one of the most nagging problems facing HD Radio station engineers today - drift of time alignment between the analog and HD-1 audio. The DSPrecision platform used in the M4DDM uses a newly developed DSP-based architecture that significantly improves the M4DDM measurement accuracy. The new TimeLock algorithm, for example, can quickly resolve up to 14 seconds of program diversity differential with accuracy to one audio sample.
Using its selective off-air tuner, the M4DDM measures the MPS analog and HD-1 digital audio diversity and generates a continuous stream of correction vectors to keep the analog and digital audio perfectly time and level aligned. These correction vectors can be processed internally by the M4DDM to delay the digital program audio or can be sent via Ethernet to an HD Radio Embedded Exporter, such as Harris' new HDE-200 to provide the necessary delay. Additionally, the M4DDM has digital audio outputs for confidence monitoring and alarm outputs for loss of program audio as well as HD Radio data and RBDS data payloads.
The M4DDM is compatible with all existing HD Radio NE-IBOC Gen I and Gen II installations as well as new installations using Embedded Exporters from Broadcast Electronics, Continental RVR, Harris and Nautel running iBiquity MPS Framework version 4.3 or later.
The algorithm used in the M4DDM is based on a new digital audio correlation methodology developed by Harris and DaySequerra and represents a substantial improvement compared to the original HD Radio correlation algorithm now in use. While the original algorithm can only correlate audio that is already within 300ms of alignment, the new DDM algorithm is capable of resolving up to 14 seconds of program diversity differential and is much more robust against processing differences and other artifacts. The TimeLock algorithm determines the actual delay between the analog and HD Radio HD-1 digital audio signals, calculates the number of audio samples that must be added or subtracted to synchronize the two audio streams, determines the level offset required, and then generates a correction message. This M4DDM correction message can be formatted as a Harris proprietary RTADD control protocol message to be sent to Harris' HDE-200 Exporter via an Ethernet network connection; the RTADD Delay Control issues the correction to the Exporter's Analog Program Delay.
For more information on this product, please visit the DaySequerra website.
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