Multicasting
DEFINITION
Multicasting is the ability for an HD Radio Technology-enabled FM station to broadcast additional supplemental program service(s) (SPS) simultaneously with the main program service (MPS) using the station’s current frequency allocation. The configuration of the supplemental program services can be set by the broadcaster to support various program formats. There is currently no multicasting available for AM stations.
A maximum of three multicast channels (SPS) can be broadcast in addition to the main program service (MPS). The MPS channel is often referred to as the HD-1 channel and the SPS/multicast channels are known as the HD-2, HD-3, and HD-4 channels. Broadcasters who are part of an HD Radio data services network, such as BTC or TTN, should consult the service providers and their documentation to ensure bandwidth allocations comply with the applicable data providers’ requirements.
The different program types for the multiple channels on a station are maintained by iBiquity. For audio and data program types, please refer to these online resources:
- NRSC Supplemental Information
- http://www.nrscstandards.org/SG/nrsc-4-B.pdf
A suggested best practice is to inform iBiquity with a quarterly checklist of the program types on the main and supplemental channels. For an example of such a quarterly checklist, please refer to these online resources:
www.hdradio.local/broadcasters
There are two FM modes available to use when multicasting:
- MP1 Mode has a usable bandwidth of 96 kbps and is referred to as the Hybrid mode.
- MP3 Mode has a usable bandwidth of 120 kbps and is referred to as the Extended Hybrid mode.
The table below shows how the multicast channels can be divided depending on the station’s programming needs.
| HD-1 (MPS) | HD-2 (SPS1) | HD-3 (SPS2) | HD-4 (SPS3) | Mode |
| 48 | 48 | Hybrid – 96 kbps | ||
| 64 | 32 | Hybrid – 96 kbps | ||
| 48 | 24 | 24 | Hybrid – 96 kbps | |
| 48 | 48 | 24 | Extended Hybrid – 120 kbps | |
| 64 | 32 | 24 | Extended Hybrid – 120 kbps | |
| 48 | 24 | 24 | 24 | Extended Hybrid – 120 kbps |
| 32 | 32 | 32 | 24 | Extended Hybrid – 120 kbps |
IMPLEMENTATION
The audio bitrates can be scaled back to include the data bandwidth requirements for features such as Artist Experience, Real-time Traffic, etc. However, Importer Version 4.x or higher is required to split audio and data on the extended sidebands in MP3 mode.
Consider the following principles when using the MP3 Extended Hybrid Mode:
- The 96-kbps bandwidth in the primary subcarrier must be divided into channels separately from the 24-kbps bandwidth in the extended sidebands.
- An additional 20% of digital power must be available from the transmitter.
- Analog FM modulation should be kept within the FCC allowed limits; excessive audio processing and/or over-modulation will decrease the performance of the IBOC extended subcarriers.
AUDIO PROCESSING AND PROGRAMMING
- A station should adjust all parameters specific to each program in order to establish a consistent Applied Sound Experience across all programs. For a list of Applied Sound Experience types, please refer to this online resource:
http://www.ibiquity.com/broadcasters/us_regulatory/nrsc_supplemental_information
- Programming sources should be delivered via an audio compression algorithm with a bit rate sufficient so as not to introduce additional coding artifacts.
- EAS Alerts and legal FCC Station ID’s must be incorporated on each audio channel.
- Song Title and Artist text, Artist Experience, or static promotional text should be used as much as possible.
- Silence alarms and periodic monitoring should be incorporated to ensure consistent and high-quality operation of each channel.
- Programming and engineering staff should be equipped with receivers to monitor all HD Radio channels.
