When the FCC's IBOC rules took effect on Sept. 14, 2007, much of the attention was focused on the use of AM IBOC at night and the changes to the notification procedures for multicasting and the use of separate antennas. A change that has received less attention is the ability for stations to operate in extended hybrid mode. Before the rule change, stations were allowed to operate only in the hybrid mode.

Both modes of operation transmit an analog and a digital signal within a station's spectrum mask, but the extended mode provides additional bandwidth for the digital signal. In hybrid mode, an FM station can deliver up to 96kb/s of data. In extended hybrid mode, additional carriers, called extended partitions, are added on the inner edges of the digital spectrum to provide up to 146kb/s. Figure 1 shows a spectrum layout.

Figure 1. The extended hybrid mode.

Hybrid mode places 10 carries on either side of the analog FM signal. Extended hybrid mode adds one, two or four additional carrier pairs to the signal, which provides an additional data capacity of about 12.5kb/s per carrier pair. The additional 12-, 25- or 50kb/s allows extended hybrid mode to pass 108-, 121- or 146kb/s of digital data.

The extended hybrid mode should interest broadcasters who provide multicast channels, because the additional bandwidth can be used to provide a wider pipe for one or two multicast streams, or it could be used to add additional streams at lower bandwidths.

Effects to analog

In 2004, NPR Labs evaluated the effect of the extended hybrid mode to existing analog service. Those tests used the 121kb/s (two carrier pairs) hybrid operation mode. The study, titled Host Compatibility Measurements for the Extended Hybrid Mode of IBOC Digital Audio Broadcasting and written by NPR Senior Technologist John Kean, is available on the NPR Labs website (www.nprlabs.org).

The test measured the signal-to-noise ratio (SNR) from the digital carriers to the analog reception using the hybrid and extended hybrid modes. NPR evaluated car, home hi-fi, and portable/compact receivers and found that car and home suffered minimal effect on the SNR. Portable receivers did not fare so well and experienced an increase in SNR from almost nothing to nearly 8dB. NPR also evaluated the effect of extended hybrid mode on SCA operation and found that there was no real change in SNR.

Unfortunately, the test did not investigate the effects of using four carrier pairs for the maximum data rate. The results of the middle-ground test have promising results, but until the maximum data rate can be evaluated, the maximum use of extended hybrid mode may have to wait until HD Radio receiver penetration grows dramatically.

Ahead to digital

It's premature to know how broadcasters will most effectively use the additional bandwidth available in extended mode hybrid operation. Multicasting appears to be the immediate benefactor to the added bandwidth, but stations are still learning how to use the data path to its fullest potential. HD Radio is still a work in progress, so enhanced data or additional multicast channels are two of the options available today. The future may provide other opportunities for the spectrum.