Overview
NAB Labs is engaged in an all-digital AM IBOC test project with the goal of fully developing a performance record of operation in this mode. Deployment of all-digital IBOC services would require a change to the FCC rules, but to-date very little technical information on all-digital operation has been entered into the public record which is an impediment to FCC action in this regard.
Since November 2012, NAB Labs has been working with a number of broadcasters and transmission equipment manufacturers to conduct field tests of all-digital AM. As of May 2014, tests have been conducted at five locations (see graphic), and additional tests are planned to take place throughout 2014.
Click image to enlarge
NAB Labs is also in the process of developing a radio test lab for conducting all-digital AM interference testing in a laboratory environment.
Background
The HD Radio in-band/on-channel (IBOC) digital radio system, developed by iBiquity Digital Corporation and standardized by the National Radio Systems Committee (NRSC), can operate in two fundamental modes: hybrid and all-digital. The hybrid mode, which utilizes a radio signal consisting of the legacy analog signal (AM or FM), was authorized for use in the U.S. by the FCC in 2002, and at present over 2,200 radio stations are broadcasting a hybrid HD Radio signal.
iBiquity and others have contemplated that the transition to digital radio using the HD Radio system could encompass two phases: the introduction of digital radio services using the hybrid signal (spectrum shown in upper figure), which would continue to allow for reception of the main channel audio portion of the signal on legacy analog receivers, and an eventual transition from the hybrid signal to the all-digital signal (spectrum shown in lower figure), when there was sufficient penetration of HD Radio receivers in the marketplace so as not to disenfranchise listeners (on the order of 85% of listeners).
Click images to enlarge
Use of the all-digital signal would be advantageous because of its increased payload capacity and robustness, however, these advantages must be traded off against the loss of reception by analog receivers, which are numerous.
