only text information can be displayed. The narrower bandwidth needed in either all-digital mode compared to hybrid mode reduces possible interference to and from stations broadcasting on adjacent channels. However, all-digital AM lacks the analog signal for fallback when the signal is too weak for the receiver to decode the primary digital carrier. Four stations operate as all-digital / digital-only broadcasters: WWFD has had special temporary authority from the FCC since July 2018 to broadcast all-digital. WMGG (since January 2021), WFAS (since May 2021) and WSRO (since December 2021) broadcast all-digital under new rules adopted by the FCC on 27 October 2020 that allow any AM station to voluntarily choose to convert to all-digital operation. WWFD experimented with using a digital subchannel, operating a second channel (HD2) at a low data rate while reducing the data rate of the primary channel (HD1). In October 2020, the FCC concluded from WWFD's experiments: "The record does not establish that an audio stream on an HD-2 subchannel is currently technically feasible". The FCC requires stations that wish to multiplex their digital AM signals to request and receive permission to do so; in early 2020 it rejected a multiplex request from WTLC. FM The FM hybrid digital / analog mode offers four options which can carry approximately 100, 112, 125, or 150 kbit/s of data carrying (lossy) compressed digital audio depending upon the station manager's power budget and desired range of signal. HD FM also provides several pure digital modes with up to 300 kbit/s rate, and enabling extra features like surround sound. Like AM, purely-digital FM provides a "fallback" condition where it reverts to a more robust 25 kbit/s signal. FM stations can divide their datastream into sub-channels (e.g., 88.1 HD‑1, HD‑2, HD‑3) of varying audio quality. The multiple services are similar to the digital subchannels found in ATSC-compliant digital television using multiplexed broadcasting. For example, some top 40 stations have added hot AC and classic rock to their digital subchannels, to provide more variety to listeners. Stations may eventually go all-digital, thus allowing as many as three full-power channels and four low-power channels (seven total). Alternatively, they could broadcast one single channel at 300 kbit/s. FCC rules require that one channel be a simulcast of the analog signal so that when the primary digital stream cannot be decoded, a receiver can fall back to the analog signal. This requires synchronization of the