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There is a statement to describe the underlined frequency in legend, it says:

Underline indicates no voice on this frequency.

What does this mean? We still need to ident the Morse code, doesn't it? So what's the "no voice" referring to?

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Navigation receivers can be used as a fall back for regular VHF/HF radio transceivers

Before data links, there were essentially two types of radio receivers in the flight deck, and this is still the case for most GA aircraft: "COM" used for voice communication, and "NAV" used to receive navigation signals. When COM transceivers fail, the pilot is left without information from ATC, and will follow their current flight plan and clearances.

However ATC may try to send additional information and new clearances using the NAV radios. In that case this is a one way communication, as NAV aboard the aircraft have no transmitting capability.

Information provided to ATCO by FAA in JO 7110.65:

a. In the event of lost communications with an aircraft under your control jurisdiction use all appropriate means available to reestablish communications with the aircraft. These may include, but not be limited to, emergency frequencies, NAVAIDs that are equipped with voice capability, FSS, Aeronautical Radio Incorporated (ARINC), etc. [...]

b. Broadcast clearances through any available means of communications including the voice feature of NAVAIDs.

The voice feature is present on all navaids, except TACAN. It may also be used to transmit ATIS when no communication emergencies are going on.

Pilot has also instruction to use NAV receivers, e.g. in AIM 6-4-1. Two-way Radio Communications Failure, 6-4-3. Reestablishing Radio Contact:

a. In addition to monitoring the NAVAID voice feature, the pilot should attempt to reestablish communications by [...]

enter image description here
Audio mixer, source

Navaids without voice capability

No voice or underlined frequency indicates ATC can't use this navaid for to transmit voice messages.

Such navaids have a class code W (without), e.g. CHANDLER NDB in Arizona:

enter image description here
Source: AirNav

enter image description here
Source: SkyVector

See also:

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    $\begingroup$ Why do you say The voice feature is present on all navaids, except TACAN. then you talk about Navaids without voice capability? So, not all Navaids have voice feature, right? BTW, the link to the AIM is broken, here is a good one faa.gov/air_traffic/publications/atpubs/aim_html/… $\endgroup$ – rvernica Jan 30 at 4:31
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    $\begingroup$ @rvernica: I believe I wanted to say all navaids are ready for voice transmission, but only some have a voice source connected to them (e.g. microphone). One key aspect is the design must allow the modulation index to stay within acceptable range (no over modulation) when navaid signal and voice are used at the same time. For example: In the ILS, the localizer signals are modulated at 2x 20% only, to allow 50% for voice, regardless the actual use of voice. For comparison, the glide path signal which is not used for voice is modulated at 2x 40%. $\endgroup$ – mins Jan 30 at 8:08

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