Broadcaster Sent Events

Broadcasters can send events that trigger changes to the receiver/display.

Text information updates

• Text information is normally transmitted as part of the DAB audio stream (DLS) or on FM RDS (RT).
• For the stucture and purpose of ‘text information’ visit the section of the Annex.
• Additional functionality is available when sending text information, which helps optimally formatting and displaying text.
• When updating text information, do not update more frequently than once every 20 seconds. Updates more frequent than this may be discarded.

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‘CLEAR DISPLAY’ command

Some displays can’t show all your text information at once and will scroll it. When you change the text message, the previous message will continue scrolling until it’s fully displayed, which may take tens of seconds and delay your new text from appearing.

If it’s important for your new text to appear immediately, send it with the “CLEAR DISPLAY” command. This should stop whatever text is scrolling and immediately display the new text. Note that not all receivers or multiplexers support this function.

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Visual information updates

Visual information can be transmitted as part of the DAB audio stream (X-PAD/Slideshow), as a DAB data service (Packet Channel/Slideshow), or provided over IP (RadioVIS). See the Annex for more information.

Visual content will usually update on screen as soon as it’s fully received.

Radio reception can be unstable, so a single transmission might not be received. The usual approach is to repeat the transmission throughout the time you want the visual displayed, so at least one reception will be successful.

Transmission time depends on file size and available bandwidth. Acquisition can take 30 to 60 seconds and varies with reception conditions, so you can’t be certain exactly when it will arrive and display.

When updating visual information, do not update more frequently than once every 20 seconds. Updates more frequent than this may be discarded.

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More accurate control of display time

You can control more precisely when a visual appears using the “MOT TriggerTime” function, which specifies the exact UTC time you want the image displayed.

By knowing the file size and transmission speed, you can estimate how long the visual needs to transmit (once, twice, or three times) and start transmission ahead of the required time. The visual will be received and held at the receiver.

When transmitting the visual, you can specify a TriggerTime for when you want it shown, or use “now” as a special value. If you’re not sure of the exact timing, send the visual without a TriggerTime, then update the TriggerTime to “now” when you want it displayed.

flowchart LR
    id[Send Image, no TriggerTime] --> id2[Send Image, no TriggerTime] --> id3[Update TriggerTime] --> id4[Image Displayed]

TriggerTime inconsistencies

Be aware that some vehicles implement the DAB Slideshow TriggerTime function inconsistently.

Some vehicles ignore the MOT TriggerTime and show the visual as soon as it’s been received once, which may be much earlier than you intend.

To work around this:

Control Transmission of the visual

Control transmission so that all packets (chunks) of the visual except the last one are transmitted once, twice, or three times, then send the final packet once, twice, or three times very close to the desired display time. On vehicles that respect MOT TriggerTime, the visual will appear at the TriggerTime. On other vehicles, it will appear as soon as the final packet is decoded (which may be a few seconds earlier).

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Additional considerations:

• Some vehicles cannot cache multiple Slideshow images. Always make sure you’ve sent a Slideshow image shortly before triggering it.
• Some vehicles don’t understand absolute Trigger Times, only “now”. Always send a TriggerTime “now” for a visual shortly before you want it displayed.