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The mot-encoder encodes Program Associated Data (PAD) which gets embedded into MP2/AAC audio frames. It supports the transmission of DLS texts and MOT Slideshow slides. The mot-encoder is contained in the FDK-AAC-DABplus package and was contributed by CSP [1]; further improvements were made by the OpenDigitalradio team.

It can be used with both Toolame-DAB and FDK-AAC-DABplus.



Please see FDK-AAC-DABplus#Installation.


Please call mot-encoder without parameters to see all available options.

Example 1: Transmission of DLS texts (file content is UTF-8; transmit as UTF-8) using 6 bytes PAD (short X-PAD):

mot-encoder -o /tmp/pad.fifo -t dls.txt -c 15 -p 6

Example 2: Transmission of MOT Slideshow using 34 bytes PAD:

mot-encoder -o /tmp/pad.fifo -d ./slides -p 34

The necessary FIFO can be created by calling e.g.:

mkfifo /tmp/pad.fifo

If you do offline encoding of a DAB programme, it makes sense to use -s 0 - otherwise mot-encoder waits (by default) 10 realtime seconds before transmitting the next DL or slide.

Supported services

Dynamic Label Segment (DLS)

DLS texts (according to ETSI EN 300 401, ch. can be embedded into PAD and are read from a specific file. This file is read everytime before the text is prepared for transmission, therefore it can be replaced in the meantime to change the transmitted text. The specification limits the size of a DLS text to at most 128 bytes - depending on the selected charset these byte amount can be used to transmit up to 128 characters.

MOT Slideshow (MOT SLS)

The MOT Slideshow (according to ETSI EN 301 234 and ETSI TS 101 499) allows the transmission of slides in JPEG or PNG format. The images in a specified folder are therefore transmitted one after another, until all images have been processed and the procedure repeats. If one image does not fulfill the 50 KB file size limit or the recommendation of a 320x240 px resolution, it is resized before transmission and converted to JPEG to fulfill the mentioned requirements (if needed, also a black border is added).

PAD lengths

Currently the following fixed PAD lengths are supported: 6 (short X-PAD; only DLS), 23, 26, 34, 42 or 58 bytes per frame. The PAD length must be set to the same value both at mot-encoder and audio encoder command line. In the future, the PAD length will be user-definable and must be set at a single location only.

Communication with audio encoder

The communication of the mot-encoder with the audio encoder is currently done via a FIFO and therefore unidirectional. As this does not allow flow control, the mot-encoder cannot take advantage of the complete available PAD bandwidth. This will change in the future, when a bidirectional communication protocol is established.


The current protocol is in use since commits 5c6b9fb (fdk-aac-dabplus) and 182d08c (toolame-dab). On mot-encoder's side, each write to the FIFO consists of (padlen + 1) bytes, divided into the following components (all widths in bytes):

| zero padding |  X-PAD  | F-PAD | used PAD len |
|            padlen              |       1      |
               |  <used PAD len> |

X-PAD and F-PAD must already be in the reversed transmission byte order. The unused part at the beginning must be filled up with zeros and is ignored by the audio encoder. The unused PAD bytes within the audio frame result in additional bytes available to audio data.

Known receiver issues

Many older receivers only support the EBU Complete Latin based repertoire (charset 0). This is the motivation for the character set conversion from UTF-8, a modern and universal encoding, towards this widely supported character set.

In case other conversions are needed, please get in touch with the developers on the mailing list.

Brand Model Firmware version Description
Dual DAB4 dab-mmi-FS2027-0000-0021_V4.0.10.25467-1A24 DL: supports only charset 0
SANGEAN DPR-17 DPR17-vp02D-EU5V DL: supports only charset 0; texts having exact 128 bytes are not displayed
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