The AAC standard assumes two time windows (frames). Both 1024 and 960 are provided. Because of compatibility with DAB, DAB+ uses 960 frames. The reason is that the development of DAB was based on MP2 encoding.
Important to remember: even if you use AAC as STL for the MUX, a conversion (re-encoding) from 1024 to 960 frames will be performed, resulting in a small loss of quality. The only solution is to use a DAB+ encoder in the studio.
Most AAC encoders/streamers, be it hardware or software (PC), use frames of 1024 samples. Only DAB+ encoders using 960 frames can be used to connect the studio to the DAB+ MUX without loss of quality.
IMPORTANT: The encoder and decoder determine the audio quality. Encoded audio contains spectrum information that is only interpreted by the decoder. The more correctly the encoder encodes this spectrum information the better the audio will sound after decoding by a decoder.