By the Encoder.How is the bit rate determined?
DTS Core payload stream is always going to be at 24/48.
This core can be at either 1509Kb/sec or 754Kb/sec - you decide when you encode the file.
Where DTS get really clever is in the use of this core plus extensions. The extensions are either additional channels or extended frequency response in so-called "Legacy" mode. Extended frequency response up to 24/96 removes the 754Kb/sec option completely - you can *only* encode these streams at 1509Kb/sec (unless we are talking about DTS-HD files which is another subject altogether).
Additional channels is the so-called "ES" mode, and can be either 5.1 with matrix derived Cs, or 6.1 where the Cs is discrete. This can be at either 1509 or 754Kb/sec bitrate.
To encode at 24/96 or 6.1 requires either the DTS-PSE encoder or one of the new DTS-HD encoders.
What does it all mean for the end user? Quite simply the DTS decoders that are in the player or amplifier will *always* decode the core stream, regardless of what the encoded file has in it's extensions. If the decoder can decode the extended frequency response, or the additional channels, it will do so automatically. DTS' system ensures that the end user always gets the highest quality audio his system is capable of decoding without the need for anything to be set up.
Hope this helps.