DTS-HD in both flavours is backward compatible with existing DTS decoders such as that in your Onkyo, but you will only hear the 'core' of the soundtrack.
The core is usually 1509kbps, so although you won't get the full benefit of DTS HD High Resolution or Master Audio, it should sound better than DTS from DVD which commonly operates at 754kbps.
The following diagram should give you an idea of what your decoder 'sees' when dealing with the various DTS soundtracks it may encounter on Blu-ray:
Whoever talked to you about TrueHD may have meant that while DTS-HD has a core that can be extracted and used by legacy decoders, TrueHD does not. A separate Dolby Digital soundtrack is usually also present on a disc to compensate, as TrueHD decoding is not mandatory on Blu-ray players (while DTS and conventional Dolby Digital decoding are).
Adam
