What's The Difference Between Onboard Decoding And Bitstream Pass-through?
Although Dolby TrueHD and DTS-HD Master Audio aren’t compressed in the same way that the original Dolby Digital and DTS formats are, they still reside on your Blu-ray disc in condensed form, a stream of bits, or bitstream, that has to be decoded properly before it can be amplified and sent to your speakers. “Bitstream pass-through,” as it's often called, means that your Blu-ray player delivers that unaltered data to your receiver or surround sound processor, which then does all the necessary decoding and digital-to-analog conversion itself. This requires that your player and receiver/processor both be equipped with HDMI connections, and that your receiver/processor is capable of doing the decoding.
If your receiver isn’t capable of such decoding, your Blu-ray player may be able decode these formats itself, “onboard” in other words, and deliver the resulting audio as PCM audio to your receiver/processor via an HDMI, or via 5.1 channel analog audio connections.
Which method you should use depends of course on what sort of gear you have or plan to purchase. Actually, though, even if your system is capable of either form of decoding, the answer isn’t quite cut and dry. Each method has its advantages and disadvantages. Bitstream pass-through to a high-quality receiver or processor generally means that you’re going to get better quality audio and more control over parameters like bass management. But this method will keep you from hearing some of the audio features on new Blu-ray discs with picture-in-picture and sound-mixing bonus features. Conversely, onboard decoding will allow you hear such features, but most Blu-ray players don’t feature onboard decoding of DTS-HD Master Audio.