What is lossless audio?

[Dubstar]

Originally Posted by BIslander

PCM is the basic digital audio format used with CDs, DVDs, and Blu-rays. Movie masters are usually recorded as PCM. The quality of a PCM master depends on the sample rate and bit depth used to make the recording.

DD 5.1 and DTS are lossy compression codecs that were developed to save the space needed to squeeze a 5.1 movie soundtrack on a film or a CD. The PCM originals are too big to fit on films, CDs, and the audio area of DVDs. So, Dolby and DTS each developed its own codec that compresses (zips) the PCM file down to a size that will fit where it needs to go. The files have to be squeezed so much that some of the data removed during compression cannot be restored during decompression. That's why they are called lossy codecs - some of the data is lost.

TrueHD and dts-MA are lossless compression codecs. Blu-ray discs have more space. So, the PCM files don't need to be compressed as much to fit on a disc. (Actually, they don't have to be compressed at all. BDs can handle multichannel PCM tracks. But, studios still want to conserve space which can be used for more extras.) When a lossless file is decompressed, everything that was removed during compression is put back in - nothing is lost.

DVD-Audio discs are DVDs formatted with lots of space for audio instead of video. The DVD-Audio format uses MLP, which is another lossless compression codec. As Big Daddy noted, Dolby uses MLP as the foundation for TrueHD.

So, you have the original, uncompressed PCM. TrueHD, dts-MA, and MLP are all ways of losslessly compressing the original PCM to save space. When decompressed, each of those codecs produces an output identical to the PCM original. CDs are also encoded using PCM. But, because of size restrictions, CDs are limited to two channels of PCM with 44.1kHz sampling rates and 16 bits. By comparison, a DVD-Audio disc can have up to six channels at 96/24 or stereo at 192/24.

SACD is not based on PCM. It uses DSD - a completely different type of digital audio. But, it's a format that can support multiple channels of high resolution audio. So, in terms of quality, it is generally equated with uncompressed PCM.

though I never understood why lossy sound occured on DTS laserdiscs, where it seems like there would be more than enough space to encode both video and audio.