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Old 06-07-07, 10:57 AM   #2
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Default Re: I guess I'm a tad confused (regarding lossless audio)

Originally Posted by superklye
With this push for the highest resolution audio possible, I don't get what the point of multiple lossless codecs is.

Dobly True HD = losslessly compressed audio
DTS-HD MA = losslessly compressed audio
LPCM = lossless audio

It's all the same audio, right? 24-bit/96Hz or whatever (unless we're up to 32-bit and I never got the memo) and when it's decoded by the players and output as 5.1 LPCM audio...does it really matter? It's all the same audio losslessly compressed with a different codec to make it sound more impressive, correct?

Or am I missing something? Does DTS-HD MA really sound better than Dolby Tue HD and better than LPCM?
Okay, here we go. It is a bit confusing. First of all, you have to understand that there are three independent things we are talking about.

First, there is bit depth. This is in reference to your "24-bit" above. The common bit depths used these days are 16-bit, 20-bit, and 24-bit. The higher the bit depth, the higher the Signal-to-Noise ratio (SNR). 16-bit is what CD uses and is generally the highest bit-depth people can actually discern a difference from. Meaning, most can tell the difference between 8bit/12bit and 16bit, however it is much harder to notice any difference between 16bit and 20bit/24bit. This is one of the reasons high bit depth formats like DVDAUDIO failed.

Second, there is sampling rate. This is in reference to your "48khz" above. The common sampling rates used these days are 44.1KHz, 48KHz, 88.2khz, and 96KHz. The higher the sampling rate, the higher the frequency range reproduced. If you divide the sampling rate in half, you get the frequency response range. So, a CD which uses 44.1khz sampling rate has a frequency response up to 22khz. A DVD which uses 48khz sampling rate has a frequency response up to 24khz. And, an HD DVD which uses a 96khz sampling rate has a frequency response up to 48khz. But here is the kicker - humans can only hear up to around 18-19khz maximum and most speakers only reproduce up to 20-21khz. Therefore, it is pretty much accepted that sampling rates higher than 48KHz are pretty much a waste since we can't hear those frequencies and most speakers can't reproduce them anyway. Another reason why high sampling rate formats like DVDAUDIO failed.

Finally, there is compression and bitrate. This is where lossy and lossless audio comes in. Lossy compression means part of the original audible signal is thrown away, resulting in differences between the original master and the compressed final result; codecs that are lossy include Dolby Digital, Dolby Digital Plus, DTS, and DTS-HD.

Then there is lossless audio. There are two types of lossless audio, perceptual and mathematical. Companies generally do not reveal which type of lossless audio their codec is. Mathematical lossless means that there is no mathematical difference between the master and the compressed final result, if compared in a waveform they look the same. Perceptual lossless audio means that there is no audible difference between the master and the compressed final result, but if compared in a wave form they look different. For instance, if a codec threw away all frequencies above 20khz but kept everything below 20khz perfectly intact, it would be considered perceptually lossless since we cannot physically hear above 19khz. Dolby TrueHD and DTS-HD Master Audio are examples of lossless audio codecs.

Now here is the confusing part. Lossless does not mean you are necessarily getting the exact original master, because there may be some downconversion of the bit resolution prior to compression. Most movies have masters at 24bit/48khz, however for most lossless audio compressions the bit resolution is downconverted to 16-bit. Though we lose some SNR, generally we cannot tell the difference as mentioned before, and the lossless track ends up taking nearly half the space at 16bit compared to 24bit. All lossless means is that whatever you feed it, it will come out compressed the same. If you feed it crap and encode it losslessly, it will come out like crap because that is what you fed it - and the studio could still claim "lossless". Bitrate is basically the amount of space a soundtrack takes up per second. LPCM (which is not on HD DVD) is a huge bitrate hog taking up 3x as much space as a lossless TrueHD soundtrack at the same bit resolution/sampling rate with no difference in quality. DTS-HDMA is also inefficient compared to TrueHD, and can take up twice as much space as TrueHD with no difference in quality. Of the three lossless techniques, TrueHD takes up the least space by far and sounds identical to an uncompressed signal.

So, when you are talking about soundtracks on an HD DVD, the following are all possibilities:
16bit/48khz lossy (common, used by Warner often with DD+ at 640kbps bitrate)
16bit/48khz lossless (common, used by Warner often with TrueHD at 1.5mbps bitrate)
20bit/48khz lossy (common, used by Paramount with DTS-HD at 1.5mbps bitrate)
24bit/48khz lossy (common, used by Universal often with DD+ at 1.5mbps bitrate)
24bit/48khz lossless (less common, used by Universal with TrueHD at 2.8mbps bitrate)
24bit/96khz lossy (rare, used only by RnB Films on the HD DVD Chronos,

The best is probably the route Warner is going with 16bit/48khz lossless compressed as you get an efficient bitrate (1.5mbps) and one generally can't tell the difference between 16bit and 24bit anyway, so its not worth spending twice the space on a 24bit encode. 16bit/48khz is technically a bit better than CD quality, and for us CD has proved fantastic quality. And finally, compressed lossless is identical to uncompressed so there is no point wasting bitrate with uncompressed LPCM that takes up 3x as much space.

Finally in terms of codecs, in general it goes like this:

Audio quality (higher is better, < means greater/less than,<= means greater/less than or equal to, = means equal to)
DD <= DTS <= DD+ = DTS-HD < TrueHD = DTS-HDMA = LPCM (no compression)

Space taken up (higher is worse, < means greater/less than, <= means greater/less than or equal to, = means equal to)
DD <= DD+ = DTS = DTS-HD <= TrueHD < DTS-HDMA < LPCM (no compression)

So in terms of lossless codecs, TrueHD, DTS-HDMA, and LPCM all should sound identical since lossless implies audibly identical to the master being compressed from. However, though the three sound identical they all take up different amounts of space, with TrueHD being the most efficient, DTS-HDMA being in the middle, and LPCM being the biggest space hog.
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