Dolby TrueHD Advanced 96k Upsampling


A bit of an old news (but only relevant now): Dolby has added a new trick to their TrueHD encoding. It allows studios and authoring houses a way to upconvert standard 48 kHz content (the sampling rate of most movies) to 96 kHz for Blu-ray.

At #CES2013 at Dolby, I got a chance to hear the results and it was very interesting.

First, a brief primer. Digital sampling allows for the recording of frequencies up to half the rate of the sample. In other words, to record a specific frequency, you need to sample at twice that rate. So a 48 kHz sample can record sounds up to 24 kHz. This is a common movie sampling rate, slightly higher than the common music (Audio CD) rate of 44.1 kHz. Now, the human ear is said to be able to hear from 20 Hz to 20 kHz. This is marginally true, in that a baby with perfect hearing can probably hear that range. Those of you capable of reading this sentence can hear in much narrower range, 20 Hz to 18 kHz if you have been taking care of your hearing.

Now a raw look at those numbers is likely causing most of you to ask: “what do I need musical information at 24 kHz, or 48 kHz for, when I can’t hear past 15 kHz?” Good question. Here’s the thing: Digitally encoding high frequencies can cause artifacts lower in the frequency range which may be audible. These are mostly “ripples” caused by the brick wall anti-aliasing filter that prevents sounds above half the sampling frequency from being encoded.

One of the core features in the upconversion processing is similar to what Meridian uses in their high-end CD players. It’s called an “Apodizing Filter.” In order to not make you fall asleep from reading the technical details, the short version is it moves “pre-ringing” artifacts to after a audio event. Here’s how Dolby describes it: “[The upconversion process] applies an advanced apodizing filter that masks preringing artifacts introduced upstream by analog-to-digital converters used in either the recording or playback stage by shifting them into postringing. The resulting increase in postringing is inaudible as it is masked. The apodizing filter does not remove any audio from the file. Instead, the filter shifts the location of unnatural artifacts that the brickwall filtering phenomenon of signal conversion introduces into content.”

Seems interesting and theoretically and technically sound. I’ve done bit rate and codec testing at various facilities before, and their listening test demos are done pretty much exactly as I’d do them. This time there wasn’t the double blind aspect I’d had with the bit rate tests, but the setup was similar. I also closed my eyes to see if I could pick the 48 from the 96 without looking (a TV in the room displayed the rate).

Despite warnings by Dolby that the effect was subtle, I found it to be not subtle at all, as did my fellow psychoacousticians and journalists also present at the event. This is not the sort of change in sound like going from MP3 to CD, or even from CD to high-rez. The level of change is more like switching to a better DAC: subtle, but noticeable. The sound was more open, that was definite. Other effects, like better attack and decay, were very noticeable on a short Joe Satriani clip from an upcoming concert Blu-ray. The high-hat at the beginning of the track sounded more natural, less “recorded.”

Going back and forth between 48 kHz and 96 kHz with some movie clips, the 48 kHz version sounded a little more closed in. The 96 was easier to listen to at higher volumes. A clip from The Dark Knight, where Batman kidnaps the mob money guy, had high-pitched beeps (of the bombs) plus extensive glass shattering. The 48 kHz version had a touch more harshness. The 96 kHz was just cleaner, wider and more “airy”.

Bottom Line

The beauty of this new tech is there’s no new equipment to buy on your end, it’s done entirely on the mastering side. The upconversion is done before you ever touch the disc. So your current system is likely fully capable of playing back “TrueHD with Advanced 96K Upsampling.”

Will you hear a difference? Well, unfortunately the kind of A/B comparison I was able to do isn’t really possible. The tracks on the discs you’ll buy are already upconverted. Even if they put standard 48 kHz Dolby TrueHD and 96K Upsampling content on the same disc (doubtful), you won’t know if you’re listening to the same mix, or if the tracks are at the same volume (this is also true with comparing codecs on the same disc). So consider it an added bonus: you’re not paying extra for the added (albeit upconverted) resolution, it’s just there to add to your enjoyment.

Look for the logo above on discs soon. The first titles, Satchurated: Live in Montreal, San Francisco Symphony at 100, The Right Stuff and the Chinese film The Flowers of War are already released as you read this article.



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