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At Dolby, we’re always looking to improve on our current technologies and your entertainment experiences. Whether it’s scaling up surround sound from Dolby Digital 5.1 to Dolby Atmos, or finding ways to make Dolby TrueHD lossless audio sound even better, we’re never satisfied. There does come a time, though, when the advances we’re making become subtler, more complex, more intensely orchestrated by computers, and more technical.
If you’re more audio fan than audio engineer, as I am, it can all be a little dense. If you got a curious streak, again, like me, you want to know more about how it all works. This was how I felt about Dolby TrueHD’s newest addition, Advanced 96kHz Upsampling, and now that I’ve wrapped my head around it I thought I’d share it with you.
First, it’s important to establish what Dolby TrueHD with Advanced 96kHz Upsampling was designed to do. Dolby TrueHD is a lossless format - that is, it provides a bit-for-bit exact copy of the soundtrack that was played in-theater. On the face of it, that means that lossless audio can not be improved upon. So I thought, anyway.
Taking Sound Quality Beyond the In-Theater Soundtrack
Dolby’s Craig Eggers set me straight during the Fidelity Forum this spring. Lossless, he said, was lossless, of course, but simply representing sounds as they were on their original soundtrack might not be the pinnacle of quality.
Film soundtracks are generally played back at a sample rate of 48kHz a second. That means that for each second of audio, there are 48 thousand tiny slices made per second as it’s converted from analog, be it a human voice or an amplified instrument, to digital.
All those tiny bits make it possible for a continuous signal to be digitized and stored, and the number of samples is closely associated with digitizations ability to capture the full dynamic range of the source sound properly. Working with digital sound? The creation of the sound you hear probably began as analog, and even if it’s a bit of digital synthesis when it was recorded, it was recorded at a specific sample rate, which has the same effect on the audio.
(Higher sample rates sound better slowed down, for example. I remember Erik Aadahl revealing that he records all his found sounds at 192kHz, and slowed down the sound of his ID card as he spun it in a circle over his head, then slowing that sound down to create some of the sounds used for the Osprey aircraft in Transformers: Dark of the Moon. The audio sounded pristine.)
If you’re keeping score at home, I’d be willing you have one “gotcha” question in your mind: “But Thom,” you’re thinking, “If all movies are shown at 48kHz, and you can’t add fidelity to a recording later, then upsampling to 96kHz won’t get you anywhere.” And you’d be right, if all Dolby TrueHD with Advanced 96kHz Upsampling did was double the sample rate. That’s where the apodizing filter comes in.
It’s the Filter That Makes the Difference
You see, that 48kHz sample rate contributes to a problem known as “pre-ringing.” Essentially, the recorded sound accidentally announces itself with an impulse that you may not even notice when it’s there, but that you’ll notice when it’s gone. This problem most commonly affects higher frequencies, and was particularly apparent to my relative novice ear on snare drums, cymbals, and creaking sound effects.
Sound confusing? Think of it this way: your friend wants to surprise you by snapping open the door extremely quickly. The only problem? You’ve got a fan that blows air towards you whenever anyone approaches the other side of the door, ruining the surprising sound and gust of wind your prankster pal was going for.
In the case of pre-ringing, the sonic artifact essentially diminishes the impact of the sound it precedes. Snare hits are murkier, cymbals crash more flatly.
But now, if I can put on my mind-reading hat again, you’re thinking to yourself, “But Thom, why did it take you this long to address this? I’ve been listening to tainted sound for all these years and there was a fix?!”
Not until recently. And not in theaters.
That’s where the 96kHz sample rate comes into play. The apodizing filter requires it to gain the “headroom” it needs to function properly. Below 88.2kHz? No dice. Until now, there was a good chance that your stereo receiver could handle 96kHz but your movies were still coming to Blu-ray with 48kHz audio and no apodizing filter. Because movies are shown at 48kHz in theaters, their soundtracks generally don’t exist at a higher resolution. And so, the apodizing filter sat in high end components reserved for the most discerning ears in the audiophile community.
Now, it’s in your signal chain on Blu-rays mastered for Dolby TrueHD with Advanced 96kHz Upsampling, and it’s another Dolby technology you’ve got to hear to believe. Hopefully after reading this, you’ll understand it all at least as much as I do!