Online Movies Audio Face-off, Pt. 1
For years, audiophiles have bemoaned the lackluster quality of MP3 audio files, saying they compress the life out of the music. Yet people still buy, stream, and enjoy MP3 (or similarly compressed) music files by the billions, so are they really that bad?
The music analogy of lossy, compressed MP3 files versus lossless, high-resolution .WAV (or similar) files is a great starting point for discussing the audio quality of streaming movie services. Without getting too deep into the weeds, streaming sites like Vudu, Netflix, and Apple deliver an audio bitstream using Dolby Digital Plus (DD+) while Blu-ray and UltraHD discs and titles downloaded from the Kaleidescape movie store use Dolby TrueHD. (We could also have a discussion of DTS versus DTS-HD Master or DTS:X, but since no streaming services yet provide or supports these, we’ll table that for later.)
A lossy codec like DD+ compresses the original full-resolution file, discarding information the encoder deems the listener won’t miss or wouldn’t have heard to begin with. This significantly reduces the original file size, making it easier to stream. But a lossless format like TrueHD retains all of the original information, resulting in a much larger file, which creates a problem for streaming services but isn’t a factor for a disc or for content downloaded from Kaleidescape.
According to Dolby, “Digital Plus provides up to twice the efficiency of Dolby Digital while adding new features like 7.1-ch audio, support for descriptive video services, and support for Dolby Atmos. Dolby Digital Plus is widely used by streaming and broadcast services to deliver surround sound audio at lower bitrates. 5.1-ch audio in Dolby Digital Plus is typically encoded at bitrates between 192–256 kbps.” (My emphasis.)
Dolby also says, “TrueHD is a lossless audio codec used widely on HD and UHD Blu-ray Discs. Dolby TrueHD supports up to 24-bit audio and sampling rates from 44.1 kHz to 192 kHz. Dolby TrueHD supports up to 7.1 audio channels as well as Dolby Atmos immersive audio. As Dolby TrueHD is a lossless audio codec, the data rate is variable. For example, Dolby TrueHD bitrates average around 6,000 kbps for Dolby Atmos at 48 kHz with peak data rates up to a maximum of 18,000 kbps for high sampling rate content.” (Again, emphasis is mine.)
So, what does this mean?
Well, if you take the highest DD+ encode rate—256 kbps—and compare it to the average for Dolby TrueHD—6,000 kbps—you’ll see that the TrueHD audio stream has more than 23 times more data allocated to it.
Fine. But can you actually hear and appreciate the difference? In Part 2, I’ll give you the results of my comparison of the same movies streamed on Vudu and Apple TV and downloaded from Kaleidescape.
Probably the most experienced writer on custom installation in the industry, John Sciacca is
co-owner of Custom Theater & Audio in Murrells Inlet, South Carolina, & is known for his writing
for such publications as Residential Systems and Sound & Vision. Follow him on Twitter at
@SciaccaTweets and at johnsciacca.com.