Sound Matters: High Definition Audio For High Defintion Video
Squeezing the Bits
When it comes to storing digital data, the method depends on the available storage space. You can either store them in their original form or use some type of compression. Compression falls into two broad categories: lossy and lossless.
Lossless compression enables bit-perfect recovery of the original material. It was first developed for computer applications where any loss of data is unacceptable.
On the other hand, lossy data compression has been most widely used in audio/video applications. It takes advantage of the fact that we can’t hear or see every detail present in a complex mix of sound and/or pictures. If engineers can identify insignificant and redundant bits and remove them, the data file will be much smaller, with little or no perceptible degradation in what we see and hear.
This type of compression is called lossy because the discarded data are never recovered. Two factors determine whether or not there’s any noticeable degradation at the delivery end with lossy compression: the degree of compression (how much data are thrown away) and the efficiency or cleverness of the coding technique (the codec). Millions of people hear lossy encoding every day on the most popular form of such compression: MP3 music downloads.
Combined with the huge amount of data storage that even standarddefinition video demands, the space limitations on DVD require the use of lossy compression for both picture and sound. Lossy compression on the audio side is either Dolby Digital or DTS. While some home theater fans and even respected experts maintain that these lossy compression formats offer more than adequate audio, many audiophiles remain uncomfortable with the idea of throwing away data, regardless of the claimed transparency of the process.
Enter Blu-ray with its vastly expanded storage. Many early Blu-ray releases took advantage of this storage by offering 5.1 or more channels of uncompressed linear PCM audio—the digital encoding format that CDs and movie soundtrack masters use. To this day, a small number of Blu-ray Disc releases sport uncompressed PCM soundtracks. Then, as now, some of these have been mastered at 24 bits with an audio sampling rate of 48 kilohertz, or even (though rarely) 24 bits/96 kHz. With most recent films, the digital audio masters used to create a Blu-ray Disc can be as high as 24 bits/48 kHz, although films mastered in the early days of digital film sound are more often 20 bits/48 kHz or even 16 bits/48 kHz.
Only the Lossless
Uncompressed PCM soundtracks were a big hit with home theater fans, but it didn’t take long before two new audio lossless compression formats rose to dominate Blu-ray releases: Dolby TrueHD and DTS-HD Master Audio. What makes these formats unique is that they recover all of the bits in the source material and do not permanently discard data. That is, they are lossless. They require more storage space than the older, lossy Dolby Digital and DTS, but they save up to half the data space that uncompressed PCM requires, which is a significant advantage.
Dolby TrueHD, DTS-HD Master Audio, and uncompressed linear PCM all offer the same resolution. They should be capable of the same sound quality that’s bit-for-bit identical to the uncompressed master file absent compromises at the production end.
On Blu-ray, both Dolby TrueHD and DTS-HD Master Audio offer up to 7.1 channels of lossless audio at a maximum resolution of 24 bits/96 kHz. DTS-HD Master Audio can deliver 24 bits/192 kHz of lossless audio to as many as 5.1 channels. Thus far, major movie title releases at anything over 24 bits/48 kHz, in 5.1 or more channels, has been extremely rare. There are a few concert video releases at 24 bits/96 kHz. The only 24-bits/96-kHz film release we know of is Baraka, and it’s as stunning sonically as it is visually.
Dolby TrueHD is based on a technique called Meridian Lossless Packing, which Meridian Audio originally developed for use in the now largely defunct DVD-Audio format. Dolby TrueHD is not a clone of DVD-Audio, but MLP DNA is present in both. On some discs, Dolby TrueHD offers the same sort of optional dynamic range compression that’s available with Dolby Digital. It also includes Dialogue Normalization, which aims to produce a consistent dialogue level from source to source without affecting the material’s overall dynamic range or sound mix. Metadata controls both dynamic compression (selectable) and Dialogue Normalization (non-defeatable). Metadata is additional data embedded in digital content. It piggybacks on the soundtrack but is separate from it.