Today’s blog post is another excerpt from my new Kindle book Home Theater for the Internet Age. It further explores home theater surround sound (also check out Home Theater: Surround Sound Basics), diving into discrete vs. matrixed audio and lossy vs. lossless compression—topics that continue to confuse both casual fans and enthusiasts alike.
Surround sound isn’t merely a speaker arrangement for your living room, but rather a standardized system by which content producers can compose, or “encode,” their content so that consumers, with the proper equipment, can decode it to simulate a more realistic, immersive audio environment. The intent of surround sound is to create audio that radiates from all around the seating positions of the viewers, while giving content producers the ability to specify from which of those speakers a particular sound or audio stream is projected.
While basic surround sound involves three speakers in front and two in back, more sophisticated systems employ between eight and 12 speakers. The more speakers involved, the more immersive and “surrounding” the sound becomes (and, relatively speaking, the more expensive the speaker system and receiver).
Discrete vs. Matrixed Surround
This book strives to avoid the overly technical and speak in plain English. However, it’s helpful to understand the difference between discrete and matrixed surround sound formats. Discrete surround involves sound information that’s specific, or dedicated to, particular channels and speaker positions (and fully independent of other channels). Thus, if a movie features a 5.1 soundtrack, it means the producers recorded and mixed six separate sound channels, each intended for a particular speaker position in your living room.
Matrixed surround, on the other hand, involves your receiver producing sound information for six or more speakers that’s derived from a two-channel (stereo) signal. While not as good as discrete formats, matrixed surround can take standard stereo and make it pretty amazing—depending on the quality of your system. Assume you’re watching an old episode of Law & Order on cable TV and it’s encoded in stereo. If your AV receiver features a matrixed surround sound format, such as Dolby Pro Logic II or DTS Neo:6 (nearly all models for the past several years do), you can apply it to the stereo signal to direct sound to the other speakers in your home theater, not just the mains.
If a film or TV program features an audio mix that’s designed for surround sound, it will offer discrete channels. Dialog will be directed to the center channel, background noise like traffic, the din of a crowd, and sirens will come from the rear speakers, and the non-speech sounds and primary action will be directed to the main speakers. Because so much legacy content—be it music, TV episodes, or movies—features audio encoded in only stereo, you’ll find great benefit in applying matrixed surround filters to take advantage of those other speakers sitting in your living room. This is especially true for those who consume mostly TV content.
Lossy vs. Lossless Compression
When the audio portion of a video is created, it’s digitally compressed to make it smaller. Reducing the size of the data helps it stream smoothly from an internet video service (like Netflix) or fit on an optical disc (DVD or Blu-ray). However, there are different types of compression that impact the quality of the sound produced by any equipment, especially nicer systems.
Traditionally, data compression for audio has been lossy. This means that, during the compression of the audio, some information is lost—resulting in less data to play back. In a nutshell, less data equals lower sound fidelity. Different compression schemes produce distinct results in terms of sound quality. Overall, lossy compression is viewed as a bad thing. Music in MP3 (MPEG Audio Layer III) and AAC (Advanced Audio Coding) formats are good examples of lossy compression standards (and avoided by audiophiles). All music sold on standard compact discs (in CDDA format; see the Disc-Based Music chapter for more info) involves lossy compression (although of significantly higher fidelity than MP3 or AAC).
Lossless compression, on the other hand, is, well, lossless. It reproduces the original audio bit-for-bit, with no reduction in data or quality whatsoever (any decrease in fidelity reflects a deficiency in your equipment, not the audio itself). If you know an audio source is lossless, you don’t really need to learn anything else (except maybe the encoding standard employed to ensure that you can decode it on your particular Blu-ray player or AV receiver).
The downside of lossless compression is that it results in significantly larger files than lossy schemes. This is why lossless audio is currently available only on high-capacity optical disc formats (like Blu-ray, which sports six times the storage of a DVD), but not in the form of internet streaming, where even the fastest broadband connections typically lack the bandwidth to support such high bit rates. Examples of lossless audio include Dolby TrueHD and DTS-HD Master Audio (both covered below). There are also internet-based download services that sell lossless music files in popular formats like FLAC and AIFF (higher quality than even regular music CDs, let alone MP3s).
Compression Levels / Bit Rate
Content compressed in a lossy format can be encoded at a variety of quality levels (measured in “bit rate” or bits per second, and sometimes called “compression levels”). For example, a 128 Kbps (kilobit per second) MP3 of Jessie’s Girl by Rick Springfield won’t sound as good as a 320 Kbps MP3 version. The 320 Kbps file contains nearly three times the data of the 128 Kbps version, enhancing the sound quality.
This is one reason that Blu-ray discs are so popular. While the video quality of Blu-ray (1080 lines of resolution) is certainly better than that of DVD, the audio improvement is even greater. Currently, there is no better sound that you can pump through your home theater than the lossless audio track of a Blu-ray movie or lossless music files like FLAC. (When it comes to audio only, there are also high-end music formats such as SACD, DVD-Audio, and Blu-ray Audio, which are covered in the Disc-Based Music chapter.)
Based on Standards
To clarify how this compression/decompression cycle works, it’s important to understand that content producers must encode their audio to a particular standard (like a United Nations interpreter choosing a language in which to speak). As you’ll learn below, for movies, this is typically a format from Dolby or DTS. Encoding makes files smaller for transport or distribution, regardless of whether it’s lossy or lossless. Your receiver or Blu-ray player incorporates a bunch of decoders. When you play a DVD or Blu-ray disc, the receiver applies the proper decoder, basically reassembling the audio data. In this respect, your AV receiver is just a specialized computer. (Some people will tell you that the audio on a Blu-ray movie is uncompressed, which isn’t necessarily true. Typically it’s compressed, but sometimes it isn’t. Even compressed, Blu-ray involves a lossless scheme.)
It’s important to note that, simply because an audio source involves a lossless compression format, this doesn’t guarantee high-end fidelity. Technically, lossless compression simply means that the file reproduced by your playback equipment exactly matches the original, before it was compressed. If the original music file was of low quality to begin with (many movies and much music are poorly mastered or recorded in less-than-optimal live venues or studio environments), the best lossless format won’t make it sound good.
Curt Robbins is author of the following books from Amazon Kindle:
- Home Theater for the Internet Age ($9.95)
- Understanding Personal Data Security ($4.99)
- Understanding Home Theater ($4.99)
- Understanding Cutting the Cord ($4.99)
- Understanding Digital Music ($4.99)