In the first segment of Understanding Computer Audio, we looked at the basics of digital audio and some of specifications and standards users may see when looking at a computer system or audio peripherals. For Part II in the series, we look into the mysteries and complexities of surround sound and how it relates to computer audio.
The earliest form of surround sound came when audio was moved from a single channel source to stereophonic recordings. Later surround sound standards were developed by the movie industry to allow for a more immersive experience. Of course, the level of immersion by the listener is very dependent upon the number of speakers used and the number of channels in the surround sound encoding.
The most basic of surround sound configurations consists of at least four speakers. Two speakers are in front of the user and two behind. This gives the spatial appearance of audio in front of and behind the listener. Most modern systems also use a fifth speaker directly in front of the listener. This is typically used for dialog during movies to give the listener the sense that the dialogue is coming directly from the screen. Another common element in surround sound is a subwoofer. This is a large speaker that only puts out low frequency or bass audio that is non-directional. This also can give the loud booming effect.
The speaker layouts are denoted in a very specific manner when referring to surround sound. Typically it will be in the form of a dotted notation such as 5.1. The first number represents the number of distinct full range audio channels or speakers. The number after the dot represents the presence of a low frequency effect (LFE) or subwoofer channel.
There are a variety of surround standards that have been developed over the years by the movie industry and are supported by computer audio processors for delivering surround sound. For those interested in surround sound with their computer system, the support for these different standards may be very important.
Dolby Pro Logic is a simple form of surround sound. This is actually a two-channel source that has encoding for certain frequencies to be played in a set of rear speakers. This allows the Dolby Pro Logic stream to be compatible with two speaker systems or to utilize a second set of rear speakers for a more immersive experience. A newer version called Dolby Pro Logic II was developed recently, but it is not very common.
AC-3 or Dolby Digital was developed by Dolby Labs as one of the first truly digital forms of surround sound for movies. It features 5 discrete channels (2 front, 2 rear, 1 center) plus a low-frequency effects channel. The primary benefit of this format is that it uses compression to take all this data and fit it in a very compact format with very little loss in quality. In order to playback sources with Dolby Digital (such as DVD movies), the computer audio processor will need to support this as well as have 5 speakers plus subwoofer. Audio processors require at least 16-bit 96KHz processing ability for true Dolby Digital.
DTS or Digital Theater Systems was a competing standard to Dolby Digital for 5.1 surround sound for movies. It also has five distinct speakers (2 front, 2 read, 1 center) plus a subwoofer. It uses a different encoding scheme than Dolby Digital that provides less loss in audio quality but at a larger audio stream. A newer standard called DTS-ES was recently released that supports a 6.1 speaker configurations and is backward compatible with the DTS 5.1 equipment.
Direct3D is an audio standard that is used by the Microsoft DirectX engines for generating 3D positional sound from the computer system. This is primary used in computer games to simulate the 3D environments that players are traveling through. This is not necessarily an encoding format more than a set of software standards that allows for the hardware manufacturers. It can support anything from two speaker configurations all the way up to 7.1 and 8.1 speaker configurations as long as the software programs will render the audio to that number of speakers.
For the majority of consumers, the answer to this question is no. Unless the computer is going to be used to watch movies or play 3D video games, the extra cost is not justified. Also, the environment that many computers exist in is not very conducive to listening to music or movies. The fans used for cooling the components of the computer will generally cause background noise that disrupts the listening experience.
For those who do plan on watching movies in surround sound or playing video games, it is generally best to make sure that the sound solutions you purchase fully support the Dolby Digital, DTS and Direct3D standards. This will ensure that whatever media you are likely to playback on your computer will work properly for surround sound.
The specifications of computer audio and the basics of surround sound have been discussed. Of course, most computer systems don't have built in means to playback audio (notebooks being the exception). How the audio moves from a computer system to external speakers can be the difference between clear crisp audio and noise.
The RCA connector has been the standard for home stereo interconnects for a very, very long time. Each individual plug carries the signal for a single channel. This means a stereo output requires a cable with two RCA connectors. Since they have been in use for so long, there has also been a lot of development in the quality of cabling.
Of course, most computer system will not feature RCA connectors. The size of the connector is very large and the limited space of the PC card slot prevents many from being used. A 5.1 surround sound configuration would require 6 connectors. Since most computers are not hooked up to home stereo systems, the manufacturers generally opt to use the mini-jack connectors instead.
With the advent of digital media such as CD and DVD, there was a need for preserving the digital signal. Constant switching between audio and digital signals induces noise into the sound. As a result, new digital interfaces were created for PCM (Pulse Code Modulation) signals from CD players to the AC-3 and DTS connections on the DVD players. Digital coax is one of the two methods for carrying the digital signal.
Digital coax looks identical to that of a RCA connector but it has a very different signal carried over it. With the digital signal traveling across the cable, it is able to pack a complete 5.1 signal into a single digital stream across the cable that would require six individual analog RCA connectors. This makes digital coax very efficient.
Of course, the drawback to using a digital coax connector is that the equipment that the computer hooks into must also be compatible. Typically, it requires either an amplified speaker system with digital decoders built into them or a home theatre receiver with the decoders. Since the digital coax can also carry different encoded streams, the device must be able to auto detect the type of signal. This can drive up the price of the connecting equipment.
As good as digital coax is there are still some inherent problems. Digital coax is still limited to the problems of an electrical signal. They are affected by the materials they travel through and the electrical fields that are surrounded by. To combat these effects, an optical connector or SPDIF (Sony/Philips Digital Interface) was developed. This transmits the digital signal across a fiber optic cable to retain the signal integrity.
SPDIF connectors provide the cleanest form of signal transfer currently available, but there are limitations. First, it requires very specialized fiber optic cables that are very expensive. Second, the receiving equipment must also have the ability to receive the SPDIF connector. This is generally found on the home theater receivers, but it is very uncommon for amplified computer speaker sets.
This is going to be very dependent upon how the computer will be used. In most cases, the only connectors that are required will be the mini-jacks. Any sound solution that you purchase should at least have a headphone or line-out, line-in and microphone jack. These should also be reconfigurable to allow for the three to be used as outputs for surround sound. For higher quality audio for home theatre environments, it is best to make sure that the audio components on the computer have a digital coax or SPDIF line out. This will provide the highest sound quality possible.