PC Audio Basics: Connectors

Most desktop computer systems do not include built-in equipment to play audio, and most laptops offer limited speaker capabilities. The mechanism by which the audio moves from a computer system to external speakers makes the difference between clear, crisp audio and noise.

Mini-Jacks

The mini-jack is the most common form of interconnect between a computer system and speakers or stereo equipment. It uses the same 3.5-mm connectors used on portable headphones.

In addition to their size, mini-jacks are widely used for audio components. Portable audio has used these for many years, making a wide range of headphones, external mini-speakers, and amplified speakers compatible with computer audio. It is also possible to convert a mini-jack plug into the standard RCA connectors for home stereo equipment with a simple cable.

Mini-jacks lack dynamic range, though. Each mini-jack can only carry the signal for two channels or speakers. In the 5.1 surround setup, three mini-jack cables carry the signal for the six channels of audio.

RCA Connectors

The RCA connector has been the standard for home stereo interconnects for a long time. Each plug carries the signal for a single channel. So, a stereo output requires a cable with two RCA connectors. Since they have been in use for so long, there's been a lot of development in their quality.

Most computer systems do not feature RCA connectors. The size of the connector is unwieldy, and the limited space of the PC card slot prevents many from being used. Typically, no more than four could reside in a single PC slot. A 5.1 surround sound configuration requires six connectors. Since most computers are not hooked up to home stereo systems, the manufacturers generally opt to use the mini-jack connectors instead.

Digital Coax

Constant conversion between analog and digital signals induces distortions into the sound. As a result, new digital interfaces were created for Pulse Code Modulation signals from CD players to the Dolby Digital and DTS connections on the DVD players. Digital coax is one of the two methods for carrying that digital signal.

Digital coax looks identical to an RCA connector, but it has a very different signal carried over it. The digital signal traveling across the cable packs a complete multiple channel surround signal into a single digital stream across the cable that would require six individual analog RCA connectors. It makes digital coax very efficient.

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 theater receiver with the decoders. Since the digital coax can also carry different encoded streams, the device must auto-detect the type of signal. This capability drives up the price of the connecting equipment.

Digital Optical (SPD/IF or TOSLINK)

An optical connector — sometimes called SPD/IF (Sony/Philips Digital Interface) — transmits the digital signal across a fiber-optic cable to retain the signal integrity. This interface was eventually standardized into what is referred to as a TOSLINK cable and connector.

TOSLINK connectors provide the cleanest form of signal transfer currently available, but there are limitations. First, it requires specialized fiber-optic cables that are more expensive than coax cables. Second, the receiving equipment must accept the TOSLINK connector, a capability rare for amplified computer-speaker sets.

USB

The Universal Serial Bus is a standard form of connection for most PC peripherals, including headphones, headsets, and even speakers.

Devices that use the USB connector for the speakers are also in effect the sound-card device. Rather than the motherboard or sound card rendering and converting the digital signals to audio, the digital signals are sent to the USB audio device and then decoded there. This approach requires fewer connections, but it has significant downsides as well — for example, the sound-card features of the speakers may not support the proper decoding levels necessary for higher-quality audio, such as 24-bit 192 kHz audio.