Nuts and Volts - February 2008

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Audio

In the December ‘07 issue, we examined the various ways to hook up pieces of your home entertainment system to your HDTV. We specifically focused on the different video interfaces. We’ll continue now with the choices for passing audio from one device to another.

by Jeff Mazur

Once again, the most common connection by far is the standard analog stereo pair using RCA jacks and cables. With good quality cable and connectors, this method can provide excellent results. The most common issue with analog audio connections is its susceptibility to picking up hum and/or other extraneous signals, especially from components within your system (or perhaps from

the ham operator who lives next door!). To solve this issue — as well as complete the total conversion to binary 1s and 0s — there are three basic ways to pass audio signals digitally between devices: coax, optical, and HDMI.

S/PDIF (Sony/Philips Digital Interconnect Format)

Figures 2-4 are courtesy of Wikipedia, the free encyclopedia (licensed to the public under the GNU Free Documentation License).

Named after the two companies that developed this interface, S/PDIF

is a means to carry audio between devices in a digital format. The signals can be carried over standard 75 ohm coaxial cable using RCA jacks (or BNC connectors in professional equipment) or via optical fiber (glass or plastic, usually terminated with F05 connectors). See Figure 1.

The optical connection — created by Toshiba

and also known as TOSLINK — uses 1 mm fiber terminated in a 5 mm connector. While earlier cables were restricted to less than 15 feet, you can now buy high quality TOSLINK cables up to 100 feet in length. TOSLINK can carry data signals of up to 125 Mbits/s, which allows for three audio channels. However, it is usually used to carry a single pair of stereo audio signals.

As an electrical signal, S/PDIF is represented by a roughly 1V digital pulse train using Biphase Mark Code (BMC) to carry the audio data. While no specific sampling rate or bit depth is specified in the standard, audio is usually carried as either 48 kHz (DAT) or 44.1 kHz (CD) data with either 20 or 24 bit samples. We’ll describe the actual data format in a moment.

HDMI

FIGURE 1. Digital audio connections (top, coax and bottom, optical).

We’ve already discussed the HDMI interface that can carry digital video between devices. HDMI also includes support for up to eight channels of uncompressed digital audio at a 192 kHz sample rate with a