The following is an article from another of my colleagues on the Windows Vista Sound team, Kristin Carr. Kristin is a Program Manager and works with Steve Ball, who previously has shared his insights into how Windows Vista handles sound. If you have questions for Kristin, please leave a comment below.
Many people have a general idea of what S/PDIF is -- perhaps by seeing it as a label on an audio output, or on a feature list for a product. But what is it exactly, and how do you use it? This post will cover some of those details.
On a PC, the audio is stored and processed digitally until the final output stage when it is usually converted to an analog signal that directly feeds your speakers. However, there may be times when you want to transmit the signal digitally to a different device that will be used to play the sound, such as a receiver. In these cases, you may want to postpone converting the signal to an analog one, and instead transmit the signal digitally to avoid any degradation and additional noise that may occur when transmitting an analog signal.
For this purpose, S/PDIF (Sony/Philips Digital Interconnect Format) was developed. Often referred to by the name of the connector (including Toslink, RCA, or simply "Optical" or "Digital Out"), S/PDIF specifies a method of transmitting a digital signal so that it can be received and interpreted correctly by the connected device. You may ask yourself, "How complicated is it to transmit a signal? Why do we need a special protocol?" Consider that the digital signal consists of a series of bits, and within that series, the bits are grouped to correspond to a sample of audio, and an even larger subset of those are grouped to correspond to a particular channel. In order to enable a receiver to properly interpret all of those bits in the correct order, it is necessary to have a format for transmitting those bits. This is where S/PDIF comes in.
S/PDIF can be used to transmit two channels of digital audio in real time over a single connection. S/PDIF specifies a particular bit pattern that a receiver can use to latch onto the stream. Once the receiver has synced up with the stream, S/PDIF specifies the order of the audio bits and how they should be arranged in a stream so that the receiver can properly interpret it.
Microsoft, Windows Vista, Audio