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.
However, there may be times when you wish to transmit more than two channels of audio over the S/PDIF link. This is where compressed audio can be used. Audio compression is a technique used to transmit equivalent information using fewer bits. This is done through a number of techniques. Some techniques, referred to as perceptual coding, take advantage of the fact that humans can only hear certain sounds. These methods of compression usually involve discarding bits that only contribute a minimal amount to what a listener needs to recognize a given sound. Other methods take advantage of numerical redundancies in the signal in order to effectively transmit the same information in a smaller amount of space. Dolby Digital and DTS are two common types of compression. Regardless of the technique, compression enables a digital audio signal to use fewer bits to transmit the audio.
The result of this compression is that it enables you to transmit the content for up to 5.1 channels of audio over S/PDIF in space that would have only fit 2 channels if the audio had been uncompressed. This is great once the signal has been encoded (synonymous with compressed), but once a signal has been encoded, that same signal must also be decoded after it has been received so that it can be sent to speakers. This means that your receiver must be capable of decoding the compressed audio signal in order for you to hear the correct sound. This is the tradeoff necessary to allow you to transmit more than two channels of audio over S/PDIF.
Another direct consequence of transmitting a compressed audio signal instead of an uncompressed audio signal (more commonly known as PCM) is that the volume of that signal cannot be modified once it has been encoded. Because the bits in an encoded signal no longer directly correspond to the volume of that signal, it is impossible to increase the volume until it is decoded. This explains why your PC cannot control the volume of your sound when you are using Dolby Digital or DTS as the output. The connected device will be the only place where the volume can be changed.
To recap, in order to avoid the electrical interference and noise present on an analog connection, consider using S/PDIF to transmit the signal digitally. If you'd like to transmit more than two channels, consider sending encoded content which allows you up to 5.1 channels over S/PDIF. You may also want to consider HDMI, but that's a post for another time!