The I²S protocol outlines one specific type of PCM digital audio communication with defined parameters outlined in the Philips specification.
The bit clock pulses once for each discrete bit of data on the data lines. The bit clock frequency is the product of the sample rate, the number of bits per channel and the number of channels. So, for example, CD Audio with a sample frequency of 44.1 kHz, with 16 bits of precision and two channels (stereo) has a bit clock frequency of:
44.1 kHz × 16 × 2 = 1.4112 MHz
The word select clock lets the device know whether channel 1 or channel 2 is currently being sent, since I²S allows two channels to be sent on the same data line. For stereo material, the I²S specification states that left audio is transmitted on the low cycle of the word select clock and the right channel is transmitted on the high cycle. The word select clock is a 50% duty-cycle signal that has the same frequency as the sample frequency.
Data are encoded in two's complement with the MSB (most significant bit) first.:2
The bus consists of at least three lines:
Bit clock line
Word clock line - also called word select (WS) or left right clock (LRCLK)
At least one multiplexed data line
It may also include the following lines:
Master clock (typical 256 x LRCLK)
A multiplexed data line for upload
In audio equipment the I²S is sometimes used as an external link between the CD transport and a separate DAC box, as opposed to purely internal connection within one player box. This could be used as an alternative to the commonly used AES/EBU or Toslink or S/PDIF standards. There is no standard interconnecting cable for this application. Some manufacturers provide simply three BNC connectors, an 8P8C ("RJ45") socket or a DE-9 connector. Others like Audio Alchemy (now defunct) used DIN connectors.