I²S
I2S, also known as Inter-IC Sound, Integrated Interchip Sound, or IIS, is an electrical serial bus interface standard used for connecting digital audio devices together. It is most commonly used to carry PCM information between the CD transport and the DAC in a CD player. The I2S bus separates clock and data signals, resulting in a very low jitter connection.
This standard was introduced in 1986 and was last revised in 1996.
The bus consists of at least three lines:
- Bit clock line
- Word clock line (also called word select line or left right clock)
- And 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
[edit] I²S
I²S consists, as stated above, of a bit clock, a word select and the data line. The I²S protocol outlines one specific type of PCM digital audio communication with defined parameters outlined in the Philips spec.
The bit clock pulses once for each discrete bit of data on the data lines. The bit clock will operate at a frequency which is a multiple of the sample rate. The bit clock frequency multiplier depends on number of bits per channel, times the number of channels. So, for example, CD Audio with a sample frequency of 44.1 kHz, with 32 bits of precision per (2) stereo channels will have a bit clock frequency of 2.8224 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.
I²S data is sent from MSB to LSB starting on the second bit clock cycle after the word select clock transition. Transmitting MSB first allows both the Transmitting and Receiving devices to not care what the audio precision of the remote device is. If the Transmitter is sending 32 bits per channel to a device with only 24 bits of internal precision, the Receiver may simply ignore the extra bits of precision by not storing the bits past the 24th bit. Likewise, if the Transmitter is sending 16 bits per channel to a Receiving device with 24 bits of precision, the receiver will simply Zero-fill the missing bits. This feature makes it possible to mix and match components of varying precision without reconfiguration.
The first data bit following a word select clock transition is the LSB of the previous word.
In audio equipment the I²S is sometimes used as an external link between the CD transport and a separate box DAC, as opposed to purely internal connection within one box player. This is considered, by some audiophiles, to be a higher quality connection than the commonly used AES/EBU or Toslink or S/PDIF standards. There is no standard interconnecting cable for I²S. 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.
