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All You Need To Know About I2S

Author: Jenny List / Source: Hackaday

Last month we marked the 40th birthday of the CD, and it was as much an obituary as a celebration because those polycarbonate discs are fast becoming a rarity. There is one piece of technology from the CD age that is very much still with us though, and it lives on in the standard for sending serial digital audio between chips.

The protocol is called I2S and comes as a hardware peripheral on many microcontrollers. It’s a surprisingly simple interface that’s quite easy to work with and thus quite hackable, so it’s worth a bit of further investigation.

It’s A Simple Enough Interface

Don’t confuse this with the other Philips Semiconductor protocol: I2C. Inter-Integrated Circuit protocol has the initials IIC, and the double letter was shortened to come up with the “eye-squared-see” nomenclature we’ve come to love from I2C. Brought to life in 1982, this predated I2S by four years which explains the somewhat strange abbreviation for “Inter-Integrated Circuit Sound”.

The protocol has stuck around because it’s very handy for dealing with the firehose of serial data associated with high-quality digital audio. It’s so handy that you’ve likely heard of it being used for other purposes than audio, which I’ll get to in a little bit. But first, what does I2S actually do?

Timing diagram for the I2S lines, from the Philips specification document.
Timing diagram for the I2S lines, from the Philips specification document.

A digital audio source will usually create two words of data, one for the left channel and one for the right, once for every sample interval. For example, a CD audio source with a 44.1 kHz sample rate that will deliver two 16-bit words 44,100 times every second. On a single serial line this is a whopping 1,411,200 bits per second (44100 x 16 x 2).

How does that poor serial data line keep up? Well, a single serial data line cannot easily convey the word boundaries for left and right samples. It is also difficult (or impossible) to reliable retrieve a clock from it without jitter. So for transmitting audio we really need some other means of delivering those pieces of information.

I2S solves both of these problems with extra lines, providing a word select line (also sometimes called L/R clock) to select left or right samples, and a bit clock line to keep everything in sync. That’s all there is to I2S: a data line, a word clock line, and a bit clock line.

The specification was formalised by Philips in a 1986 document that made it through the company’s semiconductor division becoming NXP, but sadly has disappeared from the NXP website. Happily the Wayback Machine has it though, so it is still…

Click here to read more

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