We’ve seen it pressed into service as an AM radio transmitter, an NTSC video output, a VGA output, and even an Ethernet card. Naturally, its potential as a very fast serial output has led hardware hackers to use it for other purposes. For example, it does not specify an upper limit for the clock rate. It is both a loose and a simple specification that is easy to implement, and equally easy to abuse. What more is there to understand?Īs it happens from the point of view of the work we cover at Hackaday the digital audio origins of I 2S are only the start. All you need to know about inter-chip audio interconnects, in four handy paragraphs. The seemingly impossible: ’s feat of making an ESP8266 do wired Ethernet.So there you have it: I 2S. It’s not impossible to imagine that given some TTL chips and a resistor ladder it should be possible to build an I 2S DAC from first principles on your bench, albeit not a very high-performance example. Reading the document it becomes apparent that even in the 1980s this was not a difficult interface to work with, and it even gives basic diagrams for a transmitter and receiver. Happily the Wayback Machine has it though, so it is still available. 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. That’s all there is to I 2S: a data line, a word clock line, and a bit clock line. I 2S 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.

So for transmitting audio we really need some other means of delivering those pieces of information. It is also difficult (or impossible) to reliable retrieve a clock from it without jitter. 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. On a single serial line this is a whopping 1,411,200 bits per second (44100 x 16 x 2). 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. Timing diagram for the I 2S lines, from the Philips specification document.Ī 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. 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. The protocol has stuck around because it’s very handy for dealing with the firehose of serial data associated with high-quality digital audio. Brought to life in 1982, this predated I 2S by four years which explains the somewhat strange abbreviation for “Inter-Integrated Circuit Sound”. 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 I 2C. It’s A Simple Enough Interfaceĭon’t confuse this with the other Philips Semiconductor protocol: I 2C. 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. The protocol is called I 2S and comes as a hardware peripheral on many microcontrollers.

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.

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.

Bit serial data interface bmw parts