Audio: what do I need to do to get SPDIF data to a speaker?

Question

I'm working on a speaker project where I'd like to use a microcontroller's SPDIF output (aka IEC 60958) to drive audio data to a speaker.

I've' been doing research on the topic, and I'm having trouble grasping exactly what steps are required to convert the SPDIF data into an analog input signal for an audio amplifier.

I believe the SPDIF is a compressed data format, so some type of converter is required to decompress the SPDIF data into a standard I2S data stream. From there, I'd need to convert digital to analog, then amplify the analog for the speaker:

Microcontroller --SPDIF--> I2S Converter --I2S--> DAC --analog--> amplifier --audio--> speaker

Is there any way that this approach can be simplified? I'd like to minimize complexity/part count where possible.

Is this the standard approach for handling a SPDIF interface? Or are there simpler approaches for driving a speaker with SPDIF data?

Answer 1

The signal path outlined in the question is basically correct. Those are the necessary signal processing steps to get SPDIF audio to be output from a speaker.

In terms of a simplified approach, there are ICs which combine various pieces of the SPDIF signal path (e.g. DAC+amp), but these have various design tradeoffs that are too numerous to mention here, and the selection of which is going to be highly dependent on your particular design constraints.

The other approach you may want to explore is that of "direct digital" audio amplifier ICs (such as TI TAS5760LD, or ADI SSM2529). These amplifiers employ a signal path that looks something like:

PCM(I2S) -> [PCM to PWM Modulator] -> PWM -> [Class-D Output] -> audio -> [Speaker]

An SPDIF to PCM (e.g. I2S) converter is still required, but they avoid the intermediate analog-to-digital conversion stage, keeping the signal entirely in the digital domain until the Class-D output stage/filter. This is in theory easier to implement and has a lower parts count, but with the caveat that the available options are fairly limited.

Note that SPDIF is not compressed, rather it employs Manchester encoding to combine clock and data information into one signal. DACs generally accept synchronous serial data (I2S, left-justified, etc) where clock and data are on separate inputs . As bit of an over-simplification, the SPDIF receiver separates the clock and data information into distinct signals which can be input into the DAC.

Answer 2

You need an audio DAC, one way or another. Either a device that can directly receive S/PDIF and output analog audio, in which case it might be a more complex audio codec chip, or simply an audio DAC which only has an I2S audio input, and then you only need an S/PDIF receiver which can output the audio data over I2S bus.

S/PDIF is not a compressed data format. It is simply an interface to stream digital audio data, and it can be used to transmit either linear PCM (LPCM) sample data or encoded bitstream data. When data is not LPCM but encoded bitstream, it must first be decoded with a DSP to end up with LPCM data that can be fed to DAC.