I2S MP3 webradio streaming example

This is an example of how to use the I2S module inside the ESP8266 to output sound. In this case, it is used to output decoded MP3 data (actually, more accurately: MPEG2 layer III data): the code described here basically is a webradio streamer which can connect to an Icecast server, take the MP3 data the server sends out, decode it and output it over the I2S bus to a DAC. The MP3 decoder has been tested for bitrates up to 320KBit/s and sample rates of up to 48KHz.

Configuration options, building

All high-level options can be configured in mp3/user/playerconfig.h. Edit that file to set up your access point and a webradio stream or other source of MP3 data served over HTTP.

To build the code, try running make.sh in the mp3/ directory. Alternatively, the way to use 'make' to build this code is: make COMPILE=gcc BOOT=none APP=0 SPI_SPEED=40 SPI_MODE=QIO SPI_SIZE=1024

The resulting binaries will be in the bin/ folder. Please disregard the message that pops up at the end of the make process: the addresses it mentions are wrong. The correct addresses to load the resulting files are:

bin/eagle.flash.bin     - 0x00000
bin/eagle.irom0text.bin - 0xA0000

Needed hardware

If you want to have nice, high-quality buffered audio output, you will need to connect two ICs to your ESP: a 128KByte SPI RAM and an I2S codec. Both ICs are optional, but you will get stuttering and low-quality sound if you leave them off.

The SPI RAM is a Microchip 23LC1024 part and is used to buffer the incoming MP3 data. This guards against latency isuues that are present in all but the most quiet networks and closest connections. It is connected to the same bus as the SPI flash:

ESP pin   - 23LC1024 pin
------------------------
GPIO0     - /CS (1)
SD_D0     - SO/SI1 (2)
SD_D3     - SIO2 (3) *
gnd       - gnd (4)
SD_D1     - SI/SIO0 (5)
SD_CLK    - SCK (6)
SD_D2     - /HOLD/SIO3 (7) *
3.3V      - VCC (8)

*=optional, may also be connected to Vcc on 23LC1024 side.

One way to make these connections is to take the SSOIC version of the 23LC1024, bend up pin 1 (/CS) and piggyback it on the SPI flash chip that already is on the ESP module. Solder all the pins to the same pins on the SPI flash chip except for the bent /CS pin; use a wire to connect that to GPIO0.

As Github user milkpirate correctly remarked, GPIO0 is also used to enter programming mode on the ESP8266 and will interfere with correct flashing if kept low. The correct way of flashing the module is to make GPIO0 low, reset the ESP8266 to enter programming mode, then make GPIO0 high again. Tools like esptool.py generally follow this method to automatically enter programming mode; if you manually enter programming mode you may have to adjust your methodology.

For the I2S codec, pick whatever chip or board works for you; this code was written using a ES9023 chip, but other I2S boards and chips will probably work as well. The connections to make here are:

ESP pin   - I2S signal
----------------------
GPIO2/TX1   - LRCK
GPIO3/RX0   - DATA
GPIO15      - BCLK

Also, don't forget to hook up any supply voltages and grounds needed.

Running without the SPI RAM part

To not use the SPI RAM chip, please edit mp3/user/playerconfig.h and define FAKE_SPI_BUFF. This will use a much smaller buffer in the main memory of the ESP8266. Because the buffer is much smaller, the code will be very sensitive to network latency; also, clock synchronization with live streaming stations will not work. Expect the sound to cut out a fair amount of times unless you have a quiet network and connect to a server very close to you.

Running without the I2S DAC

To not use an I2S DAC chip, please edit mp3/user/playerconfig.h and define PWM_HACK. This uses some code to abuse the I2S module as a 5-bit PWM generator. You can now connect an amplifier to the I2S data pin (GPIO3/RX0) of the ESP module. Connecting a speaker directly may also work but is not advised: the GPIOs of the ESP are not meant to drive inductive loads directly.

Sound quality

In the default configuration, this code will output 16-bit mono audio. Stereo audio is possible but hasn't been implemented yet: a stereo synth is available but has not been modified for ESP8266 use yet. In PWM mode, the output is a dithered 5-bit PWM output. Furthermore, the ESP can decode a pretty wide range of bitrates: 96KBit to 320KBit MP3s have been tested and seem to work fine. Sound quality in general is not bad, but the scaling and clock adaption algorithms can use some improvement if the decoder needs to output real high-quality sound: Patches are welcome.

About the FreeRTOS SDK used

The MP3 example is a very memory-sensitive piece of code: the MP3 decoder uses almost all the RAM and the needed buffers for input and output buffering take up the rest: when using no external SPI RAM, only a few bytes of memory are left. The SDK libs that come with this example are libraries that have been optimized for memory usage and are known to work.

Technical details on this implementation

The biggest part of this code consists of a modified version of libmad, a fixed-point mp3 decoder. The specific version we use here has already been modified by NXP to use less memory (source: www.nxp.com/documents/application_note/AN10583.pdf) and has been massaged by Espressif to store as much constants in flash as possible in order to decrease RAM use even more. The MP3 decoder is fed from a FIFO realized in the external 23LC1024 SPI RAM. This RAM is filled from a network socket in a separate thread.

On the output side, the MP3 samples are fed into the I2S subsystem using DMA. The I2S DMA basically consists of a circular buffer consisting of a number of smaller buffers. As soon as the DMA is done emptying one of the smaller buffers into the I2S subsystem, it will fire an interrupt. This interrupt will put the buffer address in a queue.

When the MP3 decoder has a bunch of samples ready, it will pop a buffer off this queue and put the samples in it until it is full, then take the next buffer etc. The MP3 decoder generally is faster than the I2S output, so at a certain moment there will be no free buffers left. The queue system of FreeRTOS will suspend the mp3 decoding task when that happens, allowing the ESP8266 to attend to other tasks.

While the ESP8266 is able to run at 160MHz, we're leaving it at its default speed of 80MHz here: it seems that at that speed the ESP8266 is perfectly capable of decoding even 320KBit MP3 data.