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Spotify Box Lite

A device that can stream Spotify

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Inspired by the project of Evan Hailey who built a custom Linux board for his Spotify streaming box, I decided to build myself one, too, but in the easier way. Instead of making my own PCB I wanted to use the popular BeagleBone SBC, that is, the smaller variant called PocketBeagle.

inspired by the project of Evan Hailey who built a custom Linux board for his Spotify streaming box, I decided to build myself one, too, but the easier way. Instead of making my own PCB I wanted to use the popular BeagleBone SBC, that is, the smaller variant called PocketBeagle.

The standalone PocketBeagle doesn't have the audio outputs, so I would build my own add-on board with an audio codec and 3.5mm jack. There are some standard parts, like TLV320AIC that are supported in the BeagleBone device tree. By that I mean, there are existing device tree overlays that should "somehow" be invoked and the CPU will recognize the codec it's being connected to. This should make my add-on PCB a standard ALSA sound card. I don't know much about the DTOs, but hopefully, this will be a nice learning process.

The same board would host a USB A-type connector, what should be used for WIFI dongle. This would make my device a way to connect to the Internet. I would also put a 320x240 SPI-based LCD screen for display of the current song artwork.

Finally, the Spotify itself, would be reached via this deamon tool, called spotifyd that is used in the original project. If I get it right, it enables the cast option controlled from my smartphone. This seems like a nice elegant solution, but ,since I'm already having troubles with running it on my VM Ubuntu, I expect some troubles along the way.

So, that's about it. I would like to hear your opinions on the project and where do you see possible pitfalls. I am a hardware designer by profession, so I expect not so many problems on a PCB side. I am, however an less experienced linux user/SW developer, so I'm not sure how to make this spotifyd thing or device tree overlay running smoothly. But, looking forward to trying it out!

The boards are already on their way from the JLC-PCB!

  • Grounding & Wiring

    Pero12/17/2021 at 23:23 0 comments

    It turned out to be a problem in my wirings. 

    What I thought was the most logical pinout of my audio socket turned out to be wrong. When a 3.5mm jack is plugged in, the tip of it gets tied to the metallic shielding on a pin 2, what made me initially believe was reserved for the GND, that is for the sleeve of the jack. But no, it's tied to the tip.

    There also seems to be an error in the datasheet, bc after inspecting it, it turns out that pin 3 is actually jack sleeve, while pin 1 is its ring. After inspecting the connecter with a more caution this time, I found out that proper way to connect it is :

    Pin 1: R (jack ring)
    Pin 2: L (tip of the jack)
    Pin 3: GND (sleeve of the jack)
    Pin 4: NC (metal piece tied to the Pin 2)

    Unfortunately, it only improved a little my signal-to-noise ratio. After examining contacts with multimeter for the 100th time, and left without ideas, I fiddled with the jack in and out of the socket. And marvelously, found a position where the sound gets crystal clear!

    Crystal clear sound only comes out when:
    HPLOUT is tied to the R in+
    HPROUT is not connected
    GND is tied to the L in+
    Additionally, extra noise reduction is achieved when the GND from PB is tied to the PE of the amplifier (that is also tied to amp gnd at one single point)

    This is very curious, since the preamp behaves as it is a differential input amplifier and it is not.

  • Streaming Spotify music

    Pero12/13/2021 at 22:27 3 comments

    After finally getting the sound to play, at least on my headphones, the main part of my project was due: making Spotify stream from my PocketBeagle!

    This turned out to be the most simple task of all so far. I literally followed the instructions from Raspberry Pi installation page, even though, I'm not using the Raspberry Pi. I didnt even bother building the code from the source. And it worked, from the first try! How cool is that?!

    I put my Spotify user data in the config file (.config/spotifyd/spotifyd.conf ) and started the program, in no-deamon way: ./spotifyd --no-daemon. And there it was. No error messages in the kernel, just a banner with "Started A spotify playing daemon" tagline. And on my smartphone/destktop app there was an option of casting the music stream onto the PocketBeagle: 

    And it connected effortlessly. The music was on, controlled by my Spotify app. It worked! Excitement! 

    I also got the daemon service running, so that all of the stream messages run in the background. 

  • Finally, some music

    Pero12/13/2021 at 22:11 0 comments

    I'm using this project in a combination with my previous project, the HiFi Audio Amplifier, for my home music system. Unfortunately, I didn't manage to get anything more than noise and hum out of my speakers. 

    With hope that all is okay with my hardware, drivers, and the device tree, I needed a way to easily configure my codec properties, until something happens. To make a sound comes out, or, really anything, that noise was unbearable. How can I change the codec settings, like the volume, filtering, mixing etc. if Linux by default doesn't let you play with the registers once they're used by kernel? 

    Well, there comes a programming interface to configure some pieces of code in the kernel, dedicated specifically to the sound cards, called ALSA. It allows you to play any sound file, using the program called aplay, or to mix the output channels, using the program called amixer. 

    The amixer lets you change all of the user options implemented in the TLV320AIC codec, like various sound levels, mux control, AGC and so on. The block diagram of the codec comes really helpful in understanding what to configure:


    For example, to get an output from the high power outputs of the codec (HPLOUT and HPROUT) I need to route the two DACs (L and R) to their respective outputs. For that I would need to look closer into the settings like HP, HP DAC, Left HP Mixer DACL1, Right HP Mixer DACL1 and similar. My basic settings were

    • 'HP' is on, so that the HP outputs are turned on
    • 'HP DAC', that is DAC volume control is turned on to max
    • 'Left DAC Mux' is set to DAC_L2, since I want the sound directly from the DAC, no mixing with other channels
    • 'Right DAC Mux' is DAC_R2, same reason as above
    • 'Left HP Power Down mode' is set to high impedance as well as the 'Right HP Power Down mode'

    amixer is very versatile, but crazy hard to work with. It is a command line program, based on set/get procedure. You need one PocketBeagle ssh session  to play a sound file in a loop, and another session to fiddle with the amixer. I spent an immense amount of time configuring the sound card this way, until I learned there is a much more user friendly, graphic program called alsamixer, where I can configure all of it, using only up and down arrows.

    In the end, I managed to get some music playing, but not on my amplifier, where I wanted it. Only when I plugged my headphones in, I got the crisp clear sound coming out. 

  • I2S Data

    Pero12/12/2021 at 21:07 0 comments

    Hooking up my sound card to the scope revealed the following:

    • Master clock at 24 MHz
    • Word clock (a sample rate) at 44.1 kHz
    • Bit Clock at 1.41 MHz

    Knowing that desired audio file has the sample rate of 44.1 kHz, 16 bit resolution, and two channels, the bit clock of 44.1 x 16 x 2 = 1.411 MHz works out fine. Data is emitted only during the low-phase of the WCLK, which is unexpected, since the datasheet specifies that, in I2S mode, the data should be emitted in both phases (for the left channel during the low phase, and for the right channel during the high phase).

  • Device Tree Overlay

    Pero12/12/2021 at 20:31 0 comments

    When I was researching for this project, my first find and the inspiration was Bela, the audio platform based on the BeagleBone. Bela utilizes the same TLV320AIC3104 codec in their sound card, but in very advanced way  with real-time co-processor (PRU) that surpasses the kernel inner timings. That was a bit too much for me, since I need to learn how to get the basic audio working. The guy on their user forum told me that if I want to use the codec with the PocketBeagle in an easy way, I could simply invoke the BB-AUDI-02 overlay device tree overlay.

    At this point I still had no idea what device tree overlays are nor how to use them. But after sniffing a bit around, things came to their place. My learning curve was as follows:

    • First, in embedded Linux, there needs to be a way for kernel to know what hardware is the CPU connected to when the system boots up. This has been solved by something called a device tree, a piece of code that tells kernel where the drivers of that hardware are, and how to configure them. Reference.
    • Device tree is compiled together with the kernel - so every change in the hardware, or in their respective drivers, would require you to re-compile everything, a tiresome and time consuming process. That's why some smart people have developed a something called a device tree overlay, another piece of code that kinda lies on top of the existing device tree backbone and tells kernel, on the run, how the drivers should be configured. Now, you can use an existing image and play with the HW any way you want. Reference
    • In BeagleBoard eco-system there was a solutionn to easily load and unload the device tree overlays, called cape manager. And there is a guide on how to use it  for the Audio Cape, that I based my sound card on. What seemed like an easy-peasy tutorial, turned out to be a lot harder endeavor, because the Cape Manger is discontinued in the kernel versions released since 2017 or so. 
    • An U-boot Overlay was introduced instead. It is the same piece of code as a device tree overlay, but instead of being loaded on the run, it is loaded by the bootloader during the boot process. A proper usage of the U-boot overlays consists of getting a valid dts file for your hw, compiling it into the dtbo file and editing /boot/uEnv.txt file and rebooting the device. 
    • Valid dts files can be found in the BeagleBone overlay repo. There is a valid dts file for the Audio Cape (BB-BONE-AUDI-02-00A0.dts), but it wont work on my PocketBeagle. The reason is a different pinout and clock setting of the two boards, BBB and PocketBeagle. In this DigiKey Tech Forum thread there is my attempt with the help of Robert Nelson, a Linux guru at BeagleBoard.org to fix the overlay to work on the PocketBeagle. It involves pinmux modifications. 
    • Although the pinmux issues were solved with Robert's help, my cape wouldn't work. Sound card was recognized by the alsa system, but the aplay wouldn't exit when I made it play a sound file. Also, the sound would never come out. The reason turned out to be invalid clocking settings between two. But, some years ago, a user at Bela Forum surpassed this issue by modifying existing TLV320AIC3104 dto made for the PocketBeagle. The modification is essentially changing the I2C instance from i2c1 to i2c2, where the TLV320 codec is connected on our (my and Bela) cape.
    • After doing the same, and compiling the new dts file, editing /boot/uEnv.txt and rebooting, the capes got recognized, and aplay worked. There was, unfortunately, still no sound at my speakers.

  • Got the capes!

    Pero12/12/2021 at 19:06 0 comments

    The assembled PCBs arrived from China! They look pretty good, as if the JLCPCB improved their quality over the last few years since the last time I used them.

  • Testing Spotifyd on Ubuntu VM

    Pero10/20/2021 at 08:25 0 comments

    Spotifyd is a program that enables streaming Spotify remotely, like you do on  a Chromecast. It can run as a daemon service, or a normal program. After building and installing it on my Ubuntu virtual machine, it is very important to have a configuration file stored in `~/.config/spotifyd/spotifyd.config`. You need a Spotify premium user number, password, and comment out the commands you dont use, like the command that gets executed after the song is skipped. Also, the proxy server should be commented out. 

    It is important to setup the backend option properly, as well. I thought it was "alsa", but it only worked with "pulseaudio". Make sure to check what is allowed by typing `spotifyd --help`.  I was figuring out for days what was wrong when the error about "not being able to write to pulseaudio" was all over the console. 


    If you want to run it as a normal program, for dev purporses, just type `spotifyd --no-daemon`. It should connect succesfully and display:
        ''''
        Loading config from "/home/pero/.config/spotifyd/spotifyd.conf"
        No proxy specified
        Using software volume controller.
        Connecting to AP "gew1-accesspoint-a-h9cf.ap.spotify.com:443"
        Authenticated as "1116304529" !
        Country: "AT"
        ''''
    Now you can check on your smartphone Spotify app if the device is visible and can you connect to it. 

    Funny enough, if I run compiled-by-me version, than it recognize alsa normally. Pre-compiled binary, downloaded from release shelf only works with `pulseaudio`. No idea why is that so. 
    In any case, I managed to bring the strem to work when I commented out these lines in .config file:
        `device = "alsa_audio_device"`,  and 
        `control = "alsa_audio_device"`

    Now spotifyd works both as daemon and a "normal" program. Cool.

  • Audio Cape Design

    Pero10/19/2021 at 07:34 0 comments

    PocketBeagle has a SiP that contains TI AM3358 CPU that has no I2S. Instead, it has the McASP.  From the reference manual: "The multichannel audio serial port (McASP) functions as a general-purpose audio serial port optimized for the needs of multichannel audio applications. The McASP is useful for time-division multiplexed (TDM) stream, Inter-Integrated Sound (I2S) protocols, and intercomponent digital audio interface transmission (DIT)."

    Hopefully, I don't need to write the code for McASP myself. I have found existing PocketBeagle add-on boards that provide audio outputs. There are two capes that kinda support audio outputs via McASP to interface the TLV320AIC audio codec. One is the Bela (on-going art/sound platform with fancy IDE), the other one is BB-BONE-AUDI (that seem to have disappeared from the market). They both have similar/same pin connections to the codec:


    TLV320AIC3104 Connector AM3358
    MCLK P1.29 PRU0.7 - MCASP0-AHCLK
    WCLK P1.33 PRU0.1 - MCASP0-FSX
    BCLK P1.36 PWM0A -  MCASP0-ACLKX
    DIN P2.32 PRU0.2 - MCASP0-AXR0
    DOUT P2.30 PRU0.3 - MCASP0-AHCLKR
    RESET# P2.26 PMICGOOD
    SCL P1.28 I2C2_SCL
    SDA P1.26 I2C2_SDA

    HW Project

    I have modified an existing BelaMini Rev B4 Design, by adding a 3.5mm audio jack. It was a nice trip down the memory lane of using Eagle. 

    I have ordered PCBs from JLC-PCB. Since they also offer a cheap SMT assembly service too, I decided to let them do the soldering. There is a limitation, however, they only work with the components they have on LCSC stock. As you can guess, my chip TLV320AIC3104 is not stocked there. Fortunately, there was an alternative: TLV320AIC3101 - a chip with integrated speaker amplifier. Everything else is the same, and should to the job just as fine. 

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kris-makes wrote 12/13/2021 at 07:24 point

Great idea! Finally a reasonable usecase for the PocketBeagle. I've searched all the internet for what to do with my pocketbeagle, maybe this will be THE project. Really interested in the outcomes. Im only a student in electrical engineering, so im also not good with linux.

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Pero wrote 12/13/2021 at 09:56 point

glad you like it! I can hapilly report that I made the spotifyd running and I am able to stream the music with my sound card. I'm gonna publish logs about it soon.

BTW, playing with the embedded platform like the Beagle is a great way of getting good with Linux

  Are you sure? yes | no

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