Working on a stable setup

Next step for the synth module project was to get the Raspberry Pi 2 to run in a stable manner. It seems like I’m getting close or that I’m already there. First I built a new RT kernel based on the 4.1.7 release of the RPi kernel. Therefore I had to checkout an older git commit because the RPi kernel is already at 4.1.8. The 4.1.7-rt8 patchset applied cleanly and the kernel booted right away:

pi@rpi-jessie:~$ uname -a
 Linux rpi-jessie 4.1.7-rt8-v7 #1 SMP PREEMPT RT Sun Sep 27 19:41:20 CEST 2015 armv7l GNU/Linux

After cleaning up my cmdline.txt it seems to run fine without any hiccups so far. My cmdline.txt now looks like this:

dwc_otg.lpm_enable=0 dwc_otg.speed=1 console=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 rootflags=data=writeback elevator=deadline rootwait

Setting USB speed to Full Speed (so USB1.1) by using dwc_otg.speed=1 is necessary otherwise the audio coming out of my USB DAC sounds distorted.

I’m starting ZynAddSubFX as follows:

zynaddsubfx -r 48000 -b 64 -I alsa -O alsa -P 7777 -L /usr/share/zynaddsubfx/banks/SynthPiano/0040-BinaryPiano2.xiz

With a buffer of 64 frames latency is very low and so far I haven’t run into instruments that cause a lot of xruns with this buffer size. Not even the multi-layered ones from Will Godfrey.

So I guess it’s time for the next step, creating a systemd startup unit so that ZynAddSubFX starts at boot. And it would be nice if USB MIDI devices would get connected automatically. And if you could see somehow which instrument is loaded, an LCD display would be great for this. Also I’d like to have the state of the synth saved, maybe by saving an .xmz file whenever there’s a state change or on regular intervals. And the synth module will need a housing or casing. Well, let’s get the software stuff down first.

Working on a stable setup

Building a synth module using a Raspberry Pi

Ever since I did an acid set with my brother in law at the now closed bar De Vinger I’ve been playing with the idea of creating some kind of synth module out of a Raspberry Pi. The Raspberry Pi 2 should be powerful enough to run a complex synth like ZynAddSubFX. When version  2.5.1 of that synth got released the idea resurfaced again since that version allows to remote control a running headless instance of ZynAddSubFX via OSC that is running on for instance a Raspberry Pi. I looked at this functionality before a few months ago but the developer was just starting to implement this feature so it wasn’t very usable yet.

zynaddsubfx-ext-guiBut with the release of ZynAddSubFX 2.5.1 the stabilitity of the zynaddsubfx-ext-gui utility has improved to such an extent that it’s a very usable tool. In the above screenshot you can see zynaddsubfx-ext-gui running on my notebook with Ubuntu 14.04 controlling a remote instance of ZynAddSubFX running on a Raspberry Pi.

So basically all the necessary building blocks for a synth module are there. Coupled with my battered Akai MPK Mini and a cheap PCM2704 USB DAC I started setting up a test setup.

For the OS on the Raspberry Pi 2 I chose Debian Jessie as I feel Raspbian isn’t getting you the most out of your Pi. It’s running a 4.1.6 kernel with the 4.1.5-rt5 RT patch set, which applied cleanly and seems to run so far:

pi@rpi-jessie:~$ uname -a
Linux rpi-jessie 4.1.6-rt0-v7 #1 SMP PREEMPT RT Sun Sep 13 21:01:19 CEST 2015 armv7l GNU/Linux

This isn’t a very clean solution of course so let’s hope a real 4.1.6 RT patch set will happen or maybe I could give the 4.1.6 PREEMPT kernel that rpi-update installed a try. I packaged a headless ZynAddSubFX for the RPi on my notebook using pbuilder with a Jessie armhf root and installed the package for Ubuntu 14.04 from the KXStudio repos. I slightly overclocked the RPi to 1000MHz and set the CPU scaling governor to performance. The filesystem is Ext4, mounted with noatime,nobarrier,data=writeback.

To get the USB audio interface and the USB MIDI keyboard into line I had to add the following line to my /etc/modprobe.d/alsa.conf file:

options snd-usb-audio index=0,1 vid=0x08bb,0x09e8 pid=0x2704,0x007c

This makes sure the DAC gets loaded as the first audio interface, so with index 0. Before adding this line the Akai would claim index 0 and since I’m using ZynAddSubFX with ALSA it couldn’t find an audio interface. But all is fine now:

pi@rpi-jessie:~$ cat /proc/asound/cards
 0 [DAC            ]: USB-Audio - USB Audio DAC
                      Burr-Brown from TI USB Audio DAC at usb-bcm2708_usb-1.3, full speed
 1 [mini           ]: USB-Audio - MPK mini
                      AKAI PROFESSIONAL,LP MPK mini at usb-bcm2708_usb-1.5, full speed

So no JACK as the audio back-end, the output is going directly to ALSA. I’ve decided to do it this way because I will only be running one single application that uses the audio interface so basically I don’t need JACK. And JACK tends to add a bit of overhead, you barely notice this on a PC system but on small systems like the Raspberry Pi JACK can consume a noticeable amount of resources. To make ZynAddSubFX use ALSA as the back-end I’m starting it with the -O alsa option:

zynaddsubfx -r 48000 -b 256 -I alsa -O alsa -P 7777

The -r option sets the sample rate, the -b option sets the buffer size, -I is for the MIDI input and the -P option sets the UDP port on which ZynAddSubFX starts listening for OSC messages. And now that’s the cool part. If you then start zynaddsubfx-ext-gui on another machine on the network and tell it to connect to this port it starts only the GUI and sends all changes to the GUI as OSC messages to the headless instance it is connected to:

zynaddsubfx-ext-gui osc.udp://

Next up is stabilizing this setup and testing with other kernels or kernel configs as the kernel I’ve cooked up now isn’t a viable long-term solution. And I’d like to add a physical MIDI in and maybe a display like described on the Samplerbox site. And the project needs a casing of course.

Building a synth module using a Raspberry Pi

Media units

media_units_editThe MK808 Android TV stick with a PCM2704 USB audio interface runs Debian Jessie with MPD and serves as our mediaplayer for audio files. It draws its power from the USB port of our cable modem so it’s always on. Most of the time Indie Pop Rocks is playing. It’s hooked up to the network via WiFi. We use MPDroid to control it.

The Raspberry Pi runs OpenELEC with Kodi. We use this for watching all kinds of video files that we stream from our NAS (an aging WD My Book Live that runs Debian Lenny) via an NFS share. It is connected to the network via ethernet.

The Chromecast is for watching Netflix. When we just got it we had some issues with connecting it to the network but after replacing our old router with an ASUS RT-AC68U it worked flawlessly.

The Technics SL-1210MK2 with Ortofon headshell and cartridge is for listening music on vinyl, you know those round black plastic units from the past with grooves in them. It doesn’t have any network connections and doesn’t run an OS. It does send electrical current to a NAD C 325BEE Stereo Integrated Amplifier with Dali Concept 2 speakers. Yeah, I’m a 2.0 guy.

The TV is an old pre Smart TV Samsung but as it still works we probably won’t replace it for the time being. It does have CEC so we can control the TV, RPi and Chromecast with a single remote.

Media units