Author: root42

Attaching a USB sound card to the Raspberry Pi

Since my Raspberry Pi runs headless, and the analog audio output is not that great, I decided to add a USB sound card to my little machine. I took a Roland UA-1G, which I was using before on an OpenWRT machine. The device was immediately recognized:


Bus 001 Device 007: ID 0582:00ea Roland Corp.

However, ALSA will prohibit the card from becoming sound device #0, thus being the default. For that you have to comment out the following line in /etc/modprobe.d/alsa-base.conf: 


# Keep snd-usb-audio from beeing loaded as first soundcard
#options snd-usb-audio index=-2

After rebooting or restarting ALSA, the Roland will become the default sound device: 


$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: UA1G [UA-1G], device 0: USB Audio [USB Audio]
  Subdevices: 0/1
  Subdevice #0: subdevice #0
card 1: ALSA [bcm2835 ALSA], device 0: bcm2835 ALSA [bcm2835 ALSA]
  Subdevices: 7/8
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
  Subdevice #4: subdevice #4
  Subdevice #5: subdevice #5
  Subdevice #6: subdevice #6
  Subdevice #7: subdevice #7

Playing with REST: SilverShield USB power web frontend

Yesterday I started to code a small tool that lets me turn on and off power outlets on my USB controlled  power strip. You can find the humble beginnings over at github. It uses the sispmctl tool to turn on and off the sockets. It is implemented in Python using the Tornado web server. The rendering is simple, not styled and not yet HTML compliant. But it already scans for attached devices, and lets you toggle single outlets. However, today the power strip sort of died on me, and now I have to look for another project to practice REST with…

How to convert a Python list into a string in a strange way

Given a list in Python, suppose you wanted a string representation of that list. Easy enough: 


str( [ 1, 2, 3 ] )

However, suppose you did not want the standard notation, but rather apply some function to each string element or simply do away with the brackets and commas, you can use list comprehension, the join function and an empty string literal: 


''.join( str( i ) for i in [ 1, 2, 3 ] )

I already knew list comprehension, but using it in this scenario and with a string literal as an object was new to me. Anyway, using such code is probably a bad idea, since it might be hard to read! At the very least, one should stow it away in a function with a fitting name, such as jointStringRepresentationOfListItems( list ). But really, I am not even sure what I would use that for…

Update: even better is this: 


','.join( map( str, [ 1, 2, 3 ] ) )

How to turn the Raspberry Pi into a music server

I have set up my Raspberry Pi as a music and file server. My requirements were:

  • Big local storage (HD), shared over WiFi
  • Local music playing capability, remotely controllable
  • AirPlay speaker capability
The means by which I fulfilled the requirements were:
  • Platinum 1TB MyDrive and Edimax EW-7711UTn USB wireless adapter
  • mpd Music Player Daemon
  • shairport AirPort emulator
For the first part, I bought a WiFi adapter, the Edimax EW-7711UTn. This one works out of the box with Raspbian, using WPA encryption (Note: I switched to a RTL8188 based dongle by now). It identifies itself with lsusb as:
Bus 001 Device 004: ID 7392:7711 Edimax Technology Co., Ltd EW-7711UTn nLite Wireless Adapter [Ralink RT2870]
This can be easily configured using the wpa_gui tool that comes with the default Raspbian installation. Settings will be saved and restored upon reboot.
Second, I installed samba, samba-common and samba-common-bin for sharing my USB drive. The latter one is a Platinum MyDrive, which is attached to a powered Belkin 7-port USB hub, so that I only need two power supplies. One for the Raspberry Py, and one for the Hub and its attached devices. The MyDrive has been formatted with NTFS, so as to be easily mountable under Linux, OS X and Windows. I mount it using the standard /etc/fstab mechanism. Just added one line to the file:
/dev/sda1       /media/MyDrive  ntfs-3g defaults          0       0
The /etc/samba/smb.conf gets in its first iteration only one additional share, a write-for-all public share, as a big file dump: 
[BigDump]
    comment = Free for all
    read only = no
    path = /media/MyDrive/FreeForAll
    guest ok = yes
Note that literally everybody in your LAN can access this and write to it! You may want to fine tune this…
Now to the mpd. It is easily installed by doing apt-get install mpd. When configuring it via /etc/mpd.conf make sure to change the following lines:
music_directory         “/media/MyDrive/Music/”
password                “MyVeryOwnPassword@read,add,control,admin”
bind_to_address         “any”

Change the directory and password to your liking. Then restart the service or reboot you device. You can control the mpd using a magnitude of clients. For example Theremin for OS X or Mpod for iOS.
Finally, I would like to be able to use the RasPi as an AirPlay target for my Mac and my iOS devices. This can be done via shairport. There are already a lot of good howtos for shairport on the Raspberry Pi. So I refer you to one of those. Two things come to my mind, though:
  1. The Net::SDP library, required by shairport, is not available on Raspbian by default. It is best to clone the github mirror, and go by its installation instructions. Installation via CPAN fails, plus CPAN needs huge amounts of RAM.
  2. The configuration needs to be tweaked a bit. The /etc/init.d/shairport script should be tuned to a sensible name for your RasPi. 
This way, you will be able to see the RasPi in your AirPlay speakers list and it will be happily churning along.

Quoting a set of parameters for program arguments using sed

I have a script which launches another program. That program takes some command line arguments, which in turn can have parameters. Those parameters may include “;”, which has a special meaning in the Bash. To quote these, you can use the following sed command: 

$ echo "foo --bar=1 --baz=2" | sed -e 's/(--[^[:space:]]*=)([^[:space:]]*)/1"2"/g'
foo --bar="1" --baz="2"

Improved handling of background GNU Global update

Earlier this year I detailed how to run gtags of GNU Global when Emacs is idle. However, this collides with some special Emacs modes, like ediff. That mode makes its own Emacs frame layout, which can be destroyed by the xgtags incremental update buffer. So I conceived a way to disable gtags temporarily as long as ediff is active: 


;; This stuff is only needed, if you haven't got xgtags loaded yet
(autoload 'gtags-mode "gtags" "" t)
(require 'xgtags)
(add-hook 'c-mode-common-hook (lambda () (xgtags-mode 1)))
(defun global-update-incrementally () (shell-command "global -u -q" "*Messages*" "*Messages*") )

;; Call gtags update when idle for some time
(defcustom my-gtags-update-idle-time 60
  "Number of idle seconds before an incremental gtags update is launched"
  :group 'my-group
  :type 'integer
  )

;; initially allow gtags updates
(setq my-gtags-update-active t)

(run-with-idle-timer my-gtags-update-idle-time t
                     (lambda ()
                       (if (and my-gtags-update-active
                                (not (minibufferp) )
                                )
                           (progn
                             (message "Running gtags...")
                             (global-update-incrementally)
                             )
                         )
                       )
                     )

(add-hook 'ediff-quit-hook 
          (lambda () 
            (message "Activating gtags update.")
            (setq my-gtags-update-active t) 
            )
          )

(add-hook 'ediff-before-setup-hook 
          (lambda () 
            (message "Deactivating gtags update.")
            (setq my-gtags-update-active nil) 
            )
          )

Encode image sequence to MPEG4 video

Today something very simple: use ffmpeg to encode an image sequence as a MPEG4 video:

ffmpeg -f image2 -i filename-%04d.png -vcodec mpeg4 -b 6000k output.mp4

I often use blender or custom software to render out a bunch of images. Blender usually names output files filename-0000.png, where 0000 is the frame number.  So ffmpeg can generate those filenames with the usual printf style format. You can of course tweak ffmpeg’s output via a myriad of options. But this here will simply generate a 25fps mpeg4 file, which is easily embedded for example into your Keynote presentation. On OS X you can get ffmpeg for example via MacPorts: sudo port install ffmpeg, and Linux distros usually have a package for ffmpeg as well.

Using Ctrl+Arrow keys in the OS X terminal

The key combination Ctrl + left or right arrow key can be used to go a word left or right in many programs such as GNU Emacs or the bash. In the default configuration of the OS X terminal, this is not the case. That’s because Terminal.app sends the wrong key codes for bash’s default configuration. So you just need to got to the preferences (cmd+,) and set the key codes for Ctrl+left and right to 33b and 33f respectively: