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OS X application tips

Struggling with applications that open certain extension files? For example opening mp3-files causes iTunes to open them default and copy the files to it’s own library.. Get RCDefaultApp

Fast, simple mp3-player: Cog

LineIn is a simple application for OS X to enable the soft playthru of audio from input devices. In simpler terms, you can use LineIn to play sound coming in through a microphone or any other device plugged in to your Sound In port, just as was once available with OS 9’s “Play input through output device” option.

Text editor Smultron

Easy and simple DVD/CD burner

Remember that OS X internal Preview can do lot’s of stuff for images, no need to find any application for basic image manipulation.

BOSS RC-50 Loop pedal

Some snippets from BOSS RC-50 Loop Pedal:

– At software version 1.0 the MIDI sync is quite useless and it’s not working as promised. See following discussions/links:


– MIDI CC (Control Change) command can be used to control the pedal in some ways. Setting tempo is quite bizarre, the value (0..127) is not linear to tempo.  Some measurements (first value is the third byte aka second CC value, where CC#1 consists of bytes 176+1+BPMvalue and CC#80 is 176+80+127 where 127 is constant since RC-50 doesn’t take any values in CC#80 messages) :

  • 127 = 250.1 BPM
  • 115 = 230.1 BPM
  • 80 = 172.2 BPM
  • 40 = 106.1 BPM
  • 10 = 56.5 BPM

– Touching the wheel selector changes your patch and you will loose all phrases without warning. This is quite nasty. This happends even you set the patch range to 1..1. When you touch the selector wheel the patch doesn’t change but it resets the patch to what you haved saved before.

Sparkfun Button Pad SPI with Arduino

Some pitfalls and thought on SparkFun’s Button Pad SPI. I’m using it with Arduino Duemilanova.

The Button Pad SPI is NOT standard SPI. You cannot use it with Arduino’s (Atmega’s) built-in SPI. You have to write your own code to handle the communications.

  • Yes, it’s possible to hook multiple boards and read all buttons statuses with only one “SPI” line. This however might be bad idea, see below.
  • The manual confuses you with the feedback-line. That’s actually MOSI from the last board. So hook that to input pin on Arduino and read the button statuses from that.
  • You cannot hook the boards parallel to the SPI line (using separate CS lines) since the boards don’t respect the CS. They will receive data even if the CS line is down.
  • If you want to control the boards separately you must have separate CS and SCK lines. MISO can be common.
  • The boards can loose their multi-board configuration setting and think they are in one-board-environment. This sucks. Hard. You have to reprogram the boards separately, the configuration works only for the board where the “SPI” is connected to. Of course this isn’t SparkFun’s problem: you’re causing this by for example when your Arduino boots or there’s some other clitches on the “SPI”-line. I think it should be more difficult to change the setting, for example only when some dip-switch or something is connected/unconnected..
  • You cannot press more than 2 buttons on a column. However, in rows you can press and receive all 4 buttons.
  • You cannot align the boards in 2*2 shape with all having same orientation (column/row).
  • It’s not 24-bit color scheme, or at least the color values are not linear to what you see. The color value has exponential-like effect to the actual led brightness.
  • It’s still a great product for the price.
  • It looks very very sweet. In dark it looks like a candy!
  • On user inteface side these boards work just like they are supposed to work. Unlimited possibilities in the software that you write for them. Be creative, be fun!

FFT aka Fast Fourier Transform explained well

From here:

The fourier transform converts a time representation ( samples ) into a frequency representation ( i.e. the frequency spectrum of these samples ) and vice versa.

Commonly it splits a number of audio samples, say 2048, into a corresponding number of sine waves of different frequencies, so that you know the volumes and the phase for each sine.

The principle is very simple:

If you multiply a set of samples with another set of reference samples and add each result, like this:

result = sample[ 0 ] * reference[ 0 ] + sample[ 1 ] * reference[ 1 ] + … + sample[ n ] * reference[ n ]

… the result is an indicator for the similarity of both sets of samples.

And now with sines

Now, if you want to know the volume and the phase of a sine at a certain frequency inside an audio sample, you have to calculate 2 of these similarity values:

FC = 2 * PI * frequency / samplerate; // “frequency constant”

result_i = sample[ 0 ] * sin( FC * 0.f ) + sample[ 1 ] * sin( FC * 1.f )+ … + sample[ n ] * sin( FC * n.f )

result_r = sample[ 0 ] * cos( FC * 0.f ) + sample[ 1 ] * cos( FC * 1.f )+ … + sample[ n ] * cos( FC * n.f )

To get the actual phase and magnitude, you use simple pythagoras to get length and trigonometry to get the angle:

magnitude = sqrt( result_i * result_i + result_r * result_r);

phase = atan2( result_i, result_r) // I think…

This way you can determine how much of a frequency is present in an audio file etc., and which phase it has relative to the beginning.

Complex Numbers?

In Fourier Transform lingo, the sine part is oftenly called “imaginary” while the cosine part is called “rational”. They pretend to be complex numbers, but that’s more or less only a convention. The phase/magnitude representation IMHO feels more like complex numbers.

Getting the whole spectrum

Now, the idea behind fourier transforms is, if you start with a “fundamental” frequency of exactly 1 cycle over the whole length of the analysed set of samples (call it n samples) and do this with every multiple up to n/2 of the fundamental, you get a representation that thoroughly reflects the spectrum of the original sample. – If you transform it back (create a new bunch of n samples out of adding sines and cosines), it will perfectly fit the original samples!

Each “frequency slot” or multiple of the fundamental (1, 2, 3, …, n/2) is called a “bin”. (Note, there’s also a zero-bin, I’ll explain that later)

Fast Fourier Transform – FFT

This would be a very time consuming technique, if there wasn’t the Fast Fourier Transform, or FFT. The FFT is a simple, yet highly difficult to understand algorithm. It takes advantage of the fact, that if you draw all those sines in a graph, you have many lines that cross each other at certain points. And because they cross each other pretty often, you need only 1 multiply to gather information for many results needed. Hence, what FFT does is recursively reordering (“prepacking”, “butterfly”) the samples in a tricky way so that only a fraction of calculations are needed to create the spectrum for a given range of audio samples. The drawback is, FFT requires certain lengths (or “ns”) of samples, which are a power of 2: like 16, 32, 1024, 4096 samples etc.

DC offset and Nyquist in FFTs

Typically, a FFT outputs the spectrum in sines and cosines, but it also output 2 special values: A cosine representing the DC (offset of powers above zero and below zero), which is bin 0, or the fundamental frequency * 0 and a sine representing Nyquist (because at Nyquist, there can’t be a cosine!). Hence some algorithms require N/2+1 bins, others “pack” the Nyquist bin into the sine field of the DC bin.

Hope this helps, or maybe it’s a start…


Wink Urs

How to remove the gloss / glare / white area from iPhone app icon?

To remove the gloss / glare / white area from iPhone app icon that iPhone adds automatically – go to the Xcode and open up the App’s Info.plist. On right side you see list like button. Press that and you’ll get new row with selectable property name. From there pick the “Icon already includes gloss..” and check the checkbox.

So in short: it’s a property in the App info.plist.