System Update Today, Downtime

I take the unusual measure, of hosting my Web-site, and therefore also this blog, on a PC running at home. The PC I use is the Debian / Jessie (Linux) computer I name ‘Phoenix’.

This morning, I installed a major set of updates on this computer, which included 137 packages, and doing so, brought it from Debian version 8.9 to version 8.10 .

But, because this PC is also my Web-server, the required reboot also meant, that the site was offline from about 10h20 until about 10h30.

I apologize for any inconvenience.

BTW, ‘Phoenix’ had been Up, for 62 straight days.

It’s too early really to tell, whether this large an update has affected reliability adversely, but they usually do not. So for now, I’m counting this as a successful update.

Dirk

 

OpenGL | ES

I have frequently blogged about the existence of ‘OpenGL‘, which is both an open-source alternative to ‘DirectX’, and an API for communicating 3D graphics directly to a graphics card. It mainly gets used on PCs.

Well I thought I should mention, that the version of that API which gets used in smart-phones and tablets, including under Android and iOS, is actually named ‘OpenGL | ES‘ . The ‘ES’ stands for ‘Embedded Systems’.

(Update 12/09/2017 : )

Some users might wonder, if they use the Android utility I suggested above, why their devices show as having both GLES 1.x and GLES 3.y .

My personal guess would be, that the compositing only requires GLES 1.x , so that as long as the hardware provides that, Android will run. OTOH, If we’re playing “Baldur’s Gate”, then this app will require the full power of GLES 3.y …

Dirk

 

Which of my articles might paraphrase frequency-domain-based sound compression best.

I have written numerous postings about sound-compression, in which I did acknowledge that certain forms of it are based on time-domain signal-processing, but where several important sound-compression techniques are based in the frequency-domain. Given numerous postings from me, a reader might ask, ‘Which posting summarizes the blogger’s understanding of the concept best?’

And while many people directly pull up a posting, which I explicitly stated, describes something which will not work, but displays that concept as a point-of-view, to compare working concepts to, instead of recommending that posting again, I would recommend this posting.

Dirk

 

About +90⁰ Phase-Shifting

I have run into people, who believe that a signal cannot be phase-advanced in real-time, only phase-delayed. And as far as I can tell, this idea stems from the misconception, that in order for a signal to be given a phase-advance, some form of prediction would be needed. The fact that this is not true can best be visualized, when we take an analog signal, and derive another signal from it, which would be the short-term derivative of the first signal. ( :1 ) Because the derivative would be most-positive at points in its waveform where the input had the most-positive slope, and zero where the input was at its peak, we would already have derived a sine-wave for example, that will be phase-advanced 90⁰ with respect to an input sine-wave.

90-deg-phase-y

But the main reason this is not done, is the fact that a short-term derivative also acts as a high-pass filter, which progressively doubles in output amplitude, for every octave of frequencies.

What can be done in the analog domain however, is that a signal can be phase-delayed 90⁰, and the frequency-response kept uniform, and then simply inverted. The phase-diagram of each of the signal’s frequency-components will then show, the entire signal has been phase-advanced 90⁰.

90-deg-phase

(Updated 11/29/2017 : )

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