## [Updated]Get emulating while you’re waiting

We’re guessing that after yesterday’s craziness you find yourself in one of two states:

1. You’ve ordered a Raspberry Pi but it’s not going to get delivered for a while.
2. You couldn’t order a Raspberry Pi this time round but are still really keen to get started.

Well fear not because we’re going to talk you through setting up an emulator for the Raspberry Pi and it’s really rather easy.

Firstly, you’ll need to download a piece of software called Virtualbox. VirtualBox is free software and available for OS X, Windows and Linux. The software allows you to run virtual machines within your existing operating system.

The second thing you’ll need to download is the Raspberry Pi emulator which you can get here(choose the most recent version of RaspberryPi.VMware.zip).

From here getting the emulator up and running is very simple. Install VirtualBox and unzip the emulator. Inside the emulator you’ll find to files one ending .ovf and the other .vmdk. Double click the .ovf file and VirtualBox will open with a screen about “Appliance Import Settings”. Click import and wait a few minutes while it imports.

When the import has finished VirtualBox should have ‘Raspberry Pi’ in its list of machines. Select it and click start. When you come to the log in screen:

User: rpi

[update]

This is emulated linux environment will allow you to run the Raspberry Pi simulation. To do so:

Click on the X in the bottom left corner (where the Windows button is for MS users)
Find and open LX terminal. Then run the following terminal commands.

First acquire the debian image prepared for the R_Pi’s arm processor.
``` wget http://rpi.descartes.co.uk/sim-emu/debian6.tar.gz ```

Then extract the image.
```tar -zxvf debian6.tar.gz ```

Change directory to the newly created folder.
```cd debian6 ```

`./launchDebian6`

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## A brief run down of the last 15 hours

Like most of the geek community we were up before 6 this morning watching the raspberrypi.org homepage. Until 6AM nothing loaded and then at 6 a static page went up which pointed us on to places to buy the Raspberry Pi. These pages also wouldn’t load.

We got through to RS who seemed to be completely unaware of what they and their servers were caught in the middle of and were simply registering expressions of interest. Apparently this will equate to sales soon.

Meanwhile over at Farnell there was total meltdown and reports of them selling out came through by about 06.30.

Six hours later while at lunch from the day job we managed to order from Farnell. Shortly after this they themselves announced they’d sold out. We don’t know if the earlier sell out was false or if the first was units that are actually built and the second was as far as they can build to order. All we know is now both sites are pre-order only.

Several emails have come through from Farnell. Our predicted shipping date is mid-April. We hope it’s sooner.

If you were unlucky this morning don’t worry. There’s plenty that can be done with a Raspberry Pi emulator while you wait for the real thing.

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## We believe we have a launch date!

While there is still an air of mystery, the raspberry pi foundation seem to have named the 29th February as their launch date. The website recently was updated with the post:

“The Raspberry Pi Foundation will be making a big (and very positive) announcement that just might interest you at 0600h GMT on Wednesday 29 February 2012. Come to www.raspberrypi.org to find out what’s going on.”

Liz Upton (Raspberry Pi web operations guru) has refused to comment any further citing “arcane contractual reasons”.

We at R_Pi Audio fully believe this will be the launch. Set your alarms.

## Sample rate to Bit rate

We looked previously at the sampling theorem and the way that affects the frequencies which can be represented. Since the advent of audio compression algorithms all the talk has been about bit rate. Today we’ll look at these two terms and converting between the two.

As we’ve said previously, the sample rate is simply the number of times we record the signal’s value when we  convert it from analogue to digital. There are more values that become important at this point. The nature of a digital audio system is that it will only allow a sample to take a finite number of values. The number of bits (a bit is a binary one or zero) a sample is recorded with is called the sample resolution. In CD audio the sample rate is 16bits. The other thing to remember about CD audio is it is stereo.

So to get to our bit rate we multiply:

Sample Rate x Sample Resolution x Channels

For CD audio that’s:

44100 x 16 x 2 = 1411200 bits per second or 1.4 Megabits per second.

Seeing this figure makes those of us familiar with compressed audio realise the sort of data rate savings it brings. “High quality” compressed audio tends to have a bit rate of between 300-400 kilobits per second.

Posted in Audio Theory | Tagged | 1 Comment

## Two views of audio

There are two views, or domains, in which audio must be viewed to make any sense of it.

Firstly, the time domain. This is the most common way of thinking of audio. Sampled audio is a time domain representation of the sound. Viewing sound as time domain information is to view it as a changing amplitude over time.

Secondly audio can also be viewed in the frequency domain. The idea of the frequency domain is that a signal can be represented as a collection of sine and cosine waves. A frequency domain representation of the audio views it as changing amplitude over the frequency spectrum.

These two views can be switched between by using the Fourier Transform.

Posted in Audio Theory | Tagged | 1 Comment

## The Sampling Theorem

Digital audio is underpinned something called the Shannon-Nyquist sampling theorem. This theory from way back in 1949 and relates to all information but is particularly important to the study of audio.

What the theory tells us is that any continuous signal (including sound) can be represented by a series of samples taken at twice the highest frequency contained within it.

It is common to refer to the nyquist frequency ($f_n$),the highest frequency that can be represented at a given sampling frequency ($f_s$). They are related by the formula

$f_n = f_s/2$

The range of human hearing is at its very best 20 KHz. Sampling sound signals audible to humans therefore requires a sampling frequency of at least 40KHz.