This article continues a series about building a DIY digital audio recorder. Inspired by this microcontroller audio project [via], I set out to build a simple digital recording device. I chose Texas Instrument's MSP430 microcontroller for this project because it's fast (16 MHz), it's cheap ($1), and it's very low power. Read the first part, and the second part.

This week we'll progress towards a digital audio recorder by playing audio files from a SD memory card. First, we'll convert an audio file to a raw format and image it directly to a SD card. Then, we'll interface the SD card with the MSP430 and play an audio file. See it in the video:


Next time we'll extend this basic circuit to include a microphone and audio recording capabilities.

Read on to learn more about generating audio with a microcontroller.

Overview
Read the previous article in this series for the fundamentals of microcontroller audio. Learn about basic MSP430 connections and programming in the MSP430 primer. This week we'll use the tiny, inexpensive MSP430 microcontroller to play audio from a SD card.



Playing an audio file
In the previous project, I used the MSP430 pulse-width modulator (PWM) to generate simple tones. The PWM duty cycle changed the frequency, and thus tone, of the generated signal. The signal is cleaned by a simple audio filter, and amplified by powered PC speakers. Hear it in this video clip from the previous article:




Each value generates a different frequency. If we change the values about 8000 times a second (8KHz) we can reproduce telephone quality audio.

8 bit audio (each sample requires one byte) at 8KHz requires 8000 bytes of data per second. 16 bit audio, the quality associated with CD players and PCs, requires twice as much data per sample (16 bits or 2 bytes) -- 16 kilobytes/second.

With 2K of internal flash, the MSP430 can store about one-quarter second of audio. External memory is needed to store any meaningful amount of audio. There's no better source of cheap storage than flash memory chips. A raw flash memory chip would be a special order item, but you probably already have some flash memory -- SD cards! These cards are ubiquitous in digital cameras and low-end MP3 players.

Working with SD cards
NOTE!!! We are going to use the SD card as flash memory. It will not have a full (FAT) file system, and can not be read on a PC by the usual methods. It will not be possible to drag and drop a raw audio file to the card and read it with the MSP430, nor will it be possible see recordings from the file browser on a Mac or PC. Special disk tools provide access to the raw disk, but a full FAT file system implementation is too big for the tiny microcontroller. The F2013 has 128 bytes of RAM and 2 kilobytes of flash. A workable FAT implementation requires 1024 bytes of RAM and several kilobytes of flash program space.

The prototype design accesses the SD card using the simple SPI protocol. SPI is a simple three wire interface that allows a microcontroller to read and write data from external chips. If you want the dirty details on how the SD card works in SPI mode, read some of these tutorials [Tutorial 1, Tutorial 2]. A source code library from TI provides easy access to the disk without writing our own initialization and read/write routines (mmc.c/mmc.h in the project archive).



SD card storage is divided into 512 byte chunks called sectors. A full 512 byte sector must be read or written at once. The MSP430F2013 has only 128 bytes of RAM -- not enough to hold a whole sector for random access. We can still read/write each byte in order, as it's needed. This method will suffice as long as each byte is read or written sequentially.

Playing an audio file is as simple as reading a byte from the SD card and writing it to the PWM duty cycle register.

Converting audio to a raw format
My 'hello world' audio clip comes from the intro to my videos. I converted the clip to an 8KHz .wav file with Audacity and then saved it to raw text file with the free version of Switch, as described here.

1. First, install Switch.
2. Start Switch, and add your desired files to the queue by clicking "add files".
3. Now set the output format to ".raw", and then click "Encoder Options". Set the encoder format to "8 bit unsigned", sample rate to "8000", and channels to "mono".
4. Now click convert and the raw version of the audio files are created.

The raw text version of the audio file is included in the project archive.

Imaging an SD card
The MSP430 isn't powerful enough to deal with a file system, so we need the audio to start exactly at the first sector the of SD card. A simple copy->paste operation would put the file on the SD card according to the rules of the PC file system. To avoid this, we image the file directly onto the card. This will be familiar if you've ever 'imaged' a CD .iso file, say, for a bootable Linux live CD. I used DiskImage 0.9 from Durban.

Imaging the audio file onto a flash card was pretty easy with DiskImage. I put the SD card in a USB flash card reader and attached it to the USB port. I opened DiskImage and it detected the card as a logical and physical volume.

1. It's crucial to deal with the disk directly, rather than through the file system. All operations should be done to the listings under "Physical drives - raw drive, independent of partitions" box.
2. Find the physical partition that represents your SD card - it's easy to identify the correct physical drive because it should be type "Removable" and the correct size (in my case 14MB). Be sure you don't overwrite your actual hard drive, this utility can easily do that!!!
3. Click on the correct drive to highlight it.
4. In the "With selected item do" box, click the "Import from file" button. Click yes, and type "i agree" after you verify that the correct disk is selected. Click yes again.
5. Next, DiskImage prompts for the file that will be imaged onto the disk. Choose the raw audio .txt file outputted earlier.

With the raw audio imaged onto the SD card, we're ready to play it from the MSP430. Take the SD card out of the computer and put it into the card holder on the prototype.

Say Hello
In the video you can see the basic firmware functions. First, a breakpoint set at the timer shows how the audio is only updated on every other byte. If we move the breakpoint inside this switch, the debugger stops every time the audio changes. Next, execution halts each time 512 bytes are read from the SD card and the sector changes. A final breakpoint halts execution each time the sector resets to 0.

Next time
The next article will be the final functional enhancement to the digital audio recorder. We'll extend this basic circuit to include a microphone and audio recording capabilities.

Prototype Circuit
For complete details of the prototype, see the previous article in this series. This week's firmware is in the project archive.


Related links
Program a MSP430 microcontroller.
Make a singing MSP430 microcontroller.

Over the holidays I got a Nike+iPod Sport Kit so I could start tracking my treadmill work. Unfortunately, I didn't have a pair of Nike's to support the device, which requires specific shoes which have a little recessed well in the sole of the shoe to hold the pedometer/transmitter. Instead, also over the holidays, I got the shoes I prefer: a pair of New Balance 779's. What to do but DIY, right? I've compiled a list of "hacks" for the Nike+iPod, all of which will somehow attach the transmitter to your shoe. Unfortunately, all of them tend to damage the shoe in some way. Next week I'll show you how to make a simple, effective pocket for your transmitter that will not harm the shoe in any way.

Simple and cheap: The 99-cent DIY shoe mod takes a simple strip of adhesive Velcro (the hook side) and allows you to latch to your shoelaces, while also slipping the unit under your laces. My problem with this? The Velcro will ultimately chew up your laces (I treasure the laces on New Balance, so unlike the slippery junk Nike's come with) and the transmitter, while tiny, can hurt when pressed against the top of your foot in this way.

(more on the next page)

There was a fix for this problem discussed quite some time ago by our friends at Engadget. The solution they addressed suggested using a video file provided by Best Buy to force those stubborn pixels back into doing their jobs. The video file, which works by flashing red, green and blue colors on the display, seeks to dislodge stuck pixels and make them part of the team again. Mixed reports suggest that the process is at least marginally effective.

It seems to me that a person could attempt the same type of fix without having to hunt down that Best Buy video file. If I was required to try it for myself, here's what I'd do:

• In any paint program I'd create a set of solid color images including: black, white, cherry red, forest green, canary yellow, royal blue, teal, magenta and cyan.
• Save those images in a large enough image size so they won't tile if set to desktop background.
• Set those images to run as a screen saver, slide show. Cycle them as fast as possible.
• Let that screen saver run all night.

In theory, the rapid fire flashing of the intense colors should break loose those stuck pixels. I'd like to test this process myself but all my LCD pixels are performing pretty well. This process is not intended to bring dead pixels back to life, it's suggested simply for breaking loose a few stubborn ones.

One additional suggestion: If you suffer from epilepsy, you'd probably rather not attempt this fix.

So you thought that once a PC was retired from data service it just went to the junk heap to be melted down and reincarnated as a Wii controller? Well, that's not what happens all the time. Some computers and their peripherals get a full make over and go on with their lives in the form of something new. Have you ever heard of a computer mouse turned Chia pet? Well, believe it or not, I found one.

A hilarious if not artsy gallery of images resides over at Techlicious.tv , and I'll bet that you will be greatly amused with what you find there. The creations displayed are made from the parts of computers just like the one you might have sitting on your desk. The best part might be that these projects all look like just about anyone could make them if given the right tools and glue. Take a look at the video display which was converted into a hamster cage. Now there's some ingenuity for you!

I guarantee it will be worth your time to take a little look. My favorite entry in the whole group would have to be the tower case hibachi. Please feel free to tell us about PC reincarnations that you have created and tell us how you go about building these things!

While a handyman has a veritable cornucopia of tools and materials to choose from for holiday gift ideas, what about the hobby hardware hacker? Once you've got a decent soldering iron, DMM and some reliable suppliers you've got everything you need, right? Of course not. While tools and parts are nice, don't overlook some of these crazy kits out there. We've got a short list of six for you:

Robots
Blubber Bots, to be precise. Essentially little autonomous floating robots. Fun at parties. Plus, the kits are reasonable and fun. Good for teenagers.

Handheld Computer
The DIY smartphone. It's no iPhone, but Compulab has a neat little kit that'll let you build your own handheld computer. Since you can purchase components like WiFi or GPS ala carte, you only get what you need in a PDA-- a refreshing change of pace, really.

Wood Keyboard
A DIY keyboard. Sure, a wooden keyboard you put together sounds strange. And for $300 the thing should at least come with some cool mouse... But instead it comes with a saw for carving out the keys. Perfect if your DIY gift recipient "has everything." Note that manufacturer Hacoa makes a habit of coating silicon in wood. Stainable USB drive anyone?

Overweight folks, beware the watt.

I am providing a concept for you that can accomplish several admirable goals all at the same time. If you're green-minded and spend your days telling people how we need to "save the planet," then you need to read this and put your butt where your mouth is...umm, sort of.

Convergence Tech Inc. is ready to provide you with complete plans for building a bicycle powered generator yourself . This generator utilizes any bicycle that is in decent shape and can easily produce between 150 to 200 watts at 12 to 20 volts DC. Bikes with larger wheel diameters provide a better mechanical advantage than bikes with smaller wheels but Convergence Tech indicates that their concept has been employed using bicycles of various sizes and types, all with satisfactory results. When you're done creating electricity, you can easily disengage the bike and ride it to work or school.

Selecting the audio and video cards for your new computer can be a daunting task. Breaking down your computer needs and desires into terms of functionality, you can make sense of how the multitude of dedicated processing cards can be sorted out to serve your particular needs.

Dedicated processing cards are sort of like your computer's main processor, except they are designed to handle specific tasks and types of information. Video or graphics cards are responsible for how visual data is displayed on your viewing screen. Sound cards are responsible for handling the performance of your speakers and other music or sound-related working applications

Your dedicated processing cards are two of the biggest component decisions you'll encounter when making your new machine truly customized. To make good choices, you need to go back to the very beginning of your computer planning and review exactly what you want your new computer to accomplish for you. Once you are reaffirmed in your computer performance desires, then you can proceed in choosing your processing cards and make your machine truly personalized.

October 15 has been chosen as Blog Action Day, a day when the blogging community seeks to unite in voice for a common cause. The declared focus for this year involves our earthly environment and how we bloggers can aid in spreading the word about things we can all do to improve our stewardship of our world's resources. While the debate on "global warming" has been muddled to the point that the concept is now being called simply "global climate change," with scientists pointing at every conceivable cause, one core reality remains clear. That reality is the truth that our earth is a finite resource, and has always been deserving of our best efforts to take good care of it.

My Blog Action focus today is about "getting off the grid" via solar power, mainly because that's a pet concept of mine. The reason is simple: every volt of electrical energy that can be generated and utilized without involving existing utility structures reduces demand for exhaustible fuel reserves. So, in pursuit of reducing your dependence on commercially generated electrical power, here's a simple guide to solar-powered battery chargers, and other solar devices that you can buy or build yourself.

We all like making things out of miniature electronics (don't we), but I've just found a use for the world's smallest camcorder. Yes, this thing is battery powered and the size of a pack of gum, but what it lacks in size it makes up for in covert usage.

In the past few months, I've heard from many people that gas tanks of parked cars are being invaded by siphoning gas thieves. It's no wonder -- gas is not exactly cheap these days. It's hard to imagine that different thieves hit different cars all the time in the same area, so wouldn't it be cool to use one of these inside the wheel well of a parked car or truck and make a video of a thief stealing gas from your precious vehicle?

The camcorder in question records at 15 frames per second (not the best, but passable) in the 3GP format, which is most commonly used in newer multimedia cellphones. I can't begin to imagine the uses for a highly covert miniature camcorder like this, but busting gas thieves and providing evidence to the police sounds like a good starter use to me. The unit stores video on a standard microSD card and has a wide angle 3.6mm lens. While not the best specs for video recording (far from it), the size alone makes this gadget exceptionally useful.

Coming into this blog post, number four in my series about building your own computer, I finally have had some of the components delivered. I have included some pictures and information to keep you up to date, but we're not nearly ready to begin assembling parts. I'm hoping that my editors aren't getting too annoyed that I haven't started actually screwing parts together yet. Perhaps I should have told them that I didn't have $900 dollars to plunk down to buy all the parts at one time. Oh well, my intention was to walk you through this slowly any way. You can see my first three lead -in posts via this link!


This week I went through the process of choosing my hard drives. Yes, I mean drive(S), as in more than one. After getting various opinions on the subject I have decided that my new computer shall be set up with two hard drives. One will serve for daily use and the second one will be a fully updated backup kept on hand as an instant replacement just in case the first one fails. It was suggested that I could run the two hard drives in tandem, using one for the operating system and one for image storage which is a fine idea also but I have determined that I do not need that volume of data storage capacity yet and that option would still leave me needing a back up solution. I can always throw another drive in there. That's one of the advantages of building a computer from scratch.

Sure your iPod Nano is cool and it brings you hours of listening pleasure. But is it quirky and dare I say it, cute? I know, I know, totally a girl thing to say but come on girls have iPods too (the pink one).

On the forums at Craftster one poster made a really fun and creative iPod Nano cover that transformed her music player into a very fun and, yes, cute monster. She includes a basic tutorial for the crochet pattern.

If I could crochet and if I had an iPod I would sooooo make this but since I can neither do nor have I'll leave that up to you. I personally think this is super cute and that the creator of it was very creative. If you make one of these be sure to warn your iPod that its about to be eaten by a warm and cuddly monster.