Universal Active Filters: Part 2

An easy way to conceptualize active filters is thinking about audio speakers. A speaker crossover has a low-pass, high-pass and band-pass effect breaking a signal into three components based upon frequency. In the previous part of this series I took that idea and applied it to a Universal Active Filter built with a single chip opamp based chip known as the UAF-42. By the way, it’s pretty much an older expensive chip, just one I picked out for demonstration.

Using a dual-ganged potentiometer, I was able to adjust the point at which frequencies are allowed to pass or be rejected. We could display this behavior by sweeping the circuit with my sweep frequency function generator which rapidly changes the frequency from low to high while we watch what can get through the filter.

In this installment I’ll test the theory that filtering out the harmonics which make up a square wave results in a predictable degradation of the waveform until at last it is a sine wave. This sine wave occurs at the fundamental frequency of the original square wave. Here’s the video but stick with me after the break to walk through each concept covered.

Continue reading “Universal Active Filters: Part 2″

Organ Donor Gives Up a Leslie Speaker

It was about ten years ago that [Richard] received an old musical organ. Moving to a new house meant it would be cumbersome to move the organ with him, so he opted to harvest some interesting components instead. Specifically, he kept the Leslie speaker.

A Leslie speaker is a special kind of speaker mechanism that creates a tremolo effect as well as a vibrato effect. You can hear this effect in [Richard’s] video below. Simple effects like this would be easy to do on a computer nowadays, but that wasn’t the case several decades ago. Before digital electronics, musical effects were often performed by analog means. [Richard’s] Leslie speaker is a small speaker behind of a Styrofoam baffle. The baffle spins around the speaker which changes the reflection angle of the sound, producing the musical effect.

[Richard] tried hooking this speaker up to other musical instruments but found that turning off the electric motor created an audible pop over the speakers. To remedy this, he build a simple “snubber” circuit. The circuit is just a simple 240 ohm resister and a 0.05 uF capacitor. These components give the transients a path to ground, preventing the pops and clicks when the motor is powered up. Now [Richard] can use this classic piece of audio equipment for newer projects. Continue reading “Organ Donor Gives Up a Leslie Speaker”

Universal Active Filters: Part 1

Today I am experimenting with a single chip Universal Active Filter, in this case I made a small PCB for the UAF-42 from Texas Instruments. I chose this part in particular as it facilitates setting the filter frequency by changing just a pair of resistors and the somewhat critical values that are contained on the chip have been laser trimmed for accuracy. This type of active filter includes Operational Amplifiers to supply gain and it supports various configurations including simultaneous operating modes such as Band Pass, Low Pass and High Pass make it “Universal”.

Filter Basics

Speaker Crossover Example
Speaker Crossover Example

Looking at the block diagram you can see where I have inserted a dual-ganged potentiometer to change both resistors simultaneously which should allow a straight forward adjustment for our purposes here.

Looking into the components of a simple RC filter which can easily implement a simple Low Pass or High Pass filter, we see that the math is fairly straight forward and swapping the components with each other is all that is needed to change the type of filter. Continue reading “Universal Active Filters: Part 1″

Repairing Burnt Speakers with a Steady Hand

[Martin] seems to have a knack for locating lightly damaged second-hand audio gear. Over the years he’s collected various types of gear and made various repairs. His most recent project involved fixing two broken tweeter speakers.

He first he needed to test the tweeters. He had to remove them from the speaker cabinet in order to gain easier access to them. The multimeter showed them as an open-circuit, indicating that they had likely been burned. This is an issue he’s seen in the past with this brand of speaker. When too much power is pumped through the speaker, the tiny magnet wire inside over heats and burns out similar to a fuse.

The voice coil itself was bathing in an oily fluid. The idea is to help keep the coil cool so it doesn’t burn out. With that in mind, the thin wire would have likely burned somewhere outside of the cooling fluid. It turned out that it had become damaged just barely outside of the coil. [Martin] used a sharp blade to sever the connection to the coil. He then made a simple repair by soldering the magnet wire back in place using a very thin iron. We’ve seen similar work before with headphone cables.

He repeated this process on the second tweeter and put everything back together. It worked good as new. This may have ultimately been a very simple fix, but considering the amount of money [Martin] saved on these speakers, it was well worth the minimal effort.

Holiday Cheer From The ATtiny13

There are smaller microcontrollers than the ATtiny13. Some ARM chips will fit on the head of a large pin, and even in Atmel world, the ATtiny10 comes in a tiny SOT-23-6 package – a size normally reserved for surface mount transistors. The ‘tiny13, though, can be programmed with just about any ISP and comes in an 8-pin DIP. It’s the bare minimum if you’re looking to break out of the world of Arduino, and you can do some pretty cool things with it, like playing some holiday audio with an SPI Flash chip.

[Vinod] tried opening up a cheap camera pen, but in the course of disassembly a few traces broke. He was now left with a 4Mbit SPI Flash chip. This was obviously the time to investigate what could be done with a small microcontroller and a huge amount of Flash. and the Attiny13 audio player was born.

The circuit uses one PWM for audio out, and reads audio directly from the Flash chip. The UART on board the ‘tiny13 is used to update the Flash, and there’s also a switch to select between play and record. If you’re counting, that means there are 4 pins for the Flash, 2 pins for the UART, 1 for the switch, one for the audio output, and the power and ground rails, all in an 8-pin package. That’s a pretty cool way to use one pin for two different functions.

You can check out a video of the project in action below.

Continue reading “Holiday Cheer From The ATtiny13″

The Four Thousand Dollar MP3 Player

[Pat]’s friend got a Pono for Christmas, a digital audio player that prides itself on having the highest fidelity of any music player. It’s a digital audio device designed in hand with [Neil Young], a device that had a six million dollar Kickstarter, and is probably the highest-spec audio device that will be released for the foreseeable future.

The Pono is an interesting device. Where CDs have 16-bit, 44.1 kHz audio, the Pono can play modern lossless formats – up to 24-bit, 192 kHz audio. There will undoubtedly be audiophiles arguing over the merits of higher sampling rates and more bits, but there is one way to make all those arguments moot: building an MP3 player out of an oscilloscope.

Digital audio players are limited by the consumer market; there’s no economical way to put gigasamples per second into a device that will ultimately sell for a few thousand dollars. Oscilloscopes are not built for the consumer market, though, and the ADCs and DACs in a medium-range scope will always be above what a simple audio player can manage.

[Pat] figured the Tektronicx MDO3000 series scope sitting on his bench would be a great way to capture and play music and extremely high bit rates. He recorded a song to memory at a ‘lazy’ 1 Megasample per second through analog channel one. From there, a press of the button made this sample ready for playback (into a cheap, battery-powered speaker, of course).

Of course this entire experiment means nothing. the FLAC format can only handle a sampling rate of up to 655 kilosamples per second. While digital audio formats could theoretically record up to 2.5 Gigasamples per second, the question of ‘why’ would inevitably enter into the minds of audio engineers and anyone with an ounce of sense. Short of recording music from the master tapes or another analog source directly into an oscilloscope, there’s no way to obtain music at this high of a bit rate. It’s just a dumb demonstration, but it is the most expensive MP3 player you can buy.

SingLock Protects Your Valuables from Shy People

Two Cornell students have designed their own multi-factor authentication system. This system uses a PIN combined with a form of voice recognition to authenticate a user. Their system is not as simple as speaking a passphrase, though. Instead, you have to sing the correct tones into the lock.

The system runs on an ATMEL MEGA1284P. The chip is not sophisticated enough to be able to easily identify actual human speech. The team decided to focus their effort on detecting pitch instead. The result is a lock that requires you to sing the perfect sequence of pitches. We would be worried about an attacker eavesdropping and attempting to sing the key themselves, but the team has a few mechanisms in place to protect against this attack. First, the system also requires a valid PIN.  An attacker can’t deduce your PIN simply by listening from around the corner. Second, the system also maintains the user’s specific voice signature.

The project page delves much more deeply into the mathematical theory behind how the system works. It’s worth a read if you are a math or audio geek. Check out the video below for a demonstration. Continue reading “SingLock Protects Your Valuables from Shy People”