I’ve designed a series of audio samples based on classic modem technology and I’ve released them on SoundCloud under the Creative Commons attribution license.

Now for the extremely geeky party, how these samples are made. The sounds are generated with Csound using a form of frequency modulation called frequency-shift keying, a technique used by modems to transmit data to one another. Here’s a brief, and for this reason a not-to-entirely accurate, explanation of how a modem works. Modems communicate with each other by sending and receiving binary data between one another. The binary data is embedded into an audible signal by modulating a sine between two discreet frequencies at a specific time interval known as the baud rate.

To accomplish this modem-like sound, the randh opcode generates random noise at a user-specified frequency. The signal continues into the signal-to-binary converter, covered last week in Positive or Negative, creating a series of pulses consisting of 0s and 1s. These 0s and 1s are then mapped to the two desired frequency values by scaling then biasing them with the multiplication and addition opcodes. Finally, this signal is inserted into the frequency parameter of a sine wave oscillator, resulting in a sine wave whose frequency changes back and forth between two values at a timed interval determined by the randh at the top of the synthesizer graph.

Whether or not the results are authentic is kinda in the air, but isn’t necessarily the point. I’ve even played with parameters that have no basis in reality. The truth is, after the audible handshake a modem makes when connecting to another computer, I have no idea what the actual transmission sounds like and couldn’t find a reliable recording online. Based on what I’ve read, my design isn’t accurate, but close enough for sound design purposes. However, I’m thinking that designing a virtual modem simulator could be fun. Might have to pencil this project in for October.

Get the clean block diagram.

Get faux_modem.csd.

I recently ran into a situation in Csound in which I needed to know if a signal was positive or negative, and it had to work at the audio rate. I could have used if-else conditional statements, though these only work at k-rate, and would have required implementing a User-Defined Opcode utilizing the setksmps opcode. Perhaps I was lazy at the time, or in the mood for some simple hacking fun, but I decided to forgo this obvious solution for something else. And I wanted to do it without using conditionals.

Get positive_or_negative.csd.

The first thing I tried was using the > (greater than) opcode. In some other languages, an expression such as (0.8 > 0) will return a value of 1, while a (-0.8 > 0) will yield a 0. Unfortunately, this opcode is not designed this way in Csound. So I kept looking.

I eventually came up with a two opcode solution utilizing limit and ceil(). First, the signal passes through the limiter, clipping the waveform between the value range of 0.0 and 1.0. As the signal continues through the ceil opcode, any non-zero value becomes a 1.0, while a zero value remains 0.0.

For example, processing a sine wave through this configuration creates a unipolar square wave.

Now what? That’ll have to wait for another post.