This page is the overview of the multichannel audio player 1 project. Which uses a stereo recording to play a single sound track, routing it to different performing characters by using tones to turn performers on and off.
Since then, the price of MP-3 players has dropped to a pittance, leading us to a new design: multichannel audio player 2.
WARNING: This page is part of our multichannel audio player 1 project. If you are considering this project, read all Related Pages before you start!
NOTE: This project is has been discontinued, and is provided only for historical reference. Please see multichannel audio player 2 instead.
A conventional multichannel recorder is like stereo on steroids. Instead of just two channels, you have four or more. Each of the channels can be a completely independent sound track. But the most common use is to make each channel record a different "voice" in a musical performance. After the recording is complete, you can mix the channels together to create a final product (e.g. "Turn up the drummer a little, and lower the background singers.")
The chief problem with a traditional multichannel recorder is that they are very expensive. Decades ago, multchannel recorders were used only by professional recording studios, built to high standards, and priced accordingly. Rick's position in the entertainment industry gave him access to equipment beyond the reach of many home haunters.
Since then, prices have dropped somewhat as the market has expanded - because garage bands need equipment, too. And technology has progressed, notably in the areas of computerized audio. But we wanted something simple and cheap.
We aren't going to attempt a real multichannel recorder and player. It would be a big project and we really don't need true multichannel capability.
Instead, we intend to produce a similar "good enough" effect with a small investment.
The control channel contains "DTMF" tones. These are the noises made when you dial a telephone, and are sometimes called "touch tone". Just about all modern telephones use this tone dialing method. [Before that, telephones dialed using a series of pulses.]
Our controller will listen for the DTMF tones on one audio channel and use them to figure out which characters should be speaking. The controller then routes the audio signal only to the characters who should be performing.
This illustration shows how the the control channel switches the
audio channel to the outputs:
We will use the following terminology:
Suppose that the material consists of characters singing, with instrumental accompanyment. At the end of the song, all the singers hold their last note, while the big bass drum beats "boom, boom, boom!" Lovely! But we have mixed everything down to mono, so the only music we hear is a combination of everything, bass drum and all. Our control channel merely allows us to choose which performers will make that big combined sound. So when we approach the final note, the control channel says "everybody sing", and the combined sound track comes out of everybody's mouths - and for each "boom" of the drum, their mouths open wider.
What we really need is three channels:
But all we have is stereo - two physical channels. We are embarking on this project because we don't have a third channel, or a fourth.
What we will do is combine the animation audio and control channels on a single physical recording channel (the stereo's left channel). But in order to combine the two channels, we need a way to split them apart when we play back. We have decided to use a specific tone frequency for the animation audio that can be filtered out. The outline is as follows:
We have decided that the way to do this is to add a second tone above the DTMF sounds, perhaps 3.5 kHz. This channel will be distributed to all props, in the animation audio channel. This tone can be recovered by a simple circuit in each prop, and turned into a boolean on/off bit-stream.
The boolean bit-stream can be used to send commands using a serial protocol to be defined later.
We're not designing this now, because we don't need it now. But we wanted to have the flexibility to add it alter.
Thank you for visiting. Your comments are welcome.
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