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Having discussed how our weird "host" computer would handle USB audio & video I/O (in short, it'd hand it off to the apps, etc you use), how'd I actually build these peripherals? I'll start by designing a cheap microphone or speaker.
Speakers transfer a vibrations of an analog signal to a magnet via a solenoid & through into the air via a flexible "membrane" attached to that magnet. Microphones do the reverse.
To hook it up to USB we'll at least need our USB-interface chip...
1/4?
Early on in these USB discussions I designed a chip exposing slices of a 16bit address-space to USB, including various I/O facilities. Lets use a model with no 8bit CPU.
Here it'd serve to buffer the audio in RAM so the analog signal & digital USB packets can be sent at different rates. The counters/timers I included would be enough here!
Between our USB-interface chip & the speaker/mic we'd need a low-pass filter to discard frequencies we can't encode & an amp to make it perceptible.
2/4?
At its simplest an amplifier can be a single large transistor possibly wrapped in resistors & capacitors. Whilst a low pass filter can be achieved by connecting a resistor in series & a capacitor in parallel.
Already at this stage we can adjust how much the amplifier increases the volume by, how fast data's transferred between the USB-interface chip & the microphone or speaker. Ideally we'd adjust (by disconnecting capacitors?) the low-pass filter to match the sample-rate.
3/4!
We could hook these controls up to our USB-interface chip so it control it via standard SETUP packets. The handler for these packets (given the chip I designed) would questionably qualify as a program, but still... Its just routing data!
The USB-interface chip would also serve to store metadata to describe the peripheral as a single isochronous endpoint of configurable-sample-frequency hooked up to a volume/mute feature-unit & in turn the speaker or mic.
This'll satisfy even our botloader.
4/4
P.S. I'll spend the next day or 2 designing nicer speakers & microphones! Exploring the design space.
Feel free to make recommendations, especially if you've got some (informal?) expertise in audio! My expertise is in reading the USB standards.
4.2/4 Truly fin!
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