More than just audio

25.08.2011 17:02

I recently came across this very useful IC that looks like it might be useful in a number of scenarios. This is C-Media electronics CM108 (datasheet). It's an USB audio interface chip that is used in a number of cheap USB audio dongles and headsets. For instance, SPEEDlink SL-8850 goes for around 15 € with shipping from Amazon or the local retailer.

Inside Speedlink SL-8850

It contains two audio DACs and one ADC (48 kHz sampling rate), as expected for such a chip. What makes it interesting is that it also contains an interface for a simple serial bus (I2S) that is meant for driving external AD or DA converters, but can be abused to stream arbitrary digital data from or to the PC, as long as it fits into 32 bits times 48 kHz bandwidth. Also included in the package are 4 GPIO pins and 4 debounced key inputs.

As usual there's an optional place for a serial EEPROM for USB product/vendor IDs, which can also be programmed via the CM108 itself.

From the USB host side, the data stream is exposed as a standard USB audio device (usable for instance via snd_usb_audio ALSA driver in Linux) while the GPIO pins and keys use the HID interface which can be accessed via the HID API.

Right now I'm using it to record data from the 433 MHz receiver I was writing about previously, but it appears it might be useful in other projects as well. For example here's an article about using it for an audio-frequency oscilloscope. Considering you get the chip and a few other tidbits (USB connector, two audio jacks) for such a low price it's certainly worth considering in place of a USB-enabled microcontroller when it fits the requirements.

Posted by Tomaž | Categories: Digital


Unfortunately all these simple USB audio dongles have mono mic input instead of stereo for SDR I/Q applications. Even this Lenovo X60S tablet of mine has the same audio limitation. I am using it mainly in stereo-mix loop mode to develop CW robot for Morse Runner simulator.

Posted by brodul

I would like to mention Stereo Input on USB Audio Adapter model Griffin iMic. It costs about $25 on eBay. Both input channels have DC bias 3.27V 12k (6k mono, inputs shorted). Amazon: Griffin Technology iMic - The original USB Stereo Input and Output Audio Adapter,

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