Cheap C-Media (?) sound card

This USB sound card that presents itself as C-Media is one of the cheapest sound cards available with price $1.1-$2 depending on vendor.

Device markings: HX2010-0705, production date 40th week of 2013 (or later?).

Device enumerates as composite device: sound card + HID input. HID input part is used for audio volume and mute control and it functionality is slightly broken - same as with multimedia keyboard it controls default sound card settings, not necessary settings for this specific device.
USB descriptor dump:

VID=0x0D8C PID=0x000C
Product string: C-Media USB Headphone Set
Audio Device Class + HID (composite device) 

Judging by the descriptors chip inside is identical as in previously described "C-Media" sound card version without any control keys.

Unlike the previous "C-Media" USB sound card I've tested and similar card from this thread it was easy to find point where DC signal component is not blocked:


Beware that with this simple setup oscilloscope input would be equivalent to 2V voltage source with 250k resistance in series. It may be not suitable for circuits with e.g. weak pull-ups. Input buffer would fix it but I'm not considering it economically justified as base STM32F103 board that doesn't have this problem (and is usable with miniscope v2c firmware/DLL) costs ~$4.

Other approach to building input stage: (I haven't tested it yet but it's simple and looks promising).

Don't judge a book by its cover (thanks to Daniel Montalvo) - it looks like othes type of sound card is sharing almost the same package:
Other sound card in same case Other sound card in same case Other sound card in same case
I'm not sure at the moment if it is able to process DC voltage. Since case is easy to open I think it might be worth asking seller for PCB photo before buying particular card.

Calibration and settings window:
Settings window allows to define voltage per bit and offset (in raw units) that would be applied to raw data. Default calibration settings are intented to result 0-6V range (*) when connected with 120k resistor in series and setting Volume to 0 in Windows 7.
For fine/final calibration use separate calibration function available in GUI (allowing calibrating each channel separately).
* Actually negative voltages can be measured also

Example traces:
complex waveform (generated with miniscope v2d AWG):
miniscope v4 + dsoundscope.dll
20Hz square
miniscope v4 + dsoundscope.dll
4kHz square (as a rule of thumb digital oscilloscopes are useful up to 1/10th of sampling frequency)
miniscope v4 + dsoundscope.dll


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