So, let me remind you of a few basic things from the physics course. The impedance is one of the most important characteristics that describe electrical loops, components and materials those components are made of. The impedance (Z) is usually defined as the full resistance of the device or electrical loop to the current. The full resistance to the sine-wave current is expressed by the ratio of the effective voltage (U) to the operating current (I) in this circuit:
where r and x are their active and reactive resistances.
Thus, the Microsoft team tested a long list of various audio peripherals to compile the results into a classification table. For each category (microphones, headphones, speaker systems), they tested 10 devices to reveal their impedance characteristic. The peripherals were tested in a frequency range from 40 to 20,000Hz. The devices that have their own volume control circuits were tested at their maximum volume to avoid any interference with the impedance measurements.
Obviously, the impedance character will be very distinctive for each audio device (you can see the difference in the diagrams below). That is, it is possible to define an impedance range for each class of audio peripherals. You can see diagrams of some audio devices as tested by Microsoft:
- Impedance graph for the Telex M-40 microphone;
- Impedance graph for the Plantronics Audio 40 headphones;
- Impedance graph for the CTX-55 passive speakers;
- Impedance graph for the Labtec LCS-2414 active speakers.
The figures showing the distribution of min and max impedance values for each device were presented as a diagram in the official Microsoft bulletin:
Basing on the acquired data, Microsoft worded its recommendations to the manufacturers of audio peripherals. The design of the device should take into account the following impedance criteria (it is evidently done for the WHQL certification):
- Microphone: from 400 to 1500 Ohm;
- Active speakers: from 3000 to 12000 Ohm;
- Passive speakers: from 4 to 16 Ohm;
- Headphones: from 32 to 100 Ohm.
The official documentation including the test methodology, data analysis rules as well as plans for classification and systematization of analog audio peripherals can be found at the Microsoft website. For those our readers who are using dial-up I would like to provide a direct link to the document, so that they did not get strayed in the maze of the Microsoft techno library: Analog Audio Classification Using Device Impedance Characteristics (the size of the self-extracting archive is 1.4MB).
The technology (or the auxiliary circuit in the audio codec) that allows the OS to configure audio ports was named Jack Sensing. It is now a part of the AC’97 rev.2.3 specification and is recommended to the audio codec makers. We will see its practical implementation in a while.