Line-in vs. microphone on X230 laptop

xarragon · 2015-10-13 07:05:28

Hello fellow.. archers? I purchased a refurbished Thinkpad X230 a while back, transitioning away from a desktop machine. However, this new machine has only a single 3.5 mm audio jack. Most laptops usually have two separate ones, one for mic and one for headphones. On this machine they are combined into one of those jacks found on your typical Android phone.

I want to record audio output from an Android phone. Is it possible to have this multi-input acting as a line-input? The difference between line input and microphone input are the signal levels; microphone input is much lower. It requires an amplifier. On my gaming laptop the inputs can be switched between microphone and line input via proprietary software. Is it possible to do that via ALSA in general? Is it possible on this particular hardware?

I already have stereo (3-line) male-to-make 3.5 mm cables. I guess I could force the levels from the mobile phone (which are speaker/headphone output levels) down using a voltage divider.

The hardware on this machine is:
Audio device: Intel Corporation 7 Series/C210 Series Chipset Family High Definition Audio Controller (rev 04)

The mobile phone is a Motorola Moto G (2nd gen).

When opened in alsamixer I have a "mic boost" control, is that the preamp for the microphone input? Will disabling it making the machine expect line-input levels? I will actually measure the levels using an oscillioscope to figure this out and write a guide, but perhaps someone has figured out the laptop hardware side a bit more already?

Thanks in advance,
Martin "xarragon" Persson

ewaller · 2015-10-13 14:50:22

Most microphone inputs are monaural. I am sure you would want to record stereo.
To do this, you would need a mixer/attenuator (divider) network to combine the channels and to reduce the signal. A lot. I'm guessing -30dBV (on the order of 30mV p-p). Beyond that, I am certain that the microphone response spectra is not flat and normally would require some preemphasis. To compensate for this, you would need to run the audio through an equalizer to remove the emphasis that is added by the microphone amplifier.

Might I suggest using Bluetooth rather than analog? If your laptop does not have Bluetooth, you can pick a USB Bluetooth tranceiver for very little money.

Line levels for consumer audio are -10dBV and will be just under a a volt peak to peak. Speaker levels are likely to be as much as 5V peak (2.5V p-p) on a phone output.

Arch Linux

#1 2015-10-13 07:05:28

Line-in vs. microphone on X230 laptop

#2 2015-10-13 14:50:22

Re: Line-in vs. microphone on X230 laptop

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