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doc:appunti:linux:audio:audio_recording

This is an old revision of the document!


Audio Recording With GNU/Linux

Choosing the right jack

The microphone jack

In the 70s - at the time of analog amplifiers, in the pre-computer age - the standard jack for microphones were the mono one, with the diameter of 1/4 inches (6,35 mm). Only two conductors were present: the tip for the signal and the sleeve for the ground. This is the so called TS jack:

Mono Jack 6.35 mm

The natural evolution of that jack, is the 3.5 mm TS mono jack:

Mono Jack 3.5 mm

These jacks have only two poles, because they were used mainly with dynamic microphones (see below) which do not require any external power supply, or with condenser microphones with an integrated battery, where the battery is sufficient to provide the required power to the mic.

With the advent of audio in personal computers, electret microphones have become very popular. Very often these microphones integrate a preamplifier that requires a supply voltage or they require a bias current necessary to power the internal Field Effect Transistor (FET) buffer; these currents are in the range of 2.5 - 5.0 volt. The solution adopted e.g. by the early Sound Blaster sound cards, was to use a three poles microphone socket, so requiring a 3.5 mm TRS jack (Tip, Ring and Sleeve) despite the microphone being mono.

Sometimes a separate pole for supplying the voltage is not available (e.g. when you want to connect a stereo microphone using a three poles TRS jack, or when you use a four poles TRRS jack for an headset with earphones and mic), so it is common practice to supply the bias voltage through the tip, on the same connector for the microphone signal.

In the following picture you see the de facto standard imposed by Creative Labs in the 90s on their SoundBlaster audio cards (from SB16 to AWE64), and the AC'97 standard proposed in 1997 by Intel. Notice that the bias current can be different, varying from 2.5 to 5 V, and sometimes it can be controlled in software.

Microphone jack with power supply

Socket on modern notebooks and smartphones

Recent laptops and smartphones have a single 3.5 mm socket for the headphones and the microphone. It requires a TRRS jack (meaning tip, ring, ring and sleeve) because it have to accomodate two audio lines, one microphone line and the ground. Unfortunatelly there exist two competing standards about the jack pinout, the CTIA and the OMTP one:

TRRS CTIA and OMTP

The CTIA (Cellular Telephone Industries Association) pinout is the one used in modern smartphones, whereas the Open Mobile Terminal Platform was used by old Nokia, Sony and Samsung phones (till 2012).

Headphone/microphone cable splitter

So if you have a recent notebook with the CTIA combined socket, yuo have to use a cable splitter to connect a standard microphone jack. Here it is the electrical schema of such adapter cable:

Dynamic, condenser and electret microphones

There are basically three types of microphones on the market: dynamic, condenser and electret ones. In the world of computers and smartphones, the electret ones are by far the most common type.

Dynamic

A small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached to the diaphragm. The sound wave moves the diaphragm and the coil moves in the magnetic field, producing a varying current in the coil through electromagnetic induction. No power is required: the current is carried directly on two poles to the mic input. These microphones generally have a 1/4“ mono jack.

Condenser

The diaphragm acts as one plate of a capacitor, and the vibrations produce changes in the distance between the plates. The plates are biased with a fixed charge, so the voltage maintained across the capacitor plates changes with the vibrations in the air.

This type of microphone requires a power source, provided either as phantom power by the equipment via microphone inputs or from a small battery inside the mic. Generally these type of microphones have an XLR connector when they are intended to be connected to professional audio equipment, which will provide the phantom power. If they have an internal battery they may have the old 1/4” mono jack.

As it is easy to imagine, there are many possible problems regarding the supply of phantom power to dynamic microphones (which do not require it, but can be instead damaged by it) or to condenser microphones which have a power provided by a built-in battery. Problems are exacerbated in turn by audio devices that can supply or not such power and by mechanically compatible cables and connectors, but with incompatible electrical characteristics.

Electret

Unlike capacitor ones, electret microphones do not require polarizing voltage, but generally they contain an integrated preamplifier that does require power. In this cases a TRS jack can be used, where the ring carries power via a resistor from, normally, a 5 V supply in the computer (often incorrectly called polarizing power or bias). Generally these microphones built to be plugged into computers and electronic audio devices, have a TRS 3.5 mm jack, carrying the mic signal and the required current.

External mic on the Acer Travelmate B113-E

Running Debian GNU/Linux 9 Stretch, it seems that the use of the headphone/microphone split cable does not abilitate the use of the microphone. In fac, if you run the acpi_listen tool (from the acpid Debian package), you can see when you insert the split cable:

jack/headphone HEADPHONE plug
jack/headphone HEADPHONE unplug

but nothing is reported about the plug of a microphone. Beside that, no bias current appear on the microphone jack. The solution is to run the hdajackretask tool (from the alsa-tools-gui Debian package), then force the connection of Pin 0x19:

  • Enable the Show unconnected pins option.
  • Click Pin ID: 0x19OverrideMicrophone.
  • Apply now (it asks for the root password, as it has to run a script with superuser privileges).

the running acpi_listen will print:

jack/microphone MICROPHONE plug

Now you can also measure a bias current of 2.85 V between the ground (sleeve) and the ring or the tip.

Using alsamixer you can control the capture device (hit F6 to select the HDA Intel PCH sound card), it is advisable to set Mic Boost as low as possible (eventually zero) and Capture level to 100.

Is not clear how to disable the external microphone: re-setting Pin 0x19 to Not connected does not generate the ACPI microphone unplug event, and the bias current is not cleared. The bias power disappears from the jack after a poweroff or an hybernation, so the external microphone seems to be “unplugged” after these events.

Problem: silence gaps using a cheap USB dongle

I was unable to use the microphone connector on my Acer TravelMate, so I tried to use an USB dongle, marked LinQ. The device was detected by the Linux Kernel 4.9.0:

usb 2-1.3: new full-speed USB device number 9 using ehci-pci
usb 2-1.3: New USB device found, idVendor=0c76, idProduct=1607
usb 2-1.3: New USB device strings: Mfr=0, Product=1, SerialNumber=0
usb 2-1.3: Product: USB Headphone Set
input: USB Headphone Set as /devices/pci0000:00/0000:00:1d.0
    /usb2/2-1/2-1.3/2-1.3:1.3/0003:0C76:1607.0004/input/input92
hid-generic 0003:0C76:1607.0004:
    input,hidraw2: USB HID v1.00 Device [USB Headphone Set]
    on usb-0000:00:1d.0-1.3/input3

LinQ USB audio dongle The USB audio adapter worked quite nicely at playing audio, I use it on a QNAP TS-120 NAS, transformed into a media player installing Debian GNU/Linux on it. The Linux kernel and the ALSA audio subsystem works well on setting the levels for playback.

Once started Audacity, I was able to select the microphone line of the USB dongle as a capture device:

Recording a sample track revealed an annoying problem: about every 0.4 seconds you can see a gap of silence in the audio track of about two hundredths of a second. The audio track is unusable!

Ugreen USB audio adapter

The device is detected by the Linux Kernel 4.9.0 in this way:

usb 2-1.3: new full-speed USB device number 5 using ehci-pci
usb 2-1.3: New USB device found, idVendor=0d8c, idProduct=0024
usb 2-1.3: New USB device strings: Mfr=1, Product=2, SerialNumber=0
usb 2-1.3: Product: USB Advanced Audio Device
usb 2-1.3: Manufacturer: C-Media Electronics Inc.
input: C-Media Electronics Inc. USB Advanced Audio Device as /devices/pci0000:00/0000:00:1d.0
    /usb2/2-1/2-1.3/2-1.3:1.3/0003:0D8C:0024.0004/input/input18
hid-generic 0003:0D8C:0024.0004:
    input,hidraw2: USB HID v1.11 Device [C-Media Electronics Inc. USB Advanced Audio Device]
    on usb-0000:00:1d.0-1.3/input3

Ugreen USB Audio Adapter

Launching alsamixer and hitting F6 you can select the USB Advanced Audio Device sound card. Hit F4 to view the only one capture device, i.e. the microphone input line. Press Space to enable/disable the capture and Up/Down arrows to increase/decrease the gain.

It seems that the best setting for audio recording through the mic, is to keep the capture mic gain to 100:

USB Advenced Audio Device in Alsamixer

The microphone socket is a 3.5 mm TRS type (three poles), it provides a bias current of 2.6 V on both the tip and the ring, so it is advisable, as reported in the device manual, to NOT use a simple two poles mono jack (TS type), because it will short-circuit the bias current on the ring with the ground on the sleeve.

Wireless UHF microphone

We have purchased an UHF wireless microphone on Aliexpress.com for about 25 €. It is composed by:

  • The transmitter powered by two AA batteries, featuring:
    • A lavalier microphone on a clip.
    • One small wire-antenna.
    • Frequency range: 610-675 MHz (53 channels spaced by 1.25 MHz each).
    • Modulation system: DQPSK
    • Frequency response: 60-16000 Hz
    • Transmitted power: 10 dBm
    • Working current: <= 100 mA
  • The receiver powered by two AAA batteries, featuring:
    • A 1/4“ (6.35 mm) mono jack.
    • One small telescopic antenna.
    • Receiving sensitivity: -94 dBm
    • Working current: <= 100 mA

To connect the UHF receiver to the Ugreen USB audio adapter, we built an adapter cable with this pinout:

1/4” TS female jack Signal 3.5 mm TRS male jack
Tip Microphone Tip
Not connected Ring
Sleeve Ground Sleeve

The 2.6 volt current provided by the USB adapter on th jack ring will go nowhere (not connected). The 2.6 volt current provided on the jack tip is probably not required by the UHF receiver, but it should not harm. No infromation is available about microphone impedence expected by the USB adapter, on the other hand no information is available about the impedence of the mic interface of the UHF receiver.

Despite all this empirical approach, the voice registration is quite satisfying!

Audio Recording With Android

Recording audio using an external microphone connected to an Android device can be a real nightmare. We tried the hard way using a Chinese, no-brand, wireless UHF microphone!

We can group the problems you can face in three main categories:

  • Physical connection
  • Electrical compatibility and hardware autodetection
  • Software capabilities and settings

Physical connection

Most Android devices have a 3.5 mm jack accepting a TRRS connector in CTIA standard and every brand of mobile phones produces headsets with their own standard. The vast majority of solutions uses electret microphones requiring a bias current, but use some combinations of impedence to detect the insertion of the microphone and to handle the headset button (or buttons).

So, connecting an external microphone, can be as simple as making a cable that connects the ring and the sleve of the CTIA connector to (respectively) the ground and the microphone line of the microphone. The hard part is to guess, or empirically try, if the microphone requires a bias current, if the Android device is provinding such a current and if the microphone produces a sufficient output level (actually a voltage in the order of the millivolts) that can be used by the device.

Electrical compatibility and hardware autodetection

As stated above, the first problem is powering the microphone itself, because generally we will use an electret one. It is a common practice to provide the power on the same line of the microphone signal, so requiring just two wires for both power (from the device to the mic) and for signal (from the mic to the device). The power, also called bias current, is generally in the range of 2.5 to 5.0 volt.

FIXME: How much current is provided in our test? Is the current always present or switched-on only at jack insertion?

Another parameter which should be considered is the impedence FIXME

Software capabilities and settings

FIXME Complete the article!

Wishing to connect the UHF microphone to an Android smartphone, we prepared an adapter cable. From 3.5 mm jack TRRS male to 1/4“ jack TS female, with this pinout:

1/4” TS female jack Signal 3.5 mm TRRS male jack (CTIA)
Not connected Tip
Not connected Ring
Sleeve Ground Ring
Tip Microphone Sleeve

The smartphone did not sensed the microphone, there is not much documentation about it, but it seems that the autodetection of the microphone is based upon the impedence of the mic itself, if it is too low, it is not detected.

Web References

doc/appunti/linux/audio/audio_recording.1556016572.txt.gz · Last modified: 2019/04/23 12:49 by niccolo