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doc:appunti:linux:audio:audio_recording [2019/04/19 15:35]
niccolo
doc:appunti:linux:audio:audio_recording [2019/05/22 15:51] (current)
niccolo [Electrical compatibility and hardware autodetection]
Line 1: Line 1:
-====== Audio Recording With Linux ======+====== Audio Recording With GNU/Linux ======
  
 ===== Choosing the right jack ===== ===== Choosing the right jack =====
Line 139: Line 139:
 ===== Wireless UHF microphone ===== ===== 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 **[[wp>Lavalier microphone|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
 +
 +{{wireless-mic-trasmitter.jpg?direct&240|}}
 +{{wireless-mic-receiver.jpg?direct&240|}}
 +
 +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**.
 +
 +On a Xiaomi Mi A1 smartphone, which has a **TRRS 3.5 mm jack, CTIA standard**, we measured a bias current of **2.19 volt** between the microphone (sleeve) and the ground (ring).
 +
 +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 ===== ===== Web References =====
  
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   * [[http://www.shure.com/americas/support/find-an-answer/will-bias-voltage-from-a-computer-effect-a-dynamic-microphone|Will bias voltage from a computer effect a dynamic microphone?]]   * [[http://www.shure.com/americas/support/find-an-answer/will-bias-voltage-from-a-computer-effect-a-dynamic-microphone|Will bias voltage from a computer effect a dynamic microphone?]]
   * [[http://www.epanorama.net/circuits/microphone_powering.html|Powering microphones]]   * [[http://www.epanorama.net/circuits/microphone_powering.html|Powering microphones]]
 +  * [[https://forum.xda-developers.com/showthread.php?t=2072612|External Mic on Galaxy Devices]] 
 +  * [[https://stackoverflow.com/questions/19255908/android-forcing-external-microphone-jack-input-3-5mm-to-stay-on|Android: Forcing external microphone (jack input 3.5mm) to stay on]] 
 +  * [[https://forum.xda-developers.com/showthread.php?t=1042051|App that turns off microphone AGC (Auto-Gain Control)?]] 
 +  * [[https://stackoverflow.com/questions/47394634/difference-between-voice-communication-and-mic-in-mediarecorder-audiosource|Difference between VOICE_COMMUNICATION and MIC in MediaRecorder.AudioSource]] 
 +  * [[https://stackoverflow.com/questions/47359587/what-is-the-best-audiosource-setting-for-calls|What is the best audiosource setting for calls?]] 
 +  * [[https://amp.reddit.com/r/androidapps/comments/4wo8sz/app_to_force_android_to_use_external_microphone/|App to force android to use external microphone when one is plugged in]]
doc/appunti/linux/audio/audio_recording.1555680902.txt.gz · Last modified: 2019/04/19 15:35 by niccolo