[c3lingo.org](https://c3lingo.org) is doing the great job of translating many talks of the Chaos Communication Congress and other CCC-related events to multiple languages.
But the required hardware for simultaneous translation is quite expensive to rent, even if it's "just" a specialized analog audio mixer.
The hardware unit should deliver the native audio (stage/ hall mix) to the interpreter's headset and provide a sum of all interpreter's microphone to the input of the video streaming/ recording chain.
Mixing of the final translated audio (ducking the native audio with the translation) will be done as part of the streaming/ recording chain, so the interpreter unit should just provide the sum of all microphones.
Normally just 2 interpreters will provide one translation, but demanding talks might require 3 people.
So either 3 headphone inputs/ outputs should be provided or it must be possible to daisy-chain multiple units.
The user interface of the unit should be as simple, as possible to decrease the risk of mis-configuration.
This means, that no compressor and equalizer will be added in the input group.
General requirements:
- Line input of stage/ hall mix (native language) (XLR/ 6.3 mm balanced jack combo connector)
- 3x Microphone input (XLR) (at first just dynamic microphones)
* VU-Meter for each input channel. (Perhaps with special color scheme: too quiet, good, too loud, clipping)
- [ ] ESD protection and galvanic isolation of line inputs and outputs
- [ ] VU meter
- [ ] Mute and On-Air buttons
## Electrical Design
This chapter contains some notes on the electrical design.
Currently just the sources of the used circuit designs.
### Sources and Design Considerations
#### Microphone Input
##### Preamplifier
For the microphone preamp, we are using the NE5534 low-noise opamp with a circuit design from [circuitlib microphone pre-amp](https://www.circuitlib.com/index.php/schematics/product/29-balanced-microphone-preamplifier).
##### Controllable Amplification
In a normal mixer, you would be able to lower the microphone's volume to zero.
But in our case we just need on/ off and some gain range to adjust for different microphones and loudness of different people.
TODO: Try, if the preamp's gain of about 50 dB is enough and how much gain is need in this stage.
From theoretical calculations, we might need another 30 to 40 dB here, which needs to be adjustable in a range of about 30 dB.
TODO: Try out using the amp's feedback resistor for gain control or just use a potentiometer on the input of the next stage.
TODO: Integrate the On-Air and mute buttons
For long-lasting endurance of the microphone level potentiometer, we're using one with conductive plastic as resistor element (Bourns model 91).
#### Line Input/ Input Module
The line input must not be amplified at all, because loudness control of the headphones is done by the headphone amplifier section.
But the differential line-level signal must be converted to a single-ended signal by the input stage.
The current design uses a LM833N opamp to convert the balanced signal into a single ended signal and has a second LM833N to provide some degree (+- 6 dB) of "factory" adjustment.
The second part of that circuit was taken from the [circuitlib audio mixer tutorial](https://www.circuitlib.com/index.php/tutorials/product/39-how-to-build-an-audio-mixer).
TODO: How to achieve galvanic isolation?
TODO: Use https://www.ti.com/product/INA134 for input conversion?
#### Summing
Summing is needed in two places: Creating the sum of all microphones (not adjustable, fixed output gain) and for the headphone mix (one input level adjustable).
A simple summing circuit using one operational amplifier is enough for our application, like in [circuitlib audio mixer tutorial](https://www.circuitlib.com/index.php/tutorials/product/39-how-to-build-an-audio-mixer).
#### Line Output Driver
TODO: Either use https://www.ti.com/product/DRV134 or http://www.thatcorp.com/1600-series_Balanced_Line_Driver_ICs.shtml.
TODO: How to achieve galvanic isolation?
#### Headphone Output Driver
The headphone output needs a maximum output power of about 0.1 W and should put the mono signal on both stereo channels of the TRS jack.
For the first draft, we're using one LM386 audio power amplifier.
TODO: Check, if driving the two speakers in parallel leads to any issues (besides cutting the speaker impedance in half).
#### VU Meter
Because the LM3916 LED bar graph driver is obsolete, we either have to re-create it's function with some comperators or have to use a microcontroller.
TODO: Re-create the LM3916 with some LM339 or design/ program some small uC.
## Notes
A dynamic microphone needs at least 60 dB gain in the pre-amp, because a typical signal is at about 1 - 100 uV (-118 to -78 dBu or -120 to -80 dBV).
Line level in professional audio gear is at +4 dBu, which is 1.228 V (RMS).
Because 0 dBu is defined as 1 mW at a load of 600 Ohm, which needs a voltage of 0.77 V.
Increasing the voltage by a factor of 10 is an amplification of 20 dB.
