[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.
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.
A dynamic microphone needs at least 50-60 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).
