[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)
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.
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?
#### 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).
After some long research to find single and dual channel, logarithmic scale potentiometers with the same dimensions, there are only a few options left:
1. Alps Alpine Series RK09K/D, 10 kOhm (log), 15 mm length, vertical mount: RK09K1130AJ3 (single) and RK09K12C0A19 (dual)
6 mm D-style (4.5 mm) shaft, 6 mm flattened + 2.2 mm (incl. 0.8 mm collar) = 8.2 mm from top
Notes: Knob might touch surface, assuming 2 mm front plate thickness
2. Alps Alpine Series RK09K/D, 10 kOhm (log), 30 mm length, vertical mount: RK09D117000C (single) and RK09K12C0A8K (dual)
6 mm D-style (4.5 mm) shaft, 12 mm flattened + 11.2 mm (incl. 5.0 mm collar) = 23.2 mm from top
Notes: Quite long shaft, matching knobs difficult to find
3.**Alps Alpine Series RK097**, 10 kOhm (log), 15 mm length, horizontal mount: RK097111080J (single) and RK09712200MC (dual)
6 mm D-style (4.5 mm) shaft, 7 mm flattened + 8 mm (incl. 5 mm threaded collar) = 15 mm from top
Notes: More expensive than RK09K; Thread should fix 2 mm front plate well
4. Alps Alpine Series RK097, 10 kOhm (log), 20 mm length, horizontal mount: RK097111080R (single) and RK09712200HA (dual)
6 mm D-style (4.5 mm) shaft, 12 mm flattened + 8 mm (incl. 7 mm threaded collar) = 20 mm from top
Notes: Quite long shaft, matching knobs difficult to find
Possible knobs may be:
- Rean F311/ F313 series (11/ 13 mm diameter): 9 mm hole depth (no skirt/ flush with end of flatted axis section)
Notes: Will fit option 1 (and 2, if potentiometer placed accordingly)
- Rean P670 series (12 mm diameter/ 16 mm at bottom): 7.5 mm hole depth + 4.5 mm skirt = 12 mm
Notes: Should fit option 3
- Cliff K87MAR series/ RS Pro 777-73xx: 7.5 mm hole depth + 4.5 mm skirt = 12 mm, inner size of skirt 12 mm (measured)
Notes: Should fit option 3 (boring for potentiometer nut a bit tight); Size quite like Rean R670
- Rean P300 series (11 mm diameter/ 15 mm at bottom): 9 mm hole depth + 3 mm skirt
Notes: Might also fit option 3
The On-Air button needs to be a latching SPST (or SPDT) switch with LED illumination, like:
- E-Switch LP11EE1NCSYG
The mute button needs to be a momentary SPST (normally open) or SPDT push button, like:
- NKK Switches UB15NBKW01N-C
- Apem 1413NC6/ 1415NC6
- TE Connectivity PB6B2FM3M2CAL00
- TODO: other, less expensive options highly appreciated!
The casing should be a desk console (angled surface), ideally with space at the front to mount the headset ports (XLR and 6.3 mm jack), like:
- [Bopla ATPH 1865-0250](https://www.bopla.de/gehaeusetechnik/product/alu-topline/alu-topline-gehaeuse/atph-1865-0250.html) (front might not have enough space for the connectors)
- TODO: other options appreciated
A possible UI layout for the Bopla ATPH 1865-0250 might look like this: 
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).
