Conference Interpreter Unit

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.

So the idea was born to design an easy to use hardware with the special requirements of our interpreters in mind.

Requirements

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)
  • Fader/ Potentiometer for gain control
  • On-Air switch (on/ off position, with state LED)
  • Temporary mute button (momentary switch)
• 3x Headphone output (6.3 mm mono/ stereo (2x mono) jack)
  • Should output mix of native audio, own microphone and other translator's microphone
  • Potentiometer for output volume
  • Potentiometer for volume of own microphone in the mix
  • Potentiometer for volume of other two microphones in the mix
• Outputs:
  • Sum of all translators (XLR/ 6.3 mm balanced jack combo connector)

All inputs (besides the microphones) and outputs should:

• expect/ deliver a nominal level of +6dBu (german TV broadcast standard, 0 dBu = 0,775 Veff)
• be transformer balanced and galvanically isolated

This results in a signal flow like this:

Status

Tasks

• Requirements collection
• Electrical design draft/ proof of concept (breadboard)
• Proof of concept validation
• First PCB layout
• More testing/ validation

Help is always appreciated!

Implemented Requirements

• Microphone input
• Line input
• Microphone summing and line output driver
• Adjustable headphone mix and output volume
• 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.

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: Integrate the On-Air button with it's LEDs

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.

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.

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 even though it has a quite high minimal amplification of factor 20.

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.

Notes

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).

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.

BoM

Approximate prices in Euro.

Connectors and Buttons (User Interface)

Count	Art. No.	Description	Price
1	Neutrik NCJ 6 FAH	Line Input	1,27
1	Neutrik NC3 FD-LX	Microphone Input	3,44
1	Neutrik NJ3 FP-6-C	Headphone Output	5,40
1	Neutrik NC3 MD-LX	Line Output	3,22
1	Neutrik NAC3 MPA-1	Main Power Input	3,33

Sub-Components

Count	Art. No.	Description	Price
1	Traco Power TXL 035-1515D or TOP 60533	Power Supply	~48,00

PCB Components: TODO when schematic is finished