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 (own PCB)
• 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

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?

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.

The mixer design uses inverting op-amp circuits, because they require less components and signal routing. Normally an audio mixer should take care of adding signals in-phase, because otherwise two microphones might cancel each other out. In our case, there are just two types of summing: All microphones for the interpreter unit's line-out and the individual headphone mixes. Because the stage input is quite independent of the interpreter's microphones, no special phase/ inversion considerations are needed. And the microphones connected to our unit will experience the same inversions anyways.

Line Output Driver

Line Output conversion is done by the DRV134 IC.

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

When searching for VU meter circuits, many use the LM3916 LED bar graph driver, which already has the right scaling built in. But this chip is obsolete and not produced any more, so we designed our own chain of comperators to drive a set of LEDs.

User Interface Hardware

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 (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:

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	Manufacturer + Art. No.	Description	Price
1	Neutrik NAC3 MPA-1	Mains Power Input	3.33
1	Neutrik NCJ 6 FAH	Line Input	1.27
1	Neutrik NC3 MD-LX	Line Output	3.22
3x1	Neutrik NCJ 6 FAH	Headset Microphone Input	1.27
3x1	TODO	Headphone Output	TODO
3x1	NKK UB15NBKW01N-C	Mute Button	7.50
3x1	E-Switch LP11EE1NCSYG	On-Air Button	5.00
3x1	Alps RK097111080J	10K log Mono Pot. (Gain)	2.16
3x1	Alps RK097111080J	10K log Mono Pot. (Headset Vol.)	2.16
3x2	Alps RK097111080J	10K log Mono Pot. (Headset Mix)	2.16
3x1	Alps RK09712200MC	10K log Dual Pot. (Headset Mix)	2.96
3x5	TODO	Potentiometer Knob	TODO
1	Vishay M64{Y,Z}104	100K Trim Pot.	0.95
		SUM	84.88

Sub-Components

Count	Manufacturer + Art. No.	Description	Price
1	Traco Power TXL 035-1515D or TOP 60533	Power Supply	~48.00

PCB Components: TODO when schematic is finished

Count	Manufacturer + Art. No.	Description	Price
3	NE5534	Low-noise Op-Amp	0.54
8	LM833	Generic Op-Amp	0.88
3	LM386N-4	Audio Power Amp	0.83
1	DRV143	Line Driver	4.50
TODO	TODO	El. Capacitor	TODO
TODO	TODO	Cer. Capacitor	TODO
TODO	TODO	Resistor	TODO
		SUM	TODO

VU Meter Components

Count	Art. No.	Description	Price
1	Vishay M64{Y,Z}503	50K Trim Pot.	0.95
1	LM833	Generic Op-Amp	0.88
2	1N4148	Signal Diode	0.02
1	1k Metal Film	Resistor	TODO
1	3.9k Metal Film	Resistor	TODO
1	47 uF	El. Capacitor	TODO
1	100k Metal Film	Resistor	TODO
3	LM339	Quad-Ch. Comperator	0.29
5	100 nF	Cer. Capacitor	TODO
1	1 uF	El. Capacitor	TODO
5	Vishay TLHR 5404	LED red	0.18
4	Vishay TLHY 5404	LED yellow	0.14
3	Vishay TLHG 5404	LED green	0.17
12	390R	Resistor	TODO
1	39R	Resistor	TODO
2	68R	Resistor	TODO
1	100R	Resistor	TODO
1	150R	Resistor	TODO
1	270R	Resistor	TODO
1	390R	Resistor	TODO
1	680R	Resistor	TODO
1	1K	Resistor	TODO
1	1.5K	Resistor	TODO
2	2.7K	Resistor	TODO
1	3.9K	Resistor	TODO
		SUM	TODO