147 lines
6.9 KiB
Markdown
147 lines
6.9 KiB
Markdown
# Conference Interpreter Unit
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[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.
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But the required hardware for simultaneous translation is quite expensive to rent, even if it's "just" a specialized analog audio mixer.
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So the idea was born to design an easy to use hardware with the special requirements of our interpreters in mind.
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## Requirements
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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.
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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.
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Normally just 2 interpreters will provide one translation, but demanding talks might require 3 people.
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So either 3 headphone inputs/ outputs should be provided or it must be possible to daisy-chain multiple units.
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The user interface of the unit should be as simple, as possible to decrease the risk of mis-configuration.
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This means, that no compressor and equalizer will be added in the input group.
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General requirements:
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- Line input of stage/ hall mix (native language) (XLR/ 6.3 mm balanced jack combo connector)
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- 3x Microphone input (XLR) (at first just dynamic microphones)
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* VU-Meter for each input channel. (Perhaps with special color scheme: too quiet, good, too loud, clipping)
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* Fader/ Potentiometer for gain control
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* On-Air switch (on/ off position, with state LED)
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* Temporary mute button (momentary switch)
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- 3x Headphone output (6.3 mm mono/ stereo (2x mono) jack)
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* Should output mix of native audio and own microphone
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* Potentiometer for output volume
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* Potentiometer for volume of own microphone in the mix
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- Outputs:
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* Sum of all translators (XLR/ 6.3 mm balanced jack combo connector)
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All inputs (besides the microphones) and outputs should:
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- expect/ deliver a nominal level of +6dBu (german TV broadcast standard, 0 dBu = 0,775 Veff)
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- be transformer balanced and galvanically isolated
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## Status
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### Tasks
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- [x] Requirements collection
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- [ ] Electrical design draft/ proof of concept (breadboard)
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- [ ] Proof of concept validation
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- [ ] First PCB layout
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- [ ] More testing/ validation
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Help is always appreciated!
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### Implemented Requirements
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- [x] Microphone input
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- [x] Line input
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- [x] Microphone summing and line output driver
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- [x] Adjustable headphone mix and output volume
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- [ ] ESD protection and galvanic isolation of line inputs and outputs
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- [ ] VU meter
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- [ ] Mute and On-Air buttons
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## Electrical Design
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This chapter contains some notes on the electrical design.
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Currently just the sources of the used circuit designs.
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### Sources and Design Considerations
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#### Microphone Input
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##### Preamplifier
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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).
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##### Controllable Amplification
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In a normal mixer, you would be able to lower the microphone's volume to zero.
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But in our case we just need on/ off and some gain range to adjust for different microphones and loudness of different people.
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TODO: Try, if the preamp's gain of about 50 dB is enough and how much gain is need in this stage.
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From theoretical calculations, we might need another 30 to 40 dB here, which needs to be adjustable in a range of about 30 dB.
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TODO: Try out using the amp's feedback resistor for gain control or just use a potentiometer on the input of the next stage.
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TODO: Integrate the On-Air and mute buttons
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For long-lasting endurance of the microphone level potentiometer, we're using one with conductive plastic as resistor element (Bourns model 91).
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#### Line Input/ Input Module
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The line input must not be amplified at all, because loudness control of the headphones is done by the headphone amplifier section.
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But the differential line-level signal must be converted to a single-ended signal by the input stage.
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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.
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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).
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TODO: How to achieve galvanic isolation?
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TODO: Use https://www.ti.com/product/INA134 for input conversion?
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#### Summing
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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).
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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).
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#### Line Output Driver
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TODO: Either use https://www.ti.com/product/DRV134 or http://www.thatcorp.com/1600-series_Balanced_Line_Driver_ICs.shtml.
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TODO: How to achieve galvanic isolation?
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#### Headphone Output Driver
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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.
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For the first draft, we're using one LM386 audio power amplifier.
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TODO: Check, if driving the two speakers in parallel leads to any issues (besides cutting the speaker impedance in half).
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#### VU Meter
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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.
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TODO: Re-create the LM3916 with some LM339 or design/ program some small uC.
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## Notes
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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).
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Line level in professional audio gear is at +4 dBu, which is 1.228 V (RMS).
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Because 0 dBu is defined as 1 mW at a load of 600 Ohm, which needs a voltage of 0.77 V.
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Increasing the voltage by a factor of 10 is an amplification of 20 dB.
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## BoM
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Approximate prices in Euro.
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Connectors and Buttons (User Interface)
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| Count | Art. No. | Description | Price |
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|-------|--------------------|------------------|-------|
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| 1 | Neutrik NCJ 6 FAH | Line Input | 1,27 |
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| 1 | Neutrik NC3 FD-LX | Microphone Input | 3,44 |
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| 1 | Neutrik NJ3 FP-6-C | Headphone Output | 5,40 |
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| 1 | Neutrik NC3 MD-LX | Line Output | 3,22 |
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| 1 | Neutrik NAC3 MPA-1 | Main Power Input | 3,33 |
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Sub-Components
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| Count | Art. No. | Description | Price |
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|-------|--------------------|------------------|--------|
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| 1 | Traco Power TXL 035-1515D or TOP 60533 | Power Supply | ~48,00 |
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PCB Components: TODO when schematic is finished
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