140 lines
6.6 KiB
Markdown
140 lines
6.6 KiB
Markdown
# Conference Interpreter Unit
|
|
[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.
|
|
|
|
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
|
|
- [x] 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
|
|
- [x] Microphone input
|
|
- [x] Line input
|
|
- [x] Microphone summing and line output driver
|
|
- [x] 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](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.
|
|
|
|
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](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.
|
|
|
|
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
|