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+# Digital Multilingual Interpreter System Over RS-485
+
+## Concept
+There was a need to create a multi-channel interpreter system that transmits audio data via RS-485 bus. The system includes permanently installed listener units that are wired using CAT6 cable. These units have a very simple user interface:
+
+- Channel selection buttons (+/-)
+- Volume control buttons (+/-)
+- Display to show the selected channel number and volume level
+
+Users can choose from several languages (channels), and the original language can also be selected, which is an important feature for people with severe hearing impairments.
+
+### Data Transmission
+For the audio stream we will use plain PCM samples.
+
+- Advantages of PCM: no encoding/decoding is required, which keeps the software simple.
+- Disadvantages: PCM needs a relatively large bandwidth on the communication channel.
+
+### Bus Architecture
+
+Transmitting such a high-volume data stream demands a fast communication medium. Two candidates were considered: Ethernet and high-speed RS-485.
+
+#### Ethernet
+Drawbacks:
+- The hardware and software stack are complex (requires transformer, PHY, and a full Ethernet protocol stack).
+- A star topology is needed, which means the system would require many switches.
+
+Benefits:
+- Each listener unit is galvanically isolated
+- Provides a very high bandwidth
+
+#### RS-485
+Benefits:
+- Simpler hardware and software (just a UART with an RS-485 driver)
+- Bus topology allows all units to be daisy-chained on a single cable, reducing wiring effort.
+
+Drawbacks:
+- The bus is not inherently galvanically isolated; isolation components must be added if separation is required.
+
+#### Conclusion
+Given our priorities—simplicity of hardware/software and minimal cabling—the RS-485 bus is the optimal choice, provided we employ high-speed RS-485 transceivers capable of several megabits per second to accommodate the PCM data rate. We will provide galvanic isolation only between bus sections, with each section containing several units.
+
+## Hardware
+
+### First idea
+Block diagram with the main parts:
+
+- RS-485 driver
+- STM32 ARM microcontroller with I2S audio interface
+- DAC (digital-to-analog converter)
+- Stereo power-amp (headphone amplifier)
+- Pushbuttons
+- Display
+- Power supply
+
+![Block diagram](doc/STM32_audio_Headphone_node_simple.png)
+
+### Final Hardware
+Main parts:
+- STM32H533RET microcontroller (512k flash, 272k RAM, max.250MHz, TQFP-64)
+- 2x RS485 port
+- 2x TLV320AIC3104 audio codec (ADC+DAC, mic. preamp with AGC, stereo headphone amp)
+- 8x pushbuttons
+- 2x 1.3" OLED 128x64 dotmatrix display (SH1106, SPI)
+- I2C serial EEPROM
+- 4-pol 3.5mm jack connector
+- 2x RJ-45 connector
+- power supply
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