Let’s say you want to solder an SPDIF coaxial connector instead of a SPDIF optical  input connector(TOSLINK receiver). First of all, you cannot do this directly on the PCB, because the pinout of the TOSLINK connector and the SPDIF coaxial connector are not the same. However, let’s say you desolder the TOSLINK input connector from the PCB and then use wires to connect the SPDIF coaxial connector to the pins where the TOSLINK input connector was located.

This will not work because SPDIF coaxial and TOSLINK use different signals, so an electronic circuit will need to be added between the SPDIF coaxial connector and the pins where the TOSLINK input connector was soldered

To connect an SPDIF coaxial input instead of a TOSLINK optical input to the same port/pins (e.g., on a digital receiver like the DIR9001, WM8804, CS8416, or similar), you need to convert the coaxial signal to the logic level signal that would normally come from the TOSLINK receiver. Digital receiver (e.g., CS8416, DIR9001, or WM8804) accepts a TTL-level signal. When using a TOSLINK receiver, it converts the optical signal into a TTL signal. If you instead use SPDIF coaxial, you must also convert it into a TTL signal for the chip to properly "understand" the data.

This is why an SPDIF coax-to-TTL converter or a suitable electronic circuit is needed.

The SPDIF coaxial signal is transmitted over a 75-ohm line with a 1 Vpp (volts peak-to-peak) voltage. The digital receiver expects a TTL signal of 0–3.3 V or 0–5 V, depending on the chip. Here's how to achieve this:

Components and Diagram:

1. SPDIF Coaxial Connector (RCA): For connecting the coaxial cable.
2. DC Blocking Capacitor (10 nF – 100 nF): To block any DC component.
3. Impedance-Matching Resistor (75 ohms): To maintain the characteristic impedance.
4. Schmitt Trigger or High-Performance Comparator (e.g., 74HCU04, SN74LVC1G17, or MAX3280): To convert the signal into TTL logic.
5. Power Supply (3.3V or 5V): Depending on your chip's operating voltage.

Connection Steps:

1. DC Blocking Capacitor:

• Connect the signal pin of the SPDIF coaxial connector through a capacitor (e.g., 10 nF or 100 nF, 50V) to remove the DC offset.

2. Impedance Matching:

• In parallel with the input (after the capacitor), add a 75-ohm resistor to ground (GND) to maintain the characteristic impedance.

3. Signal Conversion:

• Take the signal from the capacitor and feed it into the input of a Schmitt trigger or comparator. This will "clean up" the signal and convert it into a square TTL signal.
• The output of the Schmitt trigger goes directly to the pin on the digital receiver where the TOSLINK receiver was previously connected.

4. Power Supply:

• Provide the appropriate power supply for the Schmitt trigger or comparator. If your digital receiver operates at 3.3V, use  Schmitt trigger that works at 3.3V.

Example Schematic (Simplified with Schmitt Trigger):

• Input: SPDIF coaxial signal
• DC Blocking Capacitor: Connect between SPDIF input and the Schmitt trigger
• 75-ohm Resistor: Parallel to the input, connected to GND
• Schmitt Trigger Output: Connect directly to the digital receiver input
• Power: Ensure Schmitt trigger and digital receiver use compatible voltage levels

This setup ensures a seamless transition from SPDIF coaxial to TTL logic, replicating the behavior of the original TOSLINK receiver.

RCA signal --> |---[10 nF]---+---[Schmitt Trigger]--> Digital RX Pin

               |             |

               |           [75 ohm]

               |             |

              GND           GND

A TOSLINK receiver typically has three pins:

1. VCC – Power supply (3.3 V or 5 V)
2. GND – Ground
3. DATA – TTL digital signal
   
   
   
   

   Tihomir Haralović, M.Sc. in Physics
   
   

    Zagreb, 23 December 2024