spdif.txt HP 960630 981112 improved S/D-DIF input/output circuit using 74HCU04 =================================================== Vcc 22 kOhm S/P-DIF | +-----RR------+ input C | | --- A | +----+ | +----+ | | 0.1 uF | | | | | | | | o-----+---CC----+---+---| =1 |o---+---| =1 |o---+--- TTL/CMOS | | | | | | | | | | output -+- R C A +----+ +----+ A | R 75 Ohm A C C | | | | | GND GND GND +----------+----------------+ | | R C R 47 kOhm C 0.1 uF | | GND GND +----+ | | +--| =1 |o--+ | | | | | +----+ | S/P-DIF | | output +----+ | +----+ | --- | | | | | | 0.1 uF 330 Ohm | | TTL/CMOS ---| =1 |o--+--| =1 |o--+---CC------RR---+-----o | input | | | | | | | | | +----+ | +----+ | R -+- | | 100 Ohm R | | +----+ | | | | | | | GND GND +--| =1 |o--+ | | +----+ Vcc 74HCU04 | C C 0.1 uF | GND 74HCU04 power: Vcc = +5 V (recommended: +4..+5 V) (max.: +2..+6 V) GND = 0 V I = 20 mA (input circuit: 9..13 mA output circuit: 8 mA) minimum input signal: Upp=0.2V into 75 Ohm output signal: Upp=0.5V into 75 Ohm nominal S/P-DIF signal: Upp=0.5V (0.5V peak-peak = +/-0.25V) when terminated with 75 Ohm 74HCU04 hex inverter (inverters named A..F here) +-------_-------+ 1 -| A in Vcc |- 14 2 -| A out F in |- 13 3 -| B in F out |- 12 4 -| B out E in |- 11 5 -| C in E out |- 10 6 -| C out D in |- 9 7 -| GND D out |- 8 +---------------+ resistor: ---RR--- (carbon film 1/4 W or better) capacitor: ---CC--- (creamic, film, similar or better) ( but NO electrolytic or tantal) diode: ---CA--- (Cathode Anode) ( e.g. 1N4148 or better) The "U" in "74HCU04" means "Unbuffered" and is rather important in this circuit. Using a 74HCU04, the input circuit will give a clean logical low at the output if no input signal is present. When using a 74HC04 instead, the input circuit will oscillate when no input signal is present. This input circuit might look a bit odd and it might not seem obvious that quite a lot of thoughts went into this design. But I still have to see a better S/P-DIF input circuit based on standard components. NOTE: All usual discalimers apply - you do everything at your own risk whilst I can't guarantee for anything ... You can copy this file as long as this note is maintained. You might use this design for non-commercial projects/applications. any commercial use requires prior agreement. Heiko Purnhagen