S/PDIF output extension for Portable MiniDisc Devices

Here's a very trivial S/PDIF transmitter hack for portable MiniDiscs and other digital audio devices lacking S/PDIF output.

Board version 1.0 with +5V regulator - the schematic and PCB below are version 1.1

The s/pdif transmitter runs from +5V, 44kHz 16bit input at 384xfs, is set to consumer mode output, and doesn't transmit any COPY bit (copyright protection disabled). The board can be connected directly to a PC motherboard, or via a Toshiba TOTX173 to an optical cable feed, or with a small transformer to a coax line.

The DIT4096 digital audio transmitter is available from Texas Instruments, and you can set a free sample order. www.digikey.com has it in stock for around $4 (as of April/2004).


JP1 input connector pins:
  6 = Vdd input (+5V)
  5 = MCLK aka CLK/SCLK - system clock input
  4 = SCLK aka BCLK - serial bit clock input
  3 = SYNC aka LRCLK / left-right channel "sync" clock input
  2 = SDATA aka DIN/DOUT/DATA - serial data input
  1 = GND - minidisc ground

JP2 output connector pins:
  1 = TX+ and 5V TTL (noninverted output)
  2 = TX- (inverted output)
  3 = 3.3V TTL (output to PC SPDIF input pin)
  4 = ground (to PC ground or TOSLINK module GND)

MiniDisc or digital TV-box or ... modification:

Pinheader 'JP1' pins MCLK, SCLK, LRCLK, SDATA, GND have to be soldered directly to the corresponding pins of the combined DAC-ADC chip inside the minidisc portable. Likely candidates can be found by skimming through www.minidisc.org, service manuals, or by performing a Google search for the datasheets of all small chips found on the MiniDisc PCB somewhere near the headphone connector.

Example modification:

Connecting to the PC:

Find the S/PDIF connector on the PC motherboard (NOT the opto or coax connector, but the pin header, usually the 'connector for the optional S/PDIF bracket'). Then connect the JP2 3.3V TTL level pin 3 directly to the motherboard S/PDIF _input_ pin, and JP2 ground pin 4 to the motherboard ground. If the audio chip on the motherboard requires higher that 3.3V TLL input signals and refuses to work with the 3.3V input, try the TX+/5V TLL pin 1 instead of pin 3.

Connecting to coax or optical:

Coax or optical S/PDIF out's may be added by connecting some extras to JP2 pins 2 and 3. The additions are again very simple to do. Here's a screenshot from TI's DIT4096 datasheet:

(Note: in Figure 13, the '10pF' ceramic capacitor should probably be 10nF/0.01uF)