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).

Files:

Eagle schematic

Eagle PCB
layout

PCB
for printing (600 DPI)

PCB
parts placement

[a freeware version of the Eagle Layout
editor is available at http://www.cadsoft.de/]

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 http://www.minidisc.org/part_Manuals.html,
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)