View source

S/PDIF output buffer circuits

Here is set of output circuit which take in TTL level S/PDIF signal in and output standard S/PDIF coax cable output signal. Those output circuit are very useful building blocks for all kinds of S/PDIF related electronics projects. All the circuits do basically the same thing. They just use different components, so use the one where you can easily get components.

== Circuits ==

=== S/PDIF output buffer circuit ===

                                                           S/PDIF output
                                                           (RCA or BNC)
                             |\
 TTL level signal ----- -----| O-----680R-- ---------      ------- center pin
                       |     |/            |         |    |
                       |                   |          )||(
                       |     |\            |          )||(
                        -----| O-----680R--           )||(
                             |/            |          )||(
                                         100R        |    |
                           74HC04          |         |     ------- ground
                                            ---- ----  T1
                                                |
                                               ---

Idea for this circuit is taken from "A digital Output for the CD720" web page (was at http://www.spiceisle.com/homepages/brian/audiodiy/mods/digi-out.htm). The transformer T1 is high quality 1:1 pulse transformer (can be salvaged from old broken network interface card or similar source).

=== Another S/PDIF output circuit ===

                             |\
                        -----| O-----------
                       |     |/            |               S/PDIF output
                       |                   |  100nF        (RCA or BNC)
                       |     |\            |   ||      T1
 TTL level signal ----- -----| O----------- ---||----      ------- --75R--- center pin
                       |     |/            |   ||    |    |       |
                       |                   |          )|| |       |
                       |     |\            |     15   )||(  3    220R
                        -----| O-----------     turns )||( turns  |
                       |     |/            |          )|| |       |
                       |                   |         |    |       |
                       |     |\04          |         |     ------- -------- ground
                        -----| 0-----------          |
                             |/                     ---
 
                           74HCU04

This circuit is part of circuit "Splitter for S/PDIF coax/optical output" by T. Giesberts from Elektor Electronics magazine July/August 1995 pages 78-79. The transformer T1 is made to G2.3-FT12 ferrite ring core. Primary coil is 15 turns of 0.5 mm diameter enamelled copper wire and secondary is 3 turns of 0.5 mm diameter enamelled copper wire. The chip 74HCU04 is manufactured by [http://www.semiconductors.philips.com/ Philips]

The circuit in the magazine had 3 outputs. The two extra output were made by adding two more output coils to T1 and the output resistors.

=== Output circuit with transformer used as impedance matcher ===

This circuit is taken from Digital One V. 1.1 documentation. Digital One is from NET-Labs and is designed by by Enland Unruh.

                                                                S/PDIF output
 
                             IC1              0.1 uF        0.1 uF
                             |\                ||            ||
 TTL level signal -----------|  -----750R-- ---||----      --||--- center pin
                             |/            |   ||    |    |  ||
                                           |          )||(
                                           |          )||(
                                           |          )||(
                                           |          )||(
                                         499R        |    |
                                           |         |     ------- ground
                                            ---- ----  T1
                                                |
                                               ---

T1 is a HF-transformer which has 2:1 turns ratio. Transformer type used in example circuit is SC944-05. IC1 in this circuit consists of 8 parallel connected buffer ports from 74ACQ244 IC.

=== Output with 1:1 pulse transformer ===

This is the recommended S/PDIF output circuit from [http://www.crystal.com/ Crystal Semiconductor] CD4237 single chip multimedia audio system IC datasheet page 71. ''''''

                                                                S/PDIF output
                             IC1
                             |\
 TTL level signal -----------|  -----374R-- ---------      ------- center pin
                             |/            |         |    |
                                           |          )||(
                                           |          )||(
                                           |          )||(
                                           |          )||(
                                        90.9R        |    |
                                           |         |     ------- ground
                                            ---- ----  T1
                                                |
                                               ---

The transformer used in this circuit is 1:1 pulse transformer (the original application note suggested pulse transformer from Pulse Engineering or Schott Corporation). IC1 is a buffer IC which must be capable to supply enough current to drive the 75 &Omega; S/PDIF interface to full voltage swing specified (capable of loading 415 &Omega; load or capable of supplying 12 mA current).

=== Simple S/PDIF output without galvanic isolation ===

                             |\
                        -----| O-----------
                       |     |/            |               S/PDIF output
                       |                   |  150nF        (RCA or BNC)
              |\       |     |\            |   ||
 TTL level ---| o------ -----| O----------- ---||-----374R--- ----- center pin
 signal       |/       |     |/            |   ||            |
                       |                   |                 |
                       |     |\            |                93R1
                        -----| O-----------                  |
                       |     |/            |                  ----- ground
                       |                   |                 |
                       |     |\            |                ---
                        -----| 0-----------
                       |     |/            |
                       |                   |
                       |     |\            |
                        -----| 0-----------
                             |/
 
                           74HTU04

This circuit does not provide any isolation on the output. The TTL level signal form 74HCT04 buffer is attenuated to the levels specified in S/PDIF optical interface by using the voltage divider made of 374 &Omega; and 93.1 &Omega; resistors. This circuit is based on S/PDIF converter circuit diagram from Elektor Electronics magazine issue 4/1997 page 66.

=== Simplest TTL to S/PDIF coax interface ===

          100nF
   TTL ----||-----330ohm-- --------- coax S/PDIF signal out
   in                     |
                       100ohm
                          |
  Ground ----------------- --------- coax signal ground

This circuit interfaces directly to the TTL level output provided by some sound equipment (CD-ROM or sound card). This circuit works if the equipment which gives out TTL level has powerful enough TTL output to run this circuit (enough current output capability). If this simple circuit does not work reliably, then you need to build a circuit which includes the buffering electronics.

=== Isolation transformer for S/PDIF output ===

If your equipment (for example sound cards) has an S/PDIF output which does not have an isolation transformer at the output, you can easily add the transformer to it by putting a suitable transformer afther the original output.

Elector Electronics magazine issue 7-8/1999 recommend building the transformer for S/PDIF isolation in the following way:

The transformer must have good coupling factor, so the transformer core must be a toroidal core made of high permiability material. The prototype described in the magazine uses Philips Type TN13/7.5/5-3E25 core which has permeability rating (yt) of 4500. The primary and secondary windings consisted of 6 turns of 0.5 mm diameter enamelled copper wire laid on opposite sides of toroid. The transformer used in this prototype was described to have a bandwidth raged from 50 kHz to 17 MHz, which is more than adequate for an S/PDIF link.

The transformer should be fitted directly to the source and the receiving end must be properly terminated for reliable operation. The reason for this is that the transformer input and output impedances are not exactly 75 &Omega;.

== See also ==
* [[S/PDIF]]

== Contributors ==
* [[User:Joakim|Joakim Ögren]]

== Sources ==
* This page is based on [http://www.epanorama.net/documents/audio/spdif.html ePanorama.net: S/PDIF] by Tomi Engdahl (2002-05-28). Page published with permission from author.

[[Category:Circuit]]