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ePanorama.net - Audio Documents

SPDIF

History

Since the early 80's, a
step towards digital audio has been set by the introduction of the Compact Disc
player. In the beginning, those signals stayed inside the set, and were
converted to analog signals before leaving the cabinet. A new trend is to keep
signals into the digital domain as long as possible, because this is the only
way to keep the signal quality. To make this possible different devices must be
able communicate with one another within the digital domain. Several interfaces
exist to perform such tasks, from which one has grown to the audio standard
worldwide: IEC958 1989-03 (consumer Part) from the EBU. In Japan an
equivalent EIAJ CP-340 1987-9 is standard.

Characteristics

Standard IEC958
"Digital audio interface" from EBU (European Broadcasting Union)
details:

Audio format : linear 16 bit default, up to 24 bit
expandable

Allowed sampling frequencies (Fs) of the audio:

44.1kHz from CD

48 kHz from DAT

32 kHz from DSR

One way communication: from a transmitter to a
receiver.

Control information:

V (validity) bit : indicates if audio sample is valid.

U (user) bit
: user free coding i.e. running time song, track number.

C (channel status) bit : emphasis, sampling rate and copy permit.

P (parity) bit
: error detection bit to check for good reception.

Coding format: biphase mark except the headers
(preambles), for sync purposes.

Bandwidth occupation :
100kHz up to 6Mhz (no DC!)

Signal bitrate is 2.8Mhz
(Fs=44.1kHz), 2Mhz (Fs=32kHz) and 3.1Mhz (Fs=48kHz).

Physical
connection:

Cable: 75ohm +/-5% (l<10m) or 75ohm +/-35%
(l>10m)

Line driver:

Zout: 75ohm +/-20% (100kHz ..
6Mhz)

Vout: 0.4Vpp
.. 0.6Vpp, <0.05Vdc (75ohm terminated)

Line receiver:

Zin: 75ohm +/-5%

Vin: 0.2Vpp
.. 0.6Vpp

The interface

IEC958 is a newer standard
which supersedes AES/EBU and also S-PDIF. The S/PDIF interface (IEC-958) is a
'consumer' version of the AES/EBU-interface. The two formats are quite
compatible with each other, differing only in the subcode information and
connector. The professional format subcode contains ASCII strings for source
and destination identification, whereas the commercial format carries the SCMS.

Here is a short
comparision table of AES/EBU and S/PDIF interfaces:

AES/EBU              S/PDIF (IEC-958)

Cabling                   110 ohm shileded TP  75 ohm coaxial or fiber

Connector                 3-pin XLR            RCA (or BNC)

Signal level              3..10V               0.5..1V

Modulation                biphase-mark-code    biphase-mark-code

Subcode information       ASCII ID text        SCMS copy protection info

Max. Resolution           24 bits              20 bits (24 bit optional)

NOTE:
AES/EBU also exists in 75 ohm/BNC version (AES-3id-1995 standard). 75 ohm BNC
version of AES/EBU is very electrically similar to 75 ohm coaxial S/PDIF shown
above.

The electrical
characteristics of AES/EBU are based on on RS-422, so basically any
differential RS-422 chip will do as the receiver and transmitter chips. S/PDIF
coaxial interface is not specificaly based on any other standard I know of (but
is quite similar in signal levels and bandwidth requrements to some video
signals).

Both S/PDIF and AES/EBU
can, and do transfer 24 bit words. In AES/EBU, the last 4 bits have a defined
usage, so if anyone puts audio in there, it has to go to something that doesn't
expect the standard specifies. But in S/PDIF, there's nothing that says what
you have to use the bits for, so filling them all up with audio is acceptable.
Typical S/PDIF equipments only use 16 or 20 bit resolutions. While many
equipments use more than 16 bits in internal processing, it's not unusual for
the output to be limited to 16 bits.

Note on HDR-2 (2 pin
header) interface used in some PC products:

Many modern PC CD-ROM drives and some soundcards (SB32, AWE32, etc.) have a two
pin digital output connector in the back of the drive and they sometimes call
that interface S/PDIF. Unfortunately the electrical signal which comes from it
is not exactly what is described in S/PDIF specifications. The data format is
exactly the same, but the signal is TTL level (5Vpp) signal instead of the
normal 1Vpp signal. The output level might be selected to make the interfacing
to other digital electronics easy when signal is travelling inside the computer
(the normal output driver system and input amplifiers can be avoided). The
downnside of this is that you need to build some electronics to make the signal
from the CD-ROM drive to match what normal S/PDIF equipments expect.

Multi channel audio and S/PDIF

IEC958 was named IEC60958
at 1998. IEC60958 (The S/PDIF) can carry normal audio and IEC61937 datastreams.
IEC61937 datastreams can contain multichannel sound like MPEG2, AC3 or DTS.
When IEC61937 datastrams are transferred, the bits which normally carry audio
samples are replaced with the databits from the datastream and the headers of
the S/PDIF signal. Channel-status information contains one bit (but 1) which
tells if the data in S/PDIF frame is digital audio or some other data (DTS,
AC3, MPEG audio etc.). This bit will tell normal digital audio equipments that
they don't try to play back this data as they were audio samples. (would sound really horrible if this happens for some
reason).

The equipments which can
handle both normal audio and IEC61937 just look at those header bits to
determine what to do with the received data.

Cabling details

S/PDIF (IEC-958) uses 75
ohm coaxial cable and RCA connectors. 75 ohm coaxial cable is inexpensive,
because it is the same cable as used in video transmission (you can buy a video
cable with RCA connectors to connect you S/PDIF equipments together). Coaxial
S/PDIF connections work typically at least to 10-15 meter distances with good
75 ohm coaxial cable.

AES/EBU-interface uses the
well known symmetrical connections with transformer isolation and an output
impedance of 110 ohm. The signal-level of this interface is reasonably higher
than in the consumer version (3...10 volts). Because AES/EBU digital audio
signals are transmitted at high, video-like frequencies (at around 6MHz) and
should be handled very differently than standard analog audio lines. Commonly
used XLR-3 microphone cables have various impedance ratings (30 ohm to 90 ohm
typical) and exhibit poor digital transmission performance. The result is
signal drop out and reduced cable lengths due to severe impedance mis-matching
(VSWR) between AES/EBU 110 ohm equipment. AES/EBU signal
transmission work for few tens of meters with a good cable.

There also an optical
version of S/PDIF interface which is usually called Toslink, because uses
Toslink optical components. The transmission media is 1 mm plastic fiber and
the signals are trasmitted using visible light (red transmitting LED). The
optical signals have exactly the same format as the electrical S/PDIF signals, they are just converted to light signals (light
on/off). Because high light signal attenuation in the Toslink fiberoptic cable,
the transmission distance available using this technique is less than 10 meters
(with some equipments only few meters).

What can make difference in the sound of
digital signal ?

There are two things which
can cause differences between the sound of digital
interfaces:

1. Jitter (clock phase noise)

This
really only affects sound of the signal going directly to a DAC. If you're running into a computer,
the computer is effectively going to be reclocking everything. Same applies
also to CD-recoders, DAT tape decs and similar devices. Even modern DACs have
typically a small buffer and reclocking circuitry, so the jitter is not so big
problem nowadays that it used to be.

2. Errors

This usually causes very
significant changes in the sound, often loud popping noises but occasionally
less offensive effects. Any data loss or errors in either are a sign of a very
broken link which is probably intermittently dropping out altogether.

S/PDIF signals

The signal on the digital
output of a CD-player looks like almost perfect sine-wave, with an amplitude of
500 mVtt and a frequency of almost 3 MHz.

For each sample, two
32-bit words are transmitted, which results in a bit-rate of:

2.8224 Mbit/s  (44.1 kHz samplingrate, CD, DAT)

3.072  Mbit/s  (48 kHz sampling rate, DAT)

2.048  Mbit/s  (32 kHz sampling rate, for satellite purposes)

The output
impedance is standard 75 ohm, so ordinary coaxial cable designed for video
applications can be used. The minimal input level of S/PDIF interface is 200
mVtt which allows some cable losses. There is no real need for special quality
cable as long as the cable is made of 75 ohm coaxial cable (a good video
accessory cable works also as good S/PDIF cable).

The Coding Format

The digital signal is
coded using the 'biphase-mark-code' (BMC), which is a kind of phase-modulation.
In this system, two zero-crossings of the signal mean a logical 1 and one
zero-crossing means a logical 0.

_   _   _   _   _   _   _   _   _   _   _   _

| | | | | | | | | | | | | | | | | | | | | | | |

clock   0 ___ _| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_

___         _______     ___         ___

|   |       |       |   |   |       |   |

data    0 ___ _|   |_______|       |___|   |_______|   |___

signal           1   0   0   1   1   0   1   0   0   1   0

_   ___     _   _   ___   _     ___   _

Biphase        | | |   |   | | | | |   | | |   |   | | |

Mark    0 ___  | | |   |   | | | | |   | | |   |   | | |

signal         | | |   |   | | | | |   | | |   |   | | |

_| |_|   |___| |_| |_|   |_| |___|   |_| |___

cells           1 0 1 1 0 0 1 0 1 0 1 1 0 1 0 0 1 1 0 1 0 0

The frequency of the clock if twice the bitrate. Every bit of the original data is
represented as two logical states, which, together, form a cell. The length of
a cel ('time-slot') is equal to the length of a databit. The logical level at
the start of a bit is always inverted to the level at the end of the previous
bit. The level at the end of a bit is equal (a 0 transmitted) or inverted (a 1
transmitted) to the start of that bit.

The first 4 bits of a
32-bit word (bits 0 through 3) form a preamble which takes care of
synchronisation. This sync-pattern doesn't actually carry any data, but only
equals four databits in length. It also doesn't use the BMC, so bit patterns
which include more than two 0's or 1's in a row can occur (in fact, they always
do).

There are 3 different
sync-patterns, but they can appear in different forms, depending on the last
cell of the previous 32-bit word (parity):

Preamble    cell-order         cell-order

(last cell "0")    (last cell "1")

----------------------------------------------

"B"         11101000           00010111

"M"         11100010           00011101

"W"         11100100           00011011

Preamble B: Marks a word containing data for channel A (left)

at the start of the data-block.

Preamble M: Marks a word with data for channel A that isn't

at the start of the data-block.

Preamble W: Marks a word containing data for channel B.

(right, for stereo). When using more than 2

channels, this could also be any other channel

(except for A).

Word and Block Formats

Every sample is
transmitted as a 32-bit word (subframe). These bits are used as follows:

bits           meaning

----------------------------------------------------------

0-3            Preamble (see above; special structure)

4-7            Auxillary-audio-databits

8-27           Sample

(A 24-bit sample can be used (using bits 4-27).

A CD-player uses only 16 bits, so only bits

13 (LSB) to 27 (MSB) are used. Bits 4-12 are

set to 0).

28             Validity

(When this bit is set, the sample should not

be used by the receiver. A CD-player uses

the 'error-flag' to set this bit).

29             Subcode-data

30             Channel-status-information

31             Parity (bit 0-3 are not included)

The number
of subframes that are used depends on the number of channels that is transmitted.
A CD-player uses Channels A and B (left/right) and so each frame contains two
subframes. A block contains 192 frames and starts with a preamble
"B":

"M" Ch.1 "W" Ch.2 "B" Ch.1 "W" Ch.2 "M" Ch.1 "W" Ch.2 "M" ...

|                ||_ sub __|_ sub _||                |

|                ||                ||                |

|__ Frame 191 ___||__ Frame   0 ___||__ Frame  1 ____|

|

block-start

Channelstatus and subcode information

In each block, 384 bits of
channelstatus and subcode info are transmitted. The Channel-status bits are
equal for both subframes, so actually only 192 useful bits are transmitted:

bit            meaning

-------------------------------------------------------------

0-3            controlbits:

bit 0: 0 (is set to 1 during 4 channel transmission)

bit 1: 0=Digital audio, 1=Non-audio   (reserved to be 0 on old S/PDIF specs)

bit 2: copy-protection. Copying is allowed

when this bit is set.

bit 3: is set when pre-emphasis is used.

4-7            0 (reserved)

9-15           catagory-code:

0 = common 2-channel format

1 = 2-channel CD-format

(set by a CD-player when a subcode is

transmitted)

2 = 2-channel PCM-encoder-decoder format

others are not used

19-191         0 (reserved)

The
subcode-bits can be used by the manufacturer at will. They are used in blocks
of 1176 bits before which a sync-word of 16 "0"-bits is transmitted

Electrical Interface

The electrical interface
for S/PDIF signals can be either 75 ohm coaxial cable or optical fiber (usually
called Toslink). Usually consumer models use that coaxial cable interface and
semiprofessional/professional equipments use optical interface. The electrical
signal in the coaxial cable is about 500mVtt.

Converting between AES/EBU and S/PDIF
interfaces

There are differences in
the electrical characteristics of AES/EBU and S/PDIF interfaces:

AES/EBU uses a balanced differential line based on
XLR connectors and the signal levels are 5 volts

S/P-DIF uses a coaxial unbalanced line with RCA
connectors and the signal levels are around 0.5 volts

You can
convert one electrical interface to another with a small amount of
off-the-shelf hardware and a little time as you can see in the circuit below.

But the protocol used in
AES/EBU and S/PDIF is not exactly the same and that can cause sometimes
problems. The basic data format of AES and S/P-DIF are identical. There is a
bit in the channel status frame that tells which is which. Depending upon the
setting of that bit, some bits have different meanings. For example, the bits
used to describe de-emphasis in the AES/EBU protocol overlap the bits used to
implement the SCMS protocol in S/P-DIF land.

The big problem comes in
the fact that MANY products out there are VERY picky about what they see in the
bits, and even though a given signal may fall within the letter of the
standard, some equipment will absolutely refuse to talk to it. Many equipments are reasonably flexible and tolerant of slight
foos in the signal so the simple converters cna work on those. But a simple
converter that works fine with one piece will as likely not work with another.

What are different
types of IEC 958-interface

There are 2
implementations of IEC 958: consumer and professional. Those are referred in
standard as IEC958 Types I and II. IEC958 professional format is same as
AES/EBU but is carried over same type of coaxial or optical interface as
consumer S/PDIF. IEC958 consumer format is the S/PDIF format used in
CD-players. You can put an S-PDIF data stream on an AES/EBU physical balanced
cable, or vice versa, and still have it be valid
IEC958 data. Professional and consumer formats (Types I and II) differ only in
the subcode information. In order to do track indexing, you must have a
consumer format data stream (ie. an S/PDIF style data).

Jitter specifications of
AES/EBU interface

The AES/EBU standard for
serial digital audio uses typically 163 ns clock rate and allows up to +-20 ns
of jitter in the signal. This peaks to peak value of
40 ns is aroun 1/4 of the unit interval. D/A conversion clock jitter
requirements are considrably tighter. A draft AES/EBU standard specifies the
D/A converter clock at 1 ns jitte; however, a theoretical value for 16-bit
audio could be as small as 0.1 nsec. Small jitter D/A conversion is implemented by using separate PLL clocks for data recover
and DAC and by using a buffering between data recovery and DAC.

Conversion circuits

Here are some AES/EBU and
S/PDIF circuit collected from various sources. The following circuit will only
convert the signal levels, not other protocol details.

Remeber
that although the audio data is the same in both AES/EBU and S/PDIF interfaces,
they are indeed different formats, at least in their subcode. AES converted to coax is NOT
S/PDIF, and S/PDIF converted to XLR balanced is NOT AES. They are still their
native format, just the transmission medium has
changed. Whether they will work in your application depends on the equipment
chosen.

Some DATs have a switch
that selects one format or the other regardless of the physical interface, some
just ignore what they don't understand (usually resulting in the generally
positive benefit of ignoring SCMS encoding), and some indeed gag on the
"other" format. But simply fixing the physical interface works far
more often than it doesn't.

How to do different conversions using the
circuit below

Here are some ideas how to
make the most common conversions using the circits described below. Note: there
is no guarantee that the information in this or the circuit are correct (they
are believed to be correct but note tested by the author).

AES/EBU to S/PDIF: There is complete circuit for this

S/PDIF to AES/EBU: There is complete circuit for this

S/PDIF to optical: S/PDIF coax input circuit followed
with optical TOSLINK output

Optical to S/ODIF: Optical TOSLINK receiver followed
with S/PDIF output buffer circuit

CD-ROM digital output to normal S/PDIF: S/PDIF output
buffer circuit does this

CD-ROM digital output to optical: Connect optical
TOSLINK output circuit to CD-ROM output

For every
other conversion combination you can think of you can find circuits on the lis
below. To build an dapter you need two parts connected
after each other.

First you need an input circuit which converts the
input you want (coax,optical or AES/EBU) to TTL format (if the input is in
TTL format you don't need any input circuitry)

Connect a suitable output circuit with TTL input
after the receiving electronics to give you the output format you want
(coax,optical,AES/EBU,TTL)

After you
have slected the suitable circuit parts, built them an attached them together
you get the conversion circuit you want. If you need more than one output, you
can connect few (1-4) output modules to one input circuitry to have more then
one output.

AES/EBU to S/PDIF signal level converter

AES out:   2-------330 ohm-----+-------------   SPDIF in

|

3--+              91 ohm

|                |

1--+------+---------+-------------

|

-

ground

The idea for this circuit
is taken from articles posted to Usenet News.

If you are looking for a
professional components for 110 ohm to 75 ohm interconnection then check Canare web site where they have 110 OHM-75
OHM IMPEDANCE TRANSFORMERS.

S/PDIF to AES/EBU

_

+ 5 volt                | \

SPDIF out:          |                   +-|  O-+

_ +--4,7k-+         | |_/  |

diode ^ |  _    |     _   |  _   |

10 u (+)     | | | \   |    | \  | | \  |      100 n

O--+--+-||-+--------+-+-|  O--+--+-|  O-+-|  O-+-120 r-||-+----+-- 2

|  |    |        |   |_/   |  | |_/    |_/             |    |

|  +-||-+ 100n   _       2,2k |                       1 k   |

75r               ^ diode   |  |     IC                 |    |   AES in

|                |        --- |                       ---   |

---              ---           |       _                    220r

|      | \           100 n    |

+--+---|  O---+--120 r-||-+---+--- 3

Diode = 1N914 or 1N4148            |   |_/    |           |

|    _     |         1 k   +--- 1

|   | \    |           |   |

IC = 74HC04 or CMOS 4049        +---|  O---+          --- ---

|_/

The idea
for this circuit is taken from articles posted to Usenet News.

S/PDIF conversion circuit building blocks

This is a collection of
S/PDIF circuits found from various sources. The circuits are presented as
building blocks which have one end on S/PDIF standard signal and other end an TTL level signal. The TTL levle signal end of the
circuits is designed to be the the interface which you can use to wire
different modules together to make whatever S/PDIF converter circuit you want.
The circuit are presented as building blocks because
with this approacs you can most easily build a suitable circuit for all
conversion needs.

Here are tips for building
a conversision circuits for different uses:

Coaxial S/PDIF to optical: Select any S/PDIF coaxial
input circuit and connect it's TTL output to one
optical S/PDIF (Toslink) output circuit.

Optical S/PDIF to coaxial: Select one optical S/PDIF
input circuit and connet it's TTL output to any coaxial S/PDIF output
circuit.

CD-ROM digital out to coaxial: CD-ROMs output TTL
level S/PDIF, so get it to caoxial you just need a coaxial output circuit
where you directly connect the digital signal from your CD-ROM drive.

For other conversion needs
do a little thinking and you should find quite easily the answer what blocks to
connect to each other.

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 uou can easily get components.

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 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 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 capablw to supply enough current
to drive the 75 ohm S/PDIF interface to full votlage swing specified (capable
of loading 415 ohm 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 ohm and 93.1 ohm 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
ricuit interfaces directly to the TTL level output provided by some sound
equipment (CD-ROM or soundcard). 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 permiability rating (yt) of 4500. THe primary and secondary windongs 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 protperly
terminated for reliable operation. The reason for this is that the transformer
input and output impedances are not exactly 75 ohm.

S/PDIF input circuits

The following circuits can
be used to convert S/PDIF signals to TTL level signals which can be connected
to any suitable output circuit or directly to digital electronics.

S/PDIF coax input circuit

+----10K--+

10 nF        |         |

||          |    |\   |        |\

S/PDIF input ------+---||----100R--+----| O--+---+----| O----- TTL out

|   ||               |/       |    |/

75R                           10K

|                 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 chip 74HCU04 is manufactured by Philips

Another input circuit

+----10K----- +5V

|     /\

---     |

4700 pF       |

||          |    |\            |\

S/PDIF input ----------||----------+----| O-----------| O----- TTL out

(+/-500mV)             ||               |/            |/

7404

The capacitor works as DC
blocking capacitor. 10 kohm trimpot signal raises its
ground potential to the point that Hex Inverter IC1 recognizes the difference
between a digital 1 and 0 input. When you firt test the circuit then first
addjust 10 kohm trimmer to the center and then adjust it so that the circuit
receivers the S/PDIF signal well. If you have a voltmeter, you can adjust the
trimpot to show around 2.6V at pin 1 of the IC. The inverter buffers the input
and produces TTL 0-5V output signal. This circuit does not provivide the 75 ohm
termination resistor, so you need to add it if you have problems because of
cable signal reflections. This circuit is ppart of Coax Digital Out to MD circuit design.

Optical S/PDIF output circuit

Optical TOSLINK output

________________

4|             ___|

TTL signal -----------|            /

|           |

3|           |

+5V--4R7--+-----------|           |

|          2|   TOTX173 |

+---8k2-----|           |

|           |           |

=== 100 nF   |           |

|          1|            \___

+-----------|________________|

|

---

The 4.7
ohm resistor and 100 nF capacitor make a filter
against digital noise in +5V power source. The Toslink transmitting module used
in this circuit is TOTX173
from Toshiba. This circuit is a part of
circuit diagram from Elektor Electronics magazine issue 4/1997 page 65.

TORX173 and TOTX173 module pinouts

Toshiba optical receiver
& transmitter modules used for S/P-DIF TORX173, TOTX173 top view:

opto

+----+

6.    .5

+....+

1234

Pin  TORX173     TOTX173

1    out         GND

2    GND         R

3    Vcc         Vcc

4    GND         in

5    case(GND)   case(GND)

6    case(GND)   case(GND)

Vcc  +5V (use 0.1 uF decoupling to GND)

(use 47 uH choke to connect Pin 3 to Vcc for TORX)

R    8.2 kOhm to Vcc

TORX and
TOTX are both high active and are CMOS/TLL logic compatible.

More information and
datasheets is available at Toshiba optoelectronics
components web page TOSLINK section.

Very cheap optical S/PDIF output

With TOSLINK modules
available for as low as $10 it hardly seems worth it to bother with the trouble
of making your own TOSLINK module due to the fact that the ones from Sharp have
a built-in connector and are much easier to interface. But if you really want
to make your own optical output interface, you can do it because S/PDIF optical
(Toslink) transmitter is just basically a red LED coupled to the optical fiber.
So to build a very basic output circuit you only need a red LED and a resistor,
both available from any place where electronic components are sold.

The circuit is very
simple:

_____

TTL signal -------|_____|---------+

|

---

V  LED

---

|

Ground -----------------------+

The idea of the circuit is
that the digital data is fed through a series current limiting resistor to the
LED (typically LEDs only require 5-30mA). A digital 0 is 0V and the LED is off.
A digital 1 is +5V and the LED is on. That's it!

If for some reason the
circuit does not work, try to change the LED and also try to connecting it
better to the optical fiber. NOTE: Not all LEDs will work, so you may have to
experiment a bit. Optimally, you want a high-brightness RED LED with a typical
wavelength of 660nm.

If swapping the LED
doesn't work, you might want to try using a hex inverter (7404 or similar IC)
to buffer the signal. This might be good practice too, just in case the TTL
SPDIF output of your source device isn't designed to handle a 20~30mA load this
circuit generates.

Optical S/PDIF output circuit

Optical TOSLINK receiver

_____________

|___          |1

\         |----------------- TTL output

|        |

|TORX173 |3

|        |----+----coil47uH----- +5V

|        |    |

|        |   === 100 nF

|        |2   |

|        |----+

|        |4   |

|        |----+

|        |5   |

|        |----+

|        |6   |

__/         |----+

|____________|    |

---

The 47 microhenry coil and
100 nF capacitor make a filter against digital noise
in +5V power source. The Toslink receiving module used in this circuit is
TORX173 from Toshiba. This circuit is a
part of circuit diagram from Elektor Electronics magazine issue 4/1997 page 66.

TORX173 and TOTX173 module pinouts

Toshiba optical receiver &
transmitter modules used for S/P-DIF TORX173, TOTX173 top view:

opto

+----+

6.    .5

+....+

1234

Pin  TORX173     TOTX173

1    out         GND

2    GND         R

3    Vcc         Vcc

4    GND         in

5    case(GND)   case(GND)

6    case(GND)   case(GND)

Vcc  +5V (use 0.1 uF decoupling to GND)

(use 47 uH choke to connect Pin 3 to Vcc for TORX)

R    8.2 kOhm to Vcc

TORX and
TOTX are both high active and are CMOS/TLL logic compatible.

More information and
datasheets is available at Toshiba optoelectronics
components web page TOSLINK section.

Other related information

Who makes AES/EBU digital audio
transceiver chips ?

DSP FAQ mentions that Sony,
Crystal Semiconductor, Motorola and Yamaha have all AES/EBU tranceiver chips
available. DSP FAQ contain more
information about AES/EBU chips at http://tjev.tel.etf.hr/josip/DSP/FAQ/42.html.
Same companies which have AES/EBU chips have also SPDIF chips and some chips
can handle both formats.

Information sources

About
SP-DIF by DJ Greaves

DSP FAQ

Elektor Electronics magazine July/August 1995 pages
78-79

Public domain S/PDIF document found in in the net
(that article did not mention the author name)

David K. Fibush, Jitter Effect and Measurement in
Serial Digital Television Signals, SMPTE Journal, August 1993, pages
693-698

Various articles from usenet newsgroups
sci.electronics.design, rec.audio.pro and rec.audio.tech

Super-Cheap
TOSLINK Module web page

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