Elliott Sound Products

Project 85

S/PDIF Digital to Analogue
Converter
Randy McAnally
(Edited and additional
material by Rod Elliott - ESP)

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Introduction

S/PDIF is an acronym for Sony /Philips Digital Interface (or Sony /Philips
Digital Interconnect Format). The full specification for the S/PDIF interface
consists of hardware and software, but only the hardware side will be discussed
here - the software is looked after by the various interface ICs. The hardware
interface is the physical connection medium that is used to send data between
any two devices. Much of the confusion surrounding S/PDIF is created because
there are several different ways of sending S/PDIF data. You may come across the
following interfaces:

TTL

Transistor-Transistor Logic - used by nearly all digital logic
circuits
TTL is typically (but not always!) 5V (on), and 0V
(off). TTL is used as a matter of course within nearly all digital
devices, and almost all logic ICs are compatible with these signal levels.
TTL S/PDIF outputs are also provided on many sound cards. Many suppliers
sell add-on units to accomplish TTL to COAX or TTL to TOSLINK
conversion.

COAX

Coaxial cable - 75 ohm cable connected with RCA plugs
The
coaxial interface uses 75 ohm COAX cable with RCA (phono) connectors.
Standard audio interconnect cables will work for transmitting S/PDIF over
short distances, but anything over 0.5 metre or so should use 75 ohm
cable. The unloaded signal is nominally +/-0.5V and must be terminated
with 75 ohms on the receiving end - the resulting signal is +/-0.25V when
terminated. Naturally, audio 'speciality' shops love to sell the
'ultimate' cable for up to several hundred dollars, but a cable which you
can easily make yourself should cost no more than $10-20 using good
quality 75 ohm cable and connectors.

TOSLINK

An optical fibre connection
The TOSLINK interface uses
optical fibre cables that plug into TOSLINK modules. These modules send or
receive a TTL signal. Again, speciality cables will make no difference to
the final sound quality, so don't be caught out by the glib sales person
who insists that you have to spend serious money to get the best sound.
Good quality fibre cables are essential however, as degradation of the
optical signal will cause distortion, noise and even loss of signal in
extreme cases.

It does not matter which type of hardware interface you use, the data is the
same. The S/PDIF signal is independent of polarity, which means that you do not
have to concern yourself with absolute polarity - this make things a lot
easier.

It is worth pointing out that a complete digital 5.1 system can be made using
the decoder described here, along with the surround sound decoder described in
Project
18. A complete 'true' Dolby (or any other proprietary system) is not
possible for DIY, because the specialised ICs are generally only available to
licensed manufacturers. The simple Hafler Matrix type decoder described in
Project 18 is certainly not the same as a fully integrated system, but it does a
very good job for the most part, and has the benefit of being cheap to build,
using readily available components.

Description

This is quite possibly the simplest S/PDIF receiver and DAC available. It
uses the absolute minimum of parts, and also minimises the connections and
control functionality usually provided. It is still a serious project, and is
not recommended for beginners. As shown, the connection is COAX (but will almost
certainly handle TTL just as well). If you want a dedicated TTL to COAX
converter, there is an adapter shown at the end of this article. The spare gates
in the 74HC04 package may be used for the adapter if desired.

Power Supply

The supply is a basic 5 volt regulator circuit with dual output for both
analogue and digital components. The ferrite bead (FB1) should be rated for up
to 40 MHz, and may be left out altogether - the circuit will still work,
although EMI and noise will be higher than they should be. A copper ground plane
may be used to reduce EMI even further. ZD1 helps prevent damage to the
relatively expensive CS8414 if the regulator fails.

Figure 1 - Power Supply

As shown, ground returns from analogue and digital sections should be kept
separate for minimal noise at the output. Input voltage is limited to the specs
of the regulator, and increasing C1 allows AC to be used as well as DC. A low
noise regulator is recommended since the class-A output of the D/A converter has
a low PSRR - a standard 7805 will work but others may provide better
results.

D/A Converter

The D/A converter consists of two main components: the receiver, and the DAC.
The CS8414 is a S/PDIF "decoder" that features a built-in balanced RS422 S/PDIF
receiver (which is compatible with both COAX and TTL - for TTL, R1 must be
removed). It splits into stereo, recovers the sample rate, and connects directly
to many DACs using only 3 or 4 pins. It does not require a microcontroller
(unlike many others) which makes it ideally suited for this project. Read the
spec sheet from Crystal for more information - there is a lot of it. I have
shown probably the simplest way to use this IC, as it can be used as a starting
block for more complicated designs. Unused pins are not shown - refer to the
data sheet for complete pin information.

* Note that C1 (according to the datasheet) is not required, and may be a
short circuit to ground. Randy's circuit uses it and it works fine (and it also
eliminates ground loops), so it is up to the builder to decide whether to use it
or not. For a TTL input, R1 should be removed, and a short to ground used
instead of C1.

Figure 2 - Receiver and DAC

The actual DAC is an 8-pin Crystal CS4334. It features 128-512x over-sampling
which allows simple 1st-order lowpass filters to be used at the output, if
desired. Internal analogue filters and a switched-capacitor DAC are used, so
output filtering is not quite necessary, but it helps to reduce noise even
further. Audio up to 24-bit and 96 kHz is supported, making it very versatile.
The schematic shows how easily this DAC is connected to the CS8414 receiver. R11
and R12 are required if output relays are used, to prevent annoying "pops" when
the relays activate by keeping the output at ground potential. These resistors
can be increased if desired, although will not be as effective.

There are countless other DACs that could be used instead of the CS4334,
providing better S-N and dynamic range (up to 120dB!), although none is quite as
simple. I recommend the CS4334 for the first-time builder as the noise figure is
acceptable, especially when compared to that of most DIY amplifiers. The maximum
output is about 1.2v RMS.

Not shown (but absolutely essential) are the bypass caps from the analogue
and digital supplies to ground on both ICs. These should be 100nF ceramic types
for best high frequency performance. Make sure that the digital supply bypass
returns to digital ground, and the analogue bypass goes to the analogue ground!
The CS8414 makes this easy for you, since the digital bypass goes between pins 7
and 8, and the analogue bypass goes between pins 21 and 22. The CS4334 uses pins
6 and 7, and needs a 100nF ceramic bypass cap, with a parallel 1uF
electrolytic.

Resetting

It is not clear from reading the spec sheets, but the entire circuit must be
reset completely each time a digital signal becomes present. This is not as much
of an issue for use with computers and CD players where the S/PDIF signal is
always present, and the simple manual reset circuit may be used if desired. Note
that if this is used, it must be reset after power-up to begin decoding
audio.

Figure 3 - Reset Circuits

Using this project for digital cable converters, and other sources that turn
their S/PDIF output on and off (when changing channels, etc.), an automatic
"resetter" is necessary. This also may be required if it is not desirable to
manually press "reset" each time the unit is powered up. The circuit using the
555 timer will constantly reset the converter until a signal is present,
determined by VERF going low. When the S/PDIF signal is gone, or an error has
occurred VERF will go high, causing the 555 timer to start resetting again,
until VERF is low indicating presence of a digital signal.

If relays are not used at the output (as shown later), a slight ticking may
be heard when no digital signal is present. There are probably better ways to
reset the circuit, but this is the easiest way I have been able to do it. There
is a slight possibility that the CS4334 DAC may not be reset each time the
CS8414 does. This may cause the audio not to initialise every once and a while
(if channel surfing, etc.). If you remain patient when this happens, the audio
will usually start after about 5-10 seconds, preceded by a pop.

Auto Switching

Relays can easily be used to automatically switch the main output from an
analogue "pass-through" input to digital when a S/PDIF or similar signal is
present. This is useful when being used with some digital cable converters,
which may provide digital output only when watching certain "digital" channels.
I suspect some DVD players may also benefit from this feature, and it also
allows easy integration to just about any pre-amp or receiver, internally or
externally.

Figure 4 - Audio Switching Circuits

If 5v relays are used for switching, a small heat-sink should be used on the
regulator if supply voltage is greater than 15v. If Magnacraft "DIP141" (dip-14)
relays are used, total current draw from the entire project should not exceed
80mA when fully active and relays are engaged, making it ideally powered
directly from the +12v or +15v rail of a pre-amp power supply, or a small AC or
DC adapter.

The 74HC04 TTL inverter shown is used to invert the "VERF" output of the
CS8414 S/PDIF receiver, which is normally low when signal is present. The
inverter is also used to buffer channel status bits, although this option not
shown for simplicity. The relay circuit can be modified in countless ways to
eliminate the inverter if channel status LEDs are not desired, or better suit
individual design needs. The entire circuit can be left out completely if
desired.

Note - Although Randy included the pin numbers for
the relays, I left them off, since it is very likely that others may use a
different style. A DPDT (double pole, double throw) relay can also be used,
which means only one relay and diode. I also modified the driver ICs, using
two instead of one. CMOS ICs do not have a great current capacity, so using
two shares the LED and relay driver transistor current. Feel free to use a
BC549 transistor to drive the relay(s), provided the total current is below
100mA.

Notes

The entire circuit shown, with relays, fits easily on a 75 x 100 mm (3 x
4") board. Use of 1/8 W (or surface mount) resistors throughout is recommended
to save space, and a double sided board helps. The IC's are available from
distributors listed on http://www.crystal.com/. The ICs have
surface mount "SOIC" footprints, so careful soldering is a must. It is
possible to solder them easily with a 15 Watt iron and 0.8mm (0.032")
multicore solder. A small piece of tape may be used to hold them in place when
soldering the first couple of pins.

The Crystal website does not appear to support Netscape - Microsloth
products only - mutter, mutter :-(

This project is not compatible with Dolby Digital 2.0 or DTS formats,
which are both forms of compressed audio. It will work with any PCM stream
from CD players, DVD players and set-top boxes (with DD2.0 or DTS disabled),
and any computer sound board with digital output. If no digital output is
available on your sound card, a Sound Blaster Live Value for US$25 will do the
trick - and it has some really cool DSP as well

To add support for Dolby Digital 2.0 and compressed MPEG formats, only the
CS4334 DAC needs to be changed. A multi-standard DAC can be substituted,
however they usually require a microcontroller and are much more complicated
to use.

If the DAC is to be coupled with a DIY pre-amp or EQ, especially ones
utilising high speed op-amps (such as Project 28, etc.), great care must be taken to bypass the
feedback loop with a proper value capacitor. Without it, frequency response is
so great that artefacts of digital switching may become audible when the
shelving treble control is increased. For Project 28, a 1nF capacitor across
the 2k7 feedback resistor (Figure 1) will limit the high end to about 60 kHz
and rids the extra noise completely. This may also be a problem with some
commercial preamplifiers with tone controls, as the the treble control is
almost always shelving. Commercial parametric EQ does not normally use a
shelving high frequency control.

ESP is unable to provide technical support for this project, since the
project was contributed.

Photo of DAC
Unit

The photo above shows the author's unit, with a couple of pointers to things
of interest. The entire board is about 75 x 100 millimetres (3" x 4" for the
metrically challenged :-)

Appendix

There will be occasions where the user may want to convert COAX to TTL signal
levels. The circuit in Figure 5 will accomplish this conversion, and can use the
unused gates in U4 (a hex inverter). This is as simple as it can get, and the
circuit will not work correctly (if at all) if the levels are too low. Impedance
matching is quite accurate, and it will load the transmitter to almost exactly
+/-0.25V.

Figure 5 - COAX to TTL Converter

Just for completeness, here is a circuit for TTL to COAX conversion. Again,
standard 74HC04 inverter gates are used (the same ones as Figure 5, in fact).
The resistors at the output reduce the level to the required levels and provide
the correct impedance of 75 ohms.

Figure 6 - TTL to COAX converter

The actual output impedance is 72 ohms, and if you really wanted to, you
could add a 3.3 ohm resistor at the output. The error is too small to worry
about though. The resistors are standard E24 values, so should not be too
painful to acquire.

Schematics redrawn by ESP from Randy's original
material. Original text by Randy McAnally, introduction, additional schematics
and text material by ESP.

Note that the TTL to COAX and vice versa
schematics are based on material I located on the Web (and reproduced here by
permission), from the web site http://www.andrewkilpatrick.org/projects/spdif/ and the
appropriate credit for the idea is due to Andrew Kilpatrick (the author of that
page). This is fairly standard CMOS circuitry, and similar techniques have been
used by myself and others for many years (although for completely different
purposes).

The schematics shown in Figures 5 and 6 have been simulated to
verify correct operation, but were not tested in "real life". They should work
perfectly, based on the simulator results.

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Copyright Notice. This
article, including but not limited to all text and diagrams, is the
intellectual property of Randy McAnally and Rod Elliott, and is Copyright
© 2002. Reproduction or re-publication by any means whatsoever, whether
electronic, mechanical or electro- mechanical, is strictly prohibited
under International Copyright laws. The author ( Randy McAnally) and
editor ( Rod Elliott) grant the reader the right to use this information
for personal use only, and further allows that one (1) copy may be made
for reference while constructing the project. Commercial use is prohibited
without express written authorisation from  Randy McAnally and Rod
Elliott.

Page Created and Copyright © Randy McAnally / Rod Elliott 12
Jan 2002