e00a028.pdf

AUD IO

S/PDIF Output

For OSCAR and other digital audio-equipment

Design by T. Giesberts

The standard MP3 CD player is not provided with an S/PDIF-output that

would have enabled the transmission of digital audio signals to other

equipment. With the aid of this circuit, designed around an IC from

Crystal, an optical and coaxial S/PDIF signal is generated from a kind of

I 2 S signal in the OSCAR MP3-player.

Althoiugh the OSCAR MP3 player is

fitted with analogue audio outputs

that may be connected to other hi-ﬁ

equipment or the line input of a

sound card, it would be easier and

less lossy to transfer the signal in

digital form to another piece of

equipment with digital input, espe-

cially when recording tracks using,

for instance, a DAT- or MD-recorder.

As standard, OSCAR does not

possess an output that provides a

digital audio signal in usable form.

However, on the main circuit board

is a connection that provides digital

audio data in a ‘raw’ format. This

signal must first be correctly

encoded with additional information

bits and then transmitted with the

correct bi-phase modulation and

amplitude.

The CS8402A from Crystal is an

IC that can do all these tasks by

itself, hence the name ‘digital audio

interface transmitter’. Around this IC

is some circuitry that translates the

‘raw’ signal from OSCAR to S/PDIF.

In the first instance, this circuit

was designed for use with OSCAR,

however it is universally applicable

because almost all the hardware

modes of the CS8402A are available

to be selected. Two DIP switches

and two jumpers enable the circuit

to support various formats.

Elektor Electronics

10/2000

AUD IO

The CS8402 has already appeared

in various projects (such as the Sam-

pling Rate Converter from October

’96, 20-bit A/D Converter from

December ’96 and the S/PDIF Test

Generator from July/August ’99), so

we will limit ourselves to the most

important aspects. In any case, to

enable a better insight into the over-

all functionality of the IC, Figure 1

shows once again the block diagram

of the CS8402A. Additional informa-

tion can be found on the Cirrus Logic

web site.

M 2 M 1 M 0

23 22 21

SDATA

SCK

FSYNC

Serial

Port

Logic

Audio

Aux

Biphase

Mark

Encoder

C Bits

Mux

Line

Driver

TXP

TXN

Registers

CRC

U Bits

Validity

Timing

RST

Preamble

Parity

The circuit

3441342

FC0 FC1 C2 C3 C8 C9 C15

(L)PRO C1 TRNPT C6 C7 EM1 EM0 C9

15 5

CBL MCK

* professional mode only

000131-12

In Figure 2 you can see the entire

schematic for the converter circuit.

As previously mentioned, the

CS8402A takes care of the complete

conversion from raw audio data to

S/PDIF. Besides the TOSLINK module

for the optical output, the only other

active elements are two HCT logic

ICs (dual D ﬂip-ﬂops).

The master clock signal MCK for

the CS8402A arrives at pin 3 of K1 (or

Figure 1. Internal architecture of the CS8402A.

on the PCB pin of the same name). It

is then routed via header JP1 to pin 5

of IC1. To make the circuit as univer-

sally applicable as possible, two

divide-by-two circuits (IC2a and

IC2b) are connected to pin 3 of K1.

Normally (when transparent mode is

not selected) the master clock MCK amounts

to 128?Fs. Because of the presence of the

divide-by-twos MCK may also take the values

of 256?Fs or 512?Fs. This is selectable with

Jumper 1 (JP1). The divisors /4, /2 and /1 can

be seen next to JP1 on the PCB overlay. The

latter connects the MCK input of the CS8402A

IC4

4 Ω 7

TOTX173

8k2

432

IC2b

IC2a

100n

OPTO

47µH

5786

4321

75 Ω

100n

47µ

25V

TR1

JP1

MCK

COAX

÷ 4

IC1

MCK

÷ 2

÷ 1

MCK

TXP

TXN

SDATA

FSYNC

47n

SCK

PRO

75 Ω

FSYNC

C1/FC0

TRNPT/FC1

COAX

C6/C2

C7/C3

20 : 2 : 2

IC3a

CBL/SBC

EM1/C8

EM0/C9

C9/C15

47n

C/SBF

RST

SDATA

JP2

23456789

CS8402A

SCK

IC2

IC3

IC3b

SCK

22µ

40V

100n

4x 10k

8x 10k

IC2, IC3 = 74HCT74

000131 - 11

Figure 2. Complete circuit diagram of the S/PDIF-converter.

10/2000

Elektor Electronics

(H)PRO

AUD IO

COMPONENTS LIST

000131-1

Resistors:

R1 = 8 x 10k Ω resistor array

R2 = 10k Ω

R3,R5 = 270 Ω

R4,R6 = 75 Ω

R7 = 8k Ω 2

R8 = 4 Ω 7

R9 = 4 x 10k Ω resistor array

TR1

JP2

FSYNC

+5V

SCK

JP1

Capacitors:

C1 = 22µF 40V radial

C2,C3 = 47nF

C4,C6,C7,C8 = 100nF ceramic

C5 = 47µF 25V radial

IC3

IC2

MCK

Inductors:

L1 = 47 µH choke

Semiconductors:

IC1 = CS8402A-CP (Crystal)

IC2,IC3 = 74HCT74

IC4 = TOTX173 (Toshiba)

Miscellaneous:

JP1 = 3-way pinheader + jumper

JP2 = 2 x 3-way pinheader + jumper

K1 = 10-way boxheader

K2,K3 = cinch socket, PCB mount (e.g., T-

709G from Monacor/Monarch)

S1 = 8-way DIP switch

S2 = 4-way DIP switch

TR1 = ferrite ring core, Philips type

TN13/7,5/5-3E25, primary 20 turns,

secondary 2 x 2 turns, lacquered wire 0.5

mm dia.

PCB, order code 000131-1 (see Readers

Services pages)

Figure 3. The single-sided PCB designed for the converter. Be sure to ﬁt the three

wire links!

directly to K1 and associated PCB pin.

Depending on the mode, data may be

clocked on the rising or the falling edge of

serial clock SCK. Consequently, an additional

D ﬂip-ﬂop (IC3b) is provided to allow the pos-

sibility of inverting SCK. Jumper J2 selects

the polarity of SCK. To ensure the correct rela-

tionship between data and SCK, the data is

also clocked through a D ﬂip-ﬂop (IC3a). The

divide-by-two MCK clock signal (output Q of

IC2b) is used for this.

The decision to use HCT logic was a con-

scious one because it is possible that the

input signals originate from 3-V logic (such as

the MP3 CD-Player). The CS8402A interprets

signals greater than 2 V as a logic high level.

This arrangement avoids the need for a level

shifter. The L/R clock signal FSYNC (frame

sync) is connected directly, without level

shifting, from K1 to pin 7 of IC1.

SDATA, SCK and FSYNC together form the

audio serial port. Mode select pins M0, M1

and M2 deﬁne the format for this port. Seven

different formats may be selected

this way and the addition of Jumper

2 allows several more (refer to the

sidebar for details). A quad DIP

switch S2 and a quad resistor array

R9 are used to select the mode of

these 3 pins. Although the fourth

switch and resistor remain unused,

a quad package is used simply

because it is easier to obtain.

The CS8402A is provided with

eight pins that deﬁne the operation

of the transmitter. These are selected

with the aid of S1/R1. Every pin has

two functions depending on whether

professional- or consumer mode is

selected (pin 2, S1-8). Table 1 lists

the preferred values for use with

OSCAR. The functions shown for S1

apply to consumer mode.

The output of the CS8402A is

symmetrical. The 10 V pp drive signal

for Tr1 for the coaxial outputs per-

. The large trans-

former ratio provides a lower output

impedance and greater bandwidth

(better coupling because the primary

winding covers a large part of the

toroidal core).

The application of a transformer

for the coaxial outputs is motivated

as follows: full electrical isolation

between various devices will, to a

large extend, avoid ground loops and

other possible sources of trouble.

A second advantage of the sym-

metrical drive signal for the trans-

former is that by using a toroidal

Ω

Elektor Electronics

10/2000

mits a relatively large primary/sec-

ondary turns ratio (10:1). A coaxial

cable (transmission line) is normally

terminated at both ends into its

characteristic impedance. Reflec-

tions are avoided this way. An out-

put signal of 1 V pp is required on the

secondary side in order to deliver

0.5 V pp into 75

AUD IO

core, the primary winding can be

split easily into two equal halves.

The terminals for the primary and

secondary windings are now on

opposite sides of the core and the

connection between the two halves

of the primary winding acts as a vir-

tual ground point. This suppresses

crosstalk (and improves electrical

isolation at RF).

Resistors R3 and R5 damp the

outputs of the secondary windings

and ensure that the transformers

present an resistive load to the

CS8402A at all times. R4 and R5

mainly deﬁne the output impedance

(75 Ω). Capacitors C2 and C3 provide

RF grounding for the screen of the

coax cables.

The circuitry around the

TOSLINK module IC4 is of a con-

ventional nature and consists

mainly of power supply decoupling.

R7 is required for the internal

adjustment of the module.

winding consists of 20 turns and

the two secondary windings of two

turns each (refer to the drawing of

Figure 4 ). A 0.5-metre length of

wire should be adequate.

The connection between the cir-

cuit and the audio data source is

made with a ribbon cable plugged

into K1 (for connection to OSCAR) or

by using the PCB pins next to K1 and

individual wires for other applica-

tions. With OSCAR, the ribbon cable

connects via a 10-way insulation dis-

placement connector (IDC) to pin-

header JP6 (AUX) on the main circuit

board. Pay careful attention to the

correct orientation.

In the case of the MP3 player, the

supply voltage arrives via the ribbon

cable connected to K1. In the event

that the connections are made using

individual wires connected to the

PCB pins next to K1, a separate 5 V

power supply may be connected to

the power supply pins next to IC3.

A few remarks before finishing.

Unfortunately there was no space on

the PCB next to S2 to provide a quick

reference to the function of the indi-

vidual DIP switches. S2-1 is the LSB

M0, S2-2 is M1 and S2-3 is M2.

Between R1 and C1 you may find

enough room on the board to stick a

small label with the default settings

for the functions of S1.

Main settings

on S1 & S2.

Audio Port modes

S2-3

S2-2 S2-1

Format

0 0

FSYNC & SCK output

0 0

Left/right, 16-24 bits

0 1

Word sync, 16-24 bits

0 1

Reserved

1 0

Left/right, I 2 S compatible

1 0

LSB justiﬁed, 16 bits

1 1

LSB justiﬁed, 18 bits

1 1

MSB last, 16-24 bits

Sample rates in professional mode

S1-8

S1-4 S1-5

PRO

0 0

not deﬁned

0 0

48 kHz

0 1

44.1 kHz

0 1

32 kHz

Sample rates in consumer-mode

S1-8

S1-7 S1-6

PRO

FC1

FC0

1 0

44.1 kHz

Practical matters

1 0

48 kHz

1 1

32 kHz

A small circuit board was developed

for the S/PDIF converter. It is shown

in Figure 3 . The construction will

require little effort. Winding trans-

former Tr1 is probably the most diffi-

cult part.

The output transformer is wound

with 0.5 mm diameter enamelled

copper wire on a 13?5.5 mm

toroidal core from Philips

(TN13/7.5/5-3E25). The primary

1 1

44.1 kHz, CD-mode

Category code

S1-8

S1-2 S1-3

PRO

1 0

general format

1 0

PCM encoder/decoder

11 0

11 1

DAT

(000131)

Internet address:

Cirrus Logic: http://www.cirrus.com

(‘1’= switch closed, ‘0’=switch open)

Table 1. Default

settings for OSCAR.

Pos.

Signal description

-1 off

C15\

generation status

-2 off

C8\

category code

-3 off

C9\

category code

-4 off

C2\

copy prohibit/permit

-5 on

C3\

pre-emphasis

-6 off

FC0

sample frequency

-7 off

FC1

sample frequency

-8 on

PRO\ professional/consumer

mode

S2-1

S2-2

off

S2-3

off

S2-4

n.c.

JP1: /4

Figure 4. Showing how the transformer is wound.

JP2: SCK

10/2000

Elektor Electronics

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