Transparent Mode

In certain applications it is desirable to receive digi-

tal audio data with the STA120 and retransmit it

with the STA020. In this case, channel status, user

and validity information must pass through unal-

tered. For studio environments, AES recommends

that signal timing synchronization be maintained

throughout the studio. Frame synchronization of

digital audio signals input to and output from a

piece of equipment must be within +/-5%.

The transparent mode of the STA020 is selected

by setting TRNPT, pin 24, high. In this mode, the

CBL pin becomes an input, allowing direct con-

nection of the outputs of the STA120 to the inputs

of the STA020 as shown in Figure 18. The trans-

mitter and receiver are synchronized by the

FSYNC signal. CBL specifies the start of a new

channel status block boundry, allowing the trans-

mit block structure to be slaved to the block struc-

ture of the receiver.

In the transparent mode, C, U and V are now

transmitted with the current audio sample as

shown in Figure 5 (TRNPT high) and the dedi-

cated channel status pins are ignored.

When FSYNC is a word clock (Format 2), CBL is

sampled when left C, U, V are sampled. When

FSYNC is Left/Right, CBL is sampled when left

C, U, V are sampled. The channel status block

boundry is reset when CBL transitions from low to

high (based on two successive samples of CBL).

MCK for the STA020 is normally expected to be

128 times the sample frequency, in the trasparent

mode MCK must be 256 Fs.

Professional Mode

Setting PRO low places the STA020 in profes-

sional mode as shown in Figure 6. In professional

mode, channel status bit 0 is transmitted as a one

and bits 1, 2, 3, 4, 6, 7 and 9 can be controlled

via dedicated pins. The pins are actually the in-

verse of the identified bit.

For example, tying the C1 pin low places a one in

channel status bit 1. As shown in the application

Note, Overview of AES/EBU Digital Audio Inter-

face Data Structures, C1 indicates audio/non-

audio; C6 and C7 determine the sample fre-

quency and C9 allows the encoded channel

mode to be stereophonic. EM1 and EM0 deter-

mine emphasis and encode C2, C3, C4 as shown

in Table 2. The dedicated channel status pins are

read at the appropriate time and are logically

OR’ed with data input on the channel status port, C.

In Transparent Mode, these dedicated channel

status pins are ignored and channel status bits

are input at the C pin.

Consumer Mode

Setting PRO high places the STA020 in consumer

mode which redefines the pins as shown in Figure

7. In consumer mode, channel status bit 0 is trans-

mitted as a zero and channel status bits 2, 3, 8, 9,

15, 24 and 25 are controlled via dedicatedpins.

The pins are actually the inverse of the bit so if

pin C2 is tied high, channel status bit 2 will be

transmitted as a zero. Also, FC0 and FC1 are en-

coded versions of channel status bits 24 and 25,

which define the sample frequency.

When FC0 and FC1 are both high, the part is

placed in a CD submode which activates the CD

subcode port. This submode is described in detail

in the next section. Table 3 describes the encod-

ing of C24 and C25 through the FC1 and FC0

pins. According to AES/EBU standards, C2 is

copy prohibit/permit. C3 specifies pre-emphasis,

C8 and C9 define the category code and C15

identifies the generation status of the transmitted

material (i.e. first generation, second generation).

Table 2. Emphasis Encoding

EM1

EM0

C2

C3

C4

001

11

011

10

101

00

110

00

Table 3. Sample Frequency Encoding

FC1

FC0

C24

C25

Comments

0000

44.1kHz

0101

48kHz

1011

32kHz

1100

44.1kHz, CD Mode

MCK

CBL

C

U

V

FSYNC

DATA

PROCESSING

RXP

RXN

SCK

STA120

STA020

TXN

TXP

TRNPT

V+

D97AU605

SDATA

Figure 5. Transparent Mode Interface.

STA020

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