PNX1300/01/02/11 Data Book
Philips Semiconductors
10-2
PRELIMINARY SPECIFICATION
transmitted LSB first or MSB first in 32-clock cycle inter-
vals of the SPDO clock, a programmable clock generat-
ed by the SPDO Direct Digital Synthesizer.
10.4
IEC-958 SERIAL FORMAT
Figure 10-2
shows the serial format layout of a IEC-958
block. A block starts with a special
‘
B
’
pre-amble, and
consists of 192 frames. The sample-rate of all embedded
audio data is equal to the frame rate. Each frame con-
sists of 2 sub-frames. Sub-frame 1 always starts with a
‘
M
’
pre-amble, except for sub-frame 1 in frame 0, which
starts with a
‘
B
’
. Sub-frame 2 always starts with a
‘
W
’
pre-
amble.
When IEC-958 data carries 2-channel PCM data, one
audio sample is transmitted in each sub-frame,
‘
left
’
in
sub-frame 1 and
‘
right
’
in sub-frame 2. Each sample can
be 16 or 24 bits in length, where the MSB is always
aligned with bit slot 28 of the sub-frame. In case of more
than 20 bits/sample, the Aux field is used for the 4 LSBs.
When IEC-958 data carries non-PCM audio, such as 1 or
more streams of Dolby AC-3 encoded data and/or MPEG
audio, each sub-frame carries 16-bit data. The data of
successive frames adds up to a payload data-stream
which carries its own burst-data.This is described in [2].
Programmers should refer to the IEC-958 documents [1]
and Project 1937 document [2] for a precise description
of the required values in each field for different types of
consumer equipment. A complete discussion of this is-
sue is outside the scope of this document.
The SPDO block hardware only concerns itself with gen-
erating B, W and M preambles as well as generating the
P (parity) bit. All other bits in the sub-frame are complete-
ly determined by software and copied verbatim from
memory to output, subject only to bit-cell coding.
The programmer must construct valid IEC-958 blocks by
constructing the right sequence of 32-bit words as de-
scribed in
Section 10.7,
“
IEC-958 Memory Data Format.
”
10.5
IEC-958 BIT CELL AND PRE-AMBLE
Each data bit in IEC-958 is transmitted using bi-phase
mark encoding. In bi-phase mark encoding, each data bit
is transmitted as a cell consisting of two consecutive bi-
nary states. The first state of a cell is always inverted
from the second state of the previous cell. The second
state of a cell is identical to the first state if the data bit
value is a
“
0
”
, and inverted if the data bit value is a
“
1
”
.
Pre-ambles are coded as bi-phase mark violations,
where the first state of a cell is not the inverse of the last
state of the previous cell.
The duration of each state in a cell is called a UI (Unit In-
terval), so that each cell is 2 UI
’
s long. In SPDO, the
length of a UI is 1 SPDO clock cycle as determined by
Figure 10-2. Serial format of a IEC958 block
sub-frame 1
M
sub-frame 2
W
sub-frame 1
B
sub-frame 2
W
sub-frame 1
M
sub-frame 2
W
Start of block (indicated by unique B pre-amble)
sub-frame
sub-frame
frame 0
frame 1
sub-fram
M
frame 191
0
B, W or M
pre-amble
31
28
24
20
16
12
8
L
S
B
4
Sample data
M
S
B
Aux.
V U C P
Validity flag
User data
Channel status
Parity bit
sub-frame (2 channel PCM)
0
B, W or M
pre-amble
31
28
24
20
16
12
L
S
B
8
4
16-bit data
M
S
B
V U C P
Validity flag
User data
Channel status
Parity bit
sub-frame (non-PCM audio)
unused (0)