In Broadcast - March 2015 - 72

72

www.inbroadcast.com | Vol: 5 - Issue 3 | March 2015

InTech

SCTE 104/35 Frame Accurate
Ad Insertion Control
SCTE: "It's control signals Jim but not as we know them," says Microvideo...

Diagram 1:
Typical regional playout setup

T

here is a lot of mystique
surrounding terms such as SCTE
104 and 35, in essence they are simply
signals used to send frame accurate
switching information.
There is however more than just
a simple switching signal. SCTE 35
signals can give speciﬁc information,
which can be used to tailor content to
regional viewers.
SCTE 104 is a signal inserted into
an SDI programme stream which,
when encoded into an ASI programme
stream forms the SCTE 35 signal.
Think of the encoder as a SCTE 104 to
35 translator. SCTE 35 is, in essence,
a series of commands which signal
events, for instance advertising breaks
in a transport stream.
A typical broadcast setup might be
as in Diagram 1. The central playout
inserts SCTE 104 commands, which
are encoded and used by the regions
to playout a common programme but
with regional advertising inserted by
each local centre.
SCTE 35 signals are not dissimilar
to the signals used in older broadcast
systems. Analogue broadcasting used
an audio tone (DTMF) to signal the
ad time slot, and digital systems use
a part of the wide screen signalling
signal in SD, or a Teletext data
packet (packet 31) for SD or HD. The
difference is, with SCTE signals a lot
more information can be transmitted
and managed.

SCTE-35 packets are multiplexed
with video and audio packets in the
transport stream. There are a number
of commands within SCTE 35 (see
Table 1).
The Microvideo HDB 300 inserts a
number of control signals including
SCTE 104. All the key parameters
can be set to ensure frame accurate
control signals are inserted into the
transport stream (see Diagram 2).
Unique splice commands can be set
with predetermined pre-roll times
and a predetermined duration before
an auto return command is sent.
Alternatively a speciﬁc 'switch out' set
of parameters can be set. Complete
setting can be stored and recalled
through automation via GPI.
Set Up Recommendations
There are a few recommendations in
setting up a SCTE 104 signal:
AS Index uniquely identiﬁes the
source of the message - i.e. it's the
Automation System index that sent the
SCTE 104 message to start with. If not
required it can be left as 0 otherwise a
value of 1-255 should be used.
Minimum pre-roll time is 4000
milliseconds for a splice_request,
although you can use 0 if you do a
"spliceStart_immediate" or "spliceEnd_
immediate" message.
The speciﬁcation also suggests that
a splice end should always use a preroll even though it has been common

practice
in
the
industry for a return
command to be sent
with 0 pre-roll.
Interestingly, the
speciﬁcation
also
adds that there is an
important distinction
between
a
preroll of zero and a
splice return with
immediate bit set.
A spliceEnd_normal
indicates that the
insertion
should
have
ended
at
approximately the
time indicated, with
or without pre-roll.
It
should
be
noted that common
industry practice has been to ignore
a normal return message in favour of
ﬁnishing the playout of the content.
Any receiving device can potentially
ignore the return message, and just
ﬁnish the content, but may choose to
return early if it thinks the content will

overrun too much.
So, SCTE 104 and SCTE 35 are more
than just an embedded GPI signal,
they carry considerable data that is
used to manage remote playout, the
system is deliberately ﬂexible to allow
for differing workﬂow, but this can
add some confusion when setting up a
system for the ﬁrst time.
The simplest way to view it is
as a switch command with a built
in warning ahead of the actual
changeover, which gives time for 'preroll' or (in simple terms) disks to start
up.
The easiest way to set up a
remote playout is to establish the
requirements of the remote equipment
and match those, rather than trying
to dictate a working practice from the
central playout. The Microvideo HDB
300 is versatile enough to act as a
bridge between the two systems.
Microvideo solutions are used by
many of the world's top broadcast
companies to manage, control, brand
and protect both in-house and remote
playout.

Table 1: Commands Within SCTE 35
Splice Insert

Signals a Splice Event.

Splice Event

Signals the time and offset (pre-roll).
Splice events can be immediate although this is not
recommended due to problems caused by
not switching on Key Frames.

Splice Schedule

Used to schedule far in advance of the actual
event - not frame accurate but allows for scheduling
consistent time slots for advertisement breaks.

Splice event repetition Splice event commands are often repeated
and cancellation
to compensate for dropped packets within
the transports stream.
Segmentation

SCTE 35 can be used for varying content for instance,
for different content to different regions

Diagram 2: Two ad breaks showing auto and manual return sequences
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