ANSI SCTE 67:2017 pdf free download – Recommended Practice for Digital Program Insertion for Cable
6.2. System Delays
Most devices that operate on the video and audio components will delay the transport stream from the input to the output of the device. Encoders typically have delays of 0.5 to two or more seconds depending on if a low delay mode or multi-pass encoding mode is used. Splicers/statistical multiplexors also have delays in the same range. The satellite transmission of the transport stream also incurs some delay as well. Converting a transport stream to an adaptive bit-rate format such as [DASH] will incur significant delays
both on the preparation and playback/buffering side. While most providers prefer that there is as small amount of delay as possible, small amounts of additional delay are usually only a concern on live programs that have call-in or simulcast issues.
6.2.1. SCTE 104 Insertion Delays
Being able to insert messages with at least 4 seconds of pre-roll to allow the encoders to handle GOP structures properly, especially in AVC encoding, is a good practice. There are some events that are manually triggered and can lead to an immediate command being inserted. This will likely result in an inaccurate splice with MPEG-2 video or picture artifacts if AVC video is used.
One possible solution is to add a few seconds of HD-SDI delay before the SCTE 224 VANC inserter and craft the Triggers pre-roll value to account for that delay. This would have the effect of adding the additional pre-roll to the message but to the distributor it would be as if the command happened as an immediate command.
6.2.2. Encoder Delays
Encoder delays should be handled by the encoder properly resulting in frame accurate splicing. An encoder can pass the Trigger through immediately as a Cue Message in to the output essentially adding the encode delay to the pre-roll. In all cases the encoder needs to consider the encode delay when converting the SCTE 224 frame to the SCTE 35 PTS time.
6.2.3. Transmission Delays
Transmission delays are a factor when generating Cue Messages, although the variability of receipt and
processing within receive sites is a major reason why Cue Messages use the in-stream PTS time.
6.2.4. Splicer/Multiplexor Delays
Section 6.1 describes the suggested minimum pre-roll of 4 seconds for splice insert commands. This allows for one second of message processing before the 3 second minimum to send a SCTE 30 Splice_Request Message. In order to ensure proper splice accuracy and quality these minimums need to be adhered to.
Some splicers and servers using MPEG-2 video will attempt to complete insertions even if the timing is below the minimums as this does represent lost revenue. Some splicers use their delay and/or have faster processing capabilities to allow for lower minimums and theoretically could have slightly negative splice times due to delays.
See Section 6 of the document for stream requirements for splicing MPEG-2 video. Stream requirements for splicing AVC and other video codecs are described in SCTE 172. While the details of Out Point requirements differ, both video codecs require that In Points be intra-coded pictures with no stream dependencies on pictures prior to the In Point (a closed GOP in MPEG-2, an IDR in AVC). In time shifted environments (for example VOD), where ads may be inserted into content, the Out and In Points in the entertainment content may be coincident and, if coincident, needs to meet the requirements of both an In Point and an Out Point.