The zap time is the total duration of time from which the viewer changes the channel using a remote control to the point that the picture of the new channel is displayed. This includes the corresponding audio. These delays exist in all television systems, but they are more pronounced in digital television and systems that use the internet such as IPTV. Human interaction with the system is completely ignored in these measurements, so zap time is not the same as channel surfing.
Contents
- Factors
- Network Factors
- MPEG Acquisition Factors
- Set Top Box BufferingDecode Factors
- Zap Time Examples
- References
Zap time can be very disturbing for some viewers and for this reason it is considered an issue that must be addressed in IPTV systems.
Factors
The delays when changing the channel can be caused by several different factors. These factors can be classified according to the systems that cause them. Consequently, there are network factors, MPEG acquisition factors, and set top box buffering/decode factors.
Network Factors
- STB – IGMP Leave channel X, Join Y
- DSLAM – Stop X, Start Y
- DSL FEC/Interleave
- IGMP features used (version, fast leave, snooping, etc.)
- Availability of the channel (channel replication point)
- Multicast routing mechanisms used
- Availability of the channel (channel replication point)
Network factors tend to make up only a small portion of the overall delay, between 50 and 200ms of the overall zap time. Network quality of service (QoS) can reduce these time to minimize jitter, latency, and packet drop.
MPEG Acquisition Factors
- Wait for and parse PAT (Program Association Table)
- Wait for and parse PMT (Program Map Table)
- I-frame (MPEG 2) or IDR frame (H.264)
- One Index frame per group of pictures (GOP) – 12 to 30 (IBP) frames
- Typical frequency of I-frame – 500ms.
- Long GOP structure (2–4 seconds) saves bandwidth, but can cause significant channel change latency
Set Top Box Buffering/Decode Factors
Zap Time Examples
The various factors that affect zap time do not do so in the same way. The table below is an example of zap time in IPTV DSL:
Zap time delays are greater in IPTV television than in other technologies. For example: