The zap time is the total duration from the time viewer presses the channel change button, to the point the picture of the new channel is displayed, along with corresponding audio. These kind of delays exist in all television systems, but they are greater in digital television and systems that use the Internet like IPTV. Human interaction with the system is completely ignored in these measurements, so zap time is not the same as channel surfing.
Zap time can be very disturbing for the viewer since it has to wait too much time when decides to switch the channel. For this reason, zap time is an IPTV feature that has to be reduced in an IPTV system.[1]
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The delays in the TV channel change can be caused by different factors. They can be classified according to the systems that cause them. Consequently there are network factors, MPEG acquisition factors and Set Top Box Buffering/Decode.[2]
Actually, these are only a small component of delay in the final time. Usually they take about 50 - 200 ms of the overall zap time. Network quality of service(QoS) can reduce these time ensuring minimal jitter, latency and packet drop.
The factors that has been explained in the last section don't affect in the same way to the overall zap time. So in the table below there are an example of the zap time in IPTV DSL:
Channel Change Latency Factor | Device/Location | Typical Latency | Cumulative Latency | |
---|---|---|---|---|
1 | Send IGMP Leave for channel X | STB | < 10 ms | |
2 | Send IGMP Join for channel Y | STB | < 10 ms | |
3 | DSLAM gets Leave for channel X | DSLAM/Network | < 10 ms | |
4 | DSLAM gets Join for channel Y | DSLAM/Network | < 10 ms | ~ 20 - 40 ms |
5 | DSLAM stops channel X, and sends Channel Y | DSLAM/Network | ~ 30 – 50 ms | ~ 50 – 90 ms |
6 | DSL Latency (FEC/Interleave) | DSLAM/Network | ~ 10 ms | ~ 60 - 100 ms |
7 | Core/Agg Network Latency | Router/Network | ~ 20 – 60ms | ~ 80 – 160ms |
8 | De-jitter buffer | STB | ~ 300 ms | ~ 380 - 460 ms |
9 | Wait for PAT/PMT | STB MPEG buffer | ~ 125 ms | ~ 500 - 580 ms |
10 | Wait for ECM/CA | STB MPEG buffer | ~ 125 ms | ~ 620 - 700 ms |
11 | Wait for I-frame | STB MPEG buffer | ~ 250 ms to 2s | ~ 870 ms – 2.7s |
12 | MPEG buffer | STB MPEG buffer | ~ 1s to 2s | ~ 1.8s – 4.7s |
13 | Decode | STB | ~ 50ms | ~ 1.9s – 4.8s |
In this section some typical values of zap time are shown. Actually, in IPTV television these delays are greater than in other technologies: