The zap time is the total duration of time from which the viewer changes the channel using a

remote control In electronics, a remote control (also known as a remote or clicker) is an electronic device used to operate another device from a distance, usually wirelessly. In consumer electronics, a remote control can be used to operate devices such as ...

to the point that the picture of the new channel is displayed. This includes the corresponding audio. These delays exist in all

television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...

systems, but they are more pronounced in

digital television Digital television (DTV) is the transmission of television signals using digital encoding, in contrast to the earlier analog television technology which used analog signals. At the time of its development it was considered an innovative advanc ...

and systems that use the

internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

such as

IPTV Internet Protocol television (IPTV) is the delivery of television content over Internet Protocol (IP) networks. This is in contrast to delivery through traditional terrestrial, satellite, and cable television formats. Unlike downloaded media, ...

. Human interaction with the system is completely ignored in these measurements, so zap time is not the same as

channel surfing Channel surfing (also known as channel hopping or zapping) is the practice of quickly scanning through different television channels or radio frequencies to find something interesting to watch or listen to. Modern viewers, who may have cable or ...

. 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.IPTV over DSL systems.
''IPTV testing over DSL''
"

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 Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

factors, MPEG acquisition factors, and set top box buffering/decode factors.IPTV challenges and metrics.
''IPTV Challenges''
" IPTV
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Network Factors

* Access network latency (processing/propagation delays): #

STB State Security ( cs, Státní bezpečnost, sk, Štátna bezpečnosť) or StB / ŠtB, was the secret police force in communist Czechoslovakia from 1945 to its dissolution in 1990. Serving as an intelligence and counter-intelligence agency, it de ...

–

IGMP The Internet Group Management Protocol (IGMP) is a communications protocol used by hosts and adjacent routers on IPv4 networks to establish multicast group memberships. IGMP is an integral part of IP multicast and allows the network to direct mu ...

Leave channel X, Join Y #

DSLAM A digital subscriber line access multiplexer (DSLAM, often pronounced ''DEE-slam'') is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital co ...

– Stop X, Start Y #DSL FEC/Interleave #

features used (version, fast leave, snooping, etc.) #Availability of the channel (channel replication point) * Core/aggregation network latency: #Multicast

routing Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone netw ...

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 In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significa ...

, latency, and packet drop.

MPEG Acquisition Factors

*Analyze data to locate MPEG

program-specific information Program-specific information (PSI) is metadata about a program (channel) and part of an MPEG transport stream. The PSI data as defined by ISO/IEC 13818-1 (MPEG-2 Part 1: Systems) includes four tables: * PAT (Program Association Table) * CAT (Con ...

#Wait for and parse PAT (Program Association Table) #Wait for and parse PMT (Program Map Table) *Obtain

conditional access Conditional access (CA) is a term commonly used in relation to software and to digital television systems. Conditional access is that ‘just-in-time’ evaluation to ensure the person who is seeking access to content is authorized to access the c ...

keys (ECMs) to decrypt channel: wait for ECMs – part of PMT – 100ms to 500ms *Obtain MPEG key frame # 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

*MPEG Buffer: Encoder buffer fullness model (typical latency – 750ms to 2s). Wait until the buffer is full. * Decode/Display delay (typically about 50 ms)

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: *Analog ( Cable) ~ 0.01 - 1s *Analog ( off-air) ~ 0.01 – 3s *MPEG2 over QAM ~ 1.2 – 3s *MPEG2 over

QPSK Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...

~ 2 – 4s *MPEG2 over IP Multicast ~ 1.5 – 3.5s *H.264 over IP Multicast ~ 1.7 – 4s

References

{{Reflist

External links

ITU IPTV Focus GroupITU-T. Zap time.
Digital television es:Zapeo