“Understanding the basics of how IP Television and Internet television service works will help you make better choices and may help you to solve problems that can be caused by selecting the wrong types of technologies, equipment and services”
Converting Video Signals and Audio Signals to Digital Signals
A key first step in providing Internet Television service is converting the analog audio voice signals into a digital form (digitization) and then compressing the digitized information into a more efficient form.
Digitization is the conversion of analog signals (continually varying signals) into digital form (signals that have only two levels). To convert analog signals to digital form, the analog signal is sampled and digitized by using an analog-to-digital (pronounced A to D) converter.
process, the noise is removed. Television signal digitization involves digitization of both the audio and video signals
shows the basic audio digitization process. This diagram shows that a person creates sound pressure waves when they talk. These sound pressure waves are converted to electrical signals by a microphone. When the microphone senses a large sound pressure wave (loud audio), it produces a large (higher voltage) analog signal. To convert the analog signal to digital form, the analog signal is periodically sampled and converted to a number of pulses. The larger the analog signal is, the larger the number of pulses that are produced. The number of pulses can be counted and sent as digital numbers. This example also shows that when the digital information is transmitted, it may acquire distortion during transmission. A digital receiver that detects the high or low signal levels and uses these levels to recreate new dig
ital signals can eliminate this distortion. This conversion process is called regeneration or repeating. This regeneration progress allows digital signals to be sent at great distances without losing the quality of the audio sound.
shows the basic process used to digitize images for pictures from analog video. The image is scanned line by line from the top to bottom. For color video, the image is scanned into lines where each contains intensity (brightness) and color information. This example shows that each line is periodically sampled and converted into digital equivalent levels. This example shows that analog signals can have 256 levels (0-255) and that this can be represented by 8 bits of information (a byte). One byte of information represents the intensity and one byte of information represents the color.
Digital Media Compression – Gaining Efficiency
Digital media compression is a process of analyzing a digital signal (digitized video and/or audio) and using the analysis information to convert the high-speed digital signals that represent the actual signal shape into lower-speed digital signals that represent the actual content (such as a moving image or human voice). This process allows IPTV (What is IPTV?) service to have lower data transmission rates than standard digital video signals while providing for good quality video and audio. Digital media compression for IP television includes digital audio compression and digital video compression.
- Figure 1: shows the basic digital speech compression process. In this example, the word “HELLO” is digitized. The initial digitized bits represent every specific shape of the digitized word HELLO. This digital information is analyzed and it is determined that this entire word can be represented by three sounds: “HeH” + “LeL” + “OH.” Each of these sounds only requires a few digital bits instead of the many bits required to recreate the entire analog waveform.
- Figure 2: demonstrates the operation of the basic digital video compression system. Each video frame is digitized and then sent for digital compression. The digital compression process creates a sequence of frames (images) that start with a key frame. The key frame is digitized and used as reference points for the compression process. Between the key frames, only the differences in images are transmitted. This dramatically reduces the data transmission rate to represent a digital video signal as an uncompressed digital video signal requires over 50 Mbps compared to less than 4 Mbps for a typical digital video disk (DVD) digital video signal.
Packet Routing Methods
Packet routing involves the transmission of packets through intelligent switches (called routers) that analyze the destination address of the packet and determine a path that will help the packet travel toward its destination.
Routers learn from each other about the best routes for them to select when forwarding packets toward their destination (usually paths to other routers). Routers regularly broadcast their connection information to nearby routers and they listen for connection information from connected routers. From this information, routers build information tables (called routing tables) that help them to determine the best path for them to forward each packet to.
Routers may forward packets towards their destination simply based on their destination address or they may look at some descriptive information about the packet. This descriptive information may include special handling instructions (called a label or tag) or priority status (such as high priority for real time voice or video signals).
shows how blocks of data are divided into small packet sizes that can be sent through the Internet. After the data is divided into packets (envelopes shown in this example), a destination address along with some description about the contents is added to each packet (called in the packet header). As the packet enters into the Internet (routing boxes shown in this diagram), each router reviews the destination address in its routing table and determines which paths it can send the packet to so it will move further towards its destination.
If a current path is busy or unavailable (such as shown for packet #3), the router can forward the packets to other routers that can forward the packet towards its destination. This example shows that because some packets will travel through different paths, packets may arrive out of sequence at their destination. When the packets arrive at their destination, they can be reassembled into proper order using the packet sequence number.
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Packet Losses and Effects on Television Quality
Packet losses are the incomplete reception or intentional discarding of data packets as they are sent through a network. Packets may be lost due to broken line connections, distortion from electrical noise ,
from a lightning spike), or through intentional discarding due to congested switch conditions. Packet losses are usually measured by counting the number of data packets that have been lost in transmission compared to the total number of packets that have been transmitted.
Figure 6 shows how some packets may be lost during transmission through a communications system. This example shows that several packets enter into the Internet. The packets are forwarded toward their destination as usual. Unfortunately, a lighting strike corrupts (distorts) packet 8 and it cannot be forwarded. Packet 6 is lost (discarded) when a router has exceeded its capacity to forward packets because too many were arriving at the same time. This diagram shows that the packets are serialized to allow them to be placed in correct order at the receiving end.
When the receiving end determines a packet is missing in the sequence, it can request that another packet be retransmitted. If the time delivery of packets is critical (such as for packetized voice), it is common that packet retransmission requests are not performed and the lost packets simply result in distortion of the received information (such as poor audio quality)
Packet buffering is the process of temporarily storing (buffering) packets during the transmission of information to create a reserve of pack
ets that can be used during packet transmission delays or retransmission requests. While a packet buffer is commonly located in the receiving device, a packet buffer may also be used in the sending device to allow the rapid selection and retransmission of packets when they are requested by the receiving device. Packet buffering is commonly used in IP television systems to overcome the transmission delays and packet losses that occur when viewing IP television signals.
A packet buffer receives and adds small amounts of delay to packets so that all the packets appear to have been received without varying delays. The amount of packet buffering for IP television systems can vary from tenths of a second to tens of seconds.
Figure 7 shows how packet buffering can be used to reduce the effects of packet delays and packet loss for streaming media systems. This diagram shows that during the transmission of packets from the media server to the viewer, some of the packet transmission time varies (jitter) and some of the packets are lost during transmission. The packet buffer temporarily stores data before providing it to the media player. This provides the time necessary to time synchronize the packets and to request and replace packets that have been lost during transmission.
Converting Packets to Television Service
IP Television data packets are converted back to television signals via gateways. Gateways may interconnect IP television service to a television network (such as a hotel television system) or they may convert the signals directly to a television signal format (such as a NTSC or PAL analog television signals).
Gateways Connect the Internet to Standard Televisions
A television gateway is a communications device or assembly that transforms audio and video that is received from a television media server (IP television signal source) into a format that can be used by a viewer or different network. A television gateway usually has more intelligence (processing function) than a data network bridge as it can select the video and voice compression coders and adjust the protocols and timing between two dissimilar computer systems or IP Television networks.
how a media gateway connects a television channel to a data network (such as the Internet). This example shows that the gateway must convert audio, video and control signals into a format that can be sent through the Internet. While there is one communication channel from the gateway to the end viewer, the communication channel carries multiple media channels including video, audio, and control information. The gateway first converts video and audio signals into digital form.
These digital signals are then analyzed and compressed by a coding processor. Because end users may have viewers that have different types of coders (such as MPEG and AAC), the media gateway usually has available several different types of coding devices. This example shows that the media gateway receives requests to view information (the user or network sends a message to the media gateway). The gateway may have a database (or access to a database) that helps it determine authorized users and the addresses to send IP television signals.