Synchronous dynamic random-access memory

Synchronous dynamic random-access memory (SDRAM) is any dynamic random-access memory (DRAM) where the operation of its external pin interface is coordinated by an externally supplied clock signal. DRAM integrated circuits (ICs), between the mid-1970s and -1990s typically had an asynchronous interface, where responses to changes in control signal inputs occur as soon as they are received. SDRAM has a synchronous interface, meaning that a clock signal must be received before it responds to the control inputs. The interfaces of SDRAM ICs is therefore synchronous to the clock signal used. In the family of devices standardized by JEDEC, which are called synchronous DRAMs (SDRAMs), the clock signal is used to drive an internal finite state machine that pipelines incoming commands. The memory is divided into several independent sections of memory called banks, allowing the device to operate on several memory access commands at a time, provided the commands are independent of each other (in an interleaved fashion). This allows SDRAMs to achieve greater concurrency and higher data transfer rates than asynchronous DRAMs.

Channel memory

An automatic channel memory system (ACMS) is a system in which a digitally controlled radio tuner such as a TV set or VCR could search and memorize TV channels automatically. While more common in television, it can also be used to store presets for radio stations. This is often called a channel scan, though that may also refer to a "preview" mode which plays each station it finds for a few seconds and then moves on to the next, without affecting memory.

A typical TV device allows an automatic channel scan to be performed from a menu accessed by a button on the TV set, or sometimes only on the remote control. This applied first to analog TV sets — sometimes those with digital LED displays, or later always those with on-screen displays. These simply searched for the video carrier signal on every channel. (Before the advent of ACMS, many sets would search for the next channel every time it was changed.)

It now also applies to digital TV, which must not only find the signal itself, but also decode its metadata enough to remap channel numbers to their proper locations. In the case of the American ATSC system, the ATSC tuner uses PSIP metadata to do this. The internal channel map for digital TV stations is different from the presets or "favorites" that the user has programmed. Just as with analog TV (which worked only by turning a preset on or off for each station/channel), users of digital television adapters and other similar tuners can choose to ignore channels that are still in the channel map.

Virtual channel

In most telecommunications organizations, a virtual channel is a method of remapping the program number as used in H.222 Program Association Tables and Program Mapping Tables to a channel number that can be entered via digits on a receiver's remote control.

A "virtual channel" was first used for DigiCipher 2 in North America and then later used and referred to as a logical channel number (LCN) for private European Digital Video Broadcasting extensions widely used by the NDS Group and NorDig in other markets.

Pay television operators were the first to use either of these systems as a method of channel reassignment and/or rearrangement that suited their need to group multiple channels by their content or origin as well as to a lesser extent to localize advertising to a particular market.

Free-to-air ATSC uses the DigiCipher 2 method to maintain the same radio frequency channel allocation that the NTSC channel was using when both were simulcasting so the same number could bring up either service.

Virtual circuit

A virtual circuit (VC) is a mean of transporting data over a packet switched computer network in such a way that it appears as though there is a dedicated physical layer link between the source and destination end systems of this data. The term virtual circuit is synonymous with virtual connection and virtual channel. Before a connection or virtual circuit may be used, it has to be established, between two or more nodes or software applications, by configuring the relevant parts of the interconnecting network. After that, a bit stream or byte stream may be delivered between the nodes; hence, a virtual circuit protocol allows higher level protocols to avoid dealing with the division of data into segments, packets, or frames.

Virtual circuit communication resembles circuit switching, since both are connection oriented, meaning that in both cases data is delivered in correct order, and signalling overhead is required during a connection establishment phase. However, circuit switching provides a constant bit rate and latency, while these may vary in a virtual circuit service due to factors such as: