A modulation method specifies how the radio signal fluctuates to convey information. ATSC and DVB-T specify the modulation used for over-the-air digital television; by comparison, QAM is the modulation method used for cable. The specifications for a cable-ready television, then, might state that it supports 8VSB (for broadcast TV) and QAM (for cable TV).
8VSB is an 8-level vestigial sideband modulation. In essence, it converts a binary stream into an octal representation by amplitude-shift keying a sinusoidal carrier to one of eight levels. 8VSB is capable of transmitting three bits (23=8) per symbol; in ATSC, each symbol includes two bits from the MPEG transport stream which are trellis modulated to produce a three-bit figure. The resulting signal is then band-pass filtered with a Nyquist filter to remove redundancies in the side lobes, and then shifted up to the broadcast frequency.
Vestigial sideband modulation (VSB) is a modulation method which attempts to eliminate the spectral redundancy of pulse-amplitude modulation (PAM) signals. Modulating a carrier by a real-valued data sequence results in a sum and a difference frequency, resulting in two symmetrical carrier side-bands. The symmetry means that one of the sidebands is redundant, so removing one sideband still allows for demodulation. As filters with zero transition bandwidth cannot be realized, the filtering implemented leaves a vestige of the redundant sideband, hence the name “VSB”.
In the 6 MHz (megahertz) channel used for broadcast ATSC, 8VSB carries a symbol rate of 10.76 megabaud, a gross bit rate of 32 Mbit/s, and a net bit rate of 19.39 Mbit/s of usable data. The net bit rate is lower due to the addition of forward error correction codes. The eight signal levels are selected with the use of a trellis encoder. There are also similar modulations 2VSB, 4VSB, and 16VSB. 16VSB was notably intended to be used for ATSC digital cable, but quadrature amplitude modulation (QAM) has become the de facto industry standard instead as it is cheap and readily available.
A significant advantage of 8VSB for broadcasters is that it requires much less power to cover an area comparable to that of the earlier NTSC system, and it is reportedly better at this than the most common alternative system, COFDM. Part of the advantage is the lower peak to average power ratio needed compared to COFDM. An 8VSB transmitter needs to have a peak power capability of 6 db (four times) its average power. 8VSB is also more resistant to impulse noise. Some stations can cover the same area while transmitting at an effective radiated power of approximately 25% of analog broadcast power. While NTSC and most other analog television systems also use a vestigial sideband technique, the unwanted sideband is filtered much more effectively in ATSC 8VSB transmissions. 8VSB uses a Nyquist filter to achieve this. Reed–Solomon error correction is the primary system used to retain data integrity.
In summer of 2005, the ATSC published standards for Enhanced VSB, or E-VSB . Using forward error correction, the E-VSB standard will allow DTV reception on low power handheld receivers with smaller antennas in much the same way DVB-H does in Europe, but still using 8VSB transmission.
For some period of time, there had been a continuing lobby for changing the modulation for ATSC to COFDM, the way DVB-T is transmitted in Europe, and ISDB-T in Japan. However, the FCC has always held that 8VSB is the better modulation for use in U.S. digital television broadcasting. In a 1999 report, the Commission found that 8VSB has better threshold or carrier-to-noise (C/N) performance, has a higher data rate capability, requires less transmitter power for equivalent coverage, and is more robust to impulse and phase noise. As a result, it denied in 2000 a petition for rulemaking from Sinclair Broadcast Group requesting that broadcasters be allowed to choose between 8VSB or COFDM as is most appropriate for their area of coverage. The FCC report also acknowledged that COFDM would "generally be expected to perform better in situations where there is dynamic multipath," such as mobile operation or in the presence of trees that are moving in high winds. However, with the introduction of 5th Generation demodulators in 2005 and subsequent improvements in generations 6 and 7, the equalization span is now about −60 to +75 microseconds (a 135 microsecond spread) and has virtually eliminated multipath, both static and dynamic, in 8VSB reception. In comparison, the equalization span in COFDM is −100 to +100 microseconds (200 microsecond spread), but the application of this much guard band space for COFDM substantially reduces its useful payload. In fact, much of Europe has adopted 1280×720p as its HD standard for DVB-T1 because of its reduced payload capacity. The introduction of DVB-T2 is meant to increase the ability of terrestrial transmissions to carry 1920×1080p content. 1920×1080i has always been part of the 8VSB scheme from its inception, and its improved demodulators have had no effect on its innate payload capacity.
Because of continued adoption of the 8VSB-based ATSC standard in the U.S., and a large growing ATSC receiver population, a switch to COFDM is now essentially impossible. Most analog terrestrial transmissions in the US were turned off in June 2009, and 8VSB tuners are common to all new TVs, further complicating a future transition to COFDM.
The previously cited FCC Report also found that COFDM has better performance in dynamic and high level static multipath situations, and offers advantages for single frequency networks and mobile reception. Nonetheless, in 2001, a technical report compiled by the COFDM Technical Group concluded that COFDM did not offer any significant advantages over 8VSB. The report recommended in conclusion that receivers be linked to outdoor antennas raised to roughly 30 feet (9 m) in height. Neither 8VSB nor COFDM performed acceptably in most indoor test installations.
However, there were questions whether the COFDM receiver selected for these tests − a transmitter monitor  lacking normal front end filtering − colored these results. Retests that were performed using the same COFDM receivers with the addition of a front end band pass filter gave much improved results for the DVB-T receiver, but further testing was not pursued.
The debate over 8VSB versus COFDM modulation is still ongoing. Proponents of COFDM argue that it resists multipath far better than 8VSB. This is important property of the modulation for receiving HDTV in e.g. moving vehicles that is not possible with 8VSB. Early 8VSB DTV (digital television) receivers often had difficulty receiving a signal in urban environments. Newer 8VSB receivers, however, are better at dealing with multipath, but a moving receiver can still not receive the signal. Moreover, 8VSB modulation requires less power to transmit a signal the same distance. In less populated areas, 8VSB may outperform COFDM because of this. However, in some urban areas, as well as for mobile use, COFDM may offer better reception than 8VSB. Several "enhanced" VSB systems were in development, most notably E-VSB, A-VSB, and MPH. The deficiencies in 8VSB in regards to multipath reception can be dealt with by using additional forward error-correcting codes which decreases the useful bit rate, such as that used by ATSC-M/H for Mobile/Handheld reception. ATSC 3.0, the next major television standard in the United States, will use COFDM.
The vast majority of U.S. TV stations use COFDM for their studio to transmitter links and news gathering operations. These are point-to-point communication links and not broadcast transmissions.
The DVB-T signal is a Coded Orthaginal Frequency Divisional Multiplexing system or COFDM for short, but so is the cable (DVB-C) and Satellite (DVB-S). The DVB-T signal is implemented as QPSK QAM or Quadrature Phase Shift Keying and Quadrature Amplitude Modulation.The DVB-S signal is implemented as QPSK QFM or Quadrature Phase Shift Keying Quadrature Frequency Modulation. The DVB-C signal is QPM QAM or Quadrature Phase Modulation Quadrature Amplitude Modulation.16VSB
16VSB is an abbreviation for 16-level vestigial sideband modulation, capable of transmitting four bits (24=16) at a time.2VSB
In telecommunications, 2VSB is an abbreviation for 2-level vestigial sideband modulation, a transmission method capable of transmitting one bit (21=2) at a time.Other faster but less rugged forms include 4VSB, 8VSB, and 16VSB.32VSB
32VSB is an acronym for 32-level vestigial sideband modulation, capable of transmitting five bits (25=32) at a time.
32VSB is rarely used, because receivers have more trouble distinguishing between so many fine levels of modulation. It can, however, be useful in situations such as short last mile coaxial cable runs, which bring fiber to the curb systems into the home as regular cable TV, cable modem, and cable telephone services.
Other slower but more rugged forms include 2VSB, 4VSB, 8VSB, and 16VSB.4VSB
4VSB is an abbreviation for 4-level vestigial sideband modulation, a type of radio transmission capable of transmitting two bits of information (22=4) at a time. Other faster but less rugged forms include 8VSB and 16VSB. While 2VSB is more rugged, it is also slower.A-VSB
A-VSB or Advanced VSB is a modification of the 8VSB modulation system used for transmission of digital television using the ATSC system. One of the constraints of conventional ATSC transmission is that reliable reception is difficult or impossible when the receiver is moving at speeds associated with normal vehicular traffic. The technology was jointly developed by Samsung and Rohde & Schwarz.
A-VSB builds on the existing ATSC transmission standard to enhance DTV receivers’ ability to receive the main MPEG transport stream in dynamic environments. The system enables broadcasters to include multiple streams with additional error correction and time diversity encoding for enhanced reception. In addition, A-VSB facilitates synchronization of multiple transmission towers, which should improve coverage with higher uniform signal strength throughout a service area, even in locations that normally would be shielded by obstacles such as hills or buildings.
A-VSB incorporates three new elements: a Supplementary Reference Signal (SRS), a Scalable Turbo Stream (STS), and support for Single Frequency Networks (SFN).ATSC-M/H
ATSC-M/H (Advanced Television Systems Committee - Mobile/Handheld) is a U.S. standard for mobile digital TV that allows TV broadcasts to be received by mobile devices.ATSC-M/H is a mobile TV extension to preexisting terrestrial TV broadcasting standard ATSC A/53. It corresponds to the European DVB-H and 1seg extensions of DVB-T and ISDB-T terrestrial digital TV standards respectively. ATSC is optimized for a fixed reception in the typical North American environment and uses 8VSB modulation. The ATSC transmission method is not robust enough against Doppler shift and multipath radio interference in mobile environments, and is designed for highly directional fixed antennas. To overcome these issues, additional channel coding mechanisms are introduced in ATSC-M/H to protect the signal.ATSC standards
Advanced Television Systems Committee (ATSC) standards are a set of standards for digital television transmission over terrestrial, cable, and satellite networks. It is largely a replacement for the analog NTSC standard, and like that standard, used mostly in the United States, Mexico and Canada. Other former users of NTSC, like Japan,]] because they adopted their own system called ISDB.
The ATSC standards were developed in the early 1990s by the Grand Alliance, a consortium of electronics and telecommunications companies that assembled to develop a specification for what is now known as HDTV. The standard is now administered by the Advanced Television Systems Committee. The standard includes a number of patented elements, and licensing is required for devices that use these parts of the standard. Key among these is the 8VSB modulation system used for over-the-air broadcasts.
ATSC includes two primary high definition video formats, 1080i and 720p. It also includes standard-definition formats, although initially only HDTV services were launched in the digital format. ATSC can carry multiple channels of information on a single stream, and it is common for there to be a single high-definition signal and several standard-definition signals carried on a single 6 MHz (former NTSC) channel allocation.CEA-909
CEA-909 is the ANSI standard for 8VSB/ATSC smart antennas. The basic concept is that the smart antenna either physically rotates toward the signal, or is stationary, but has elements pointed in different directions and uses only those elements that maximize the received signal. This is accomplished by feedback from the control device, such as a digital-to-analog converter box, telling the smart antenna when the signal is stronger or weaker.
Analog televisions generally give instant feedback as the signal gets better or worse as you move the antenna. Digital television antennas can be difficult to aim correctly because of the cliff effect and because of delays in decoding and displaying the signal. Smart antennas remove the burden of positioning the antenna for digital TVs and can make the tuning process easier than it was with analog television.Digital television
Digital television (DTV) is the transmission of television signals, including the sound channel, using digital encoding, in contrast to the earlier television technology, analog television, in which the video and audio are carried by analog signals. It is an innovative advance that represents the first significant evolution in television technology since color television in the 1950s. Digital TV transmits in a new image format called high definition television (HDTV), with greater resolution than analog TV, in a widescreen aspect ratio similar to recent movies in contrast to the narrower screen of analog TV. It makes more economical use of scarce radio spectrum space; it can transmit multiple channels, up to 7, in the same bandwidth occupied by a single channel of analog television, and provides many new features that analog television cannot. A transition from analog to digital broadcasting began around 2006. Different digital television broadcasting standards have been adopted in different parts of the world; below are the more widely used standards:
Digital Video Broadcasting (DVB) uses coded orthogonal frequency-division multiplexing (OFDM) modulation and supports hierarchical transmission. This standard has been adopted in Europe, Africa, Asia, Australia, total about 60 countries.
Advanced Television System Committee (ATSC) uses eight-level vestigial sideband (8VSB) for terrestrial broadcasting. This standard has been adopted by 6 countries: United States, Canada, Mexico, South Korea, Dominican Republic and Honduras.
Integrated Services Digital Broadcasting (ISDB) is a system designed to provide good reception to fixed receivers and also portable or mobile receivers. It utilizes OFDM and two-dimensional interleaving. It supports hierarchical transmission of up to three layers and uses MPEG-2 video and Advanced Audio Coding. This standard has been adopted in Japan and the Philippines. ISDB-T International is an adaptation of this standard using H.264/MPEG-4 AVC that been adopted in most of South America and is also being embraced by Portuguese-speaking African countries.
Digital Terrestrial Multimedia Broadcasting (DTMB) adopts time-domain synchronous (TDS) OFDM technology with a pseudo-random signal frame to serve as the guard interval (GI) of the OFDM block and the training symbol. The DTMB standard has been adopted in the People's Republic of China, including Hong Kong and Macau.
Digital Multimedia Broadcasting (DMB) is a digital radio transmission technology developed in South Korea as part of the national IT project for sending multimedia such as TV, radio and datacasting to mobile devices such as mobile phones, laptops and GPS navigation systems.E-VSB
E-VSB or Enhanced VSB is an optional enhancement to the original ATSC Standards that use the 8VSB modulation system used for transmission of digital television. It is intended for improving reception where signals are weaker, including fringe reception areas, and on portable devices such as handheld televisions or mobile phones. It does not cause problems to older receivers, but they cannot take advantage of its features. E-VSB was approved by the ATSC committee in 2004. However, it has been implemented by few stations or manufacturers.For mobile applications, ATSC suffers significant signal degradation caused by the Doppler effect. Additionally, low-power handheld receivers are usually equipped with smaller antennas. These have a poor signal-to-noise ratio, which is disruptive to digital signals. The E-VSB standard provides for Reed-Solomon forward error correction to alleviate the data corruption caused by these issues.
Additionally, the standard can use either the MPEG-4 AVC or VC-1 video codecs. As these codecs have higher video compression than the original MPEG-2, they require less bandwidth.
As 8VSB lacks both link adaptation and hierarchical modulation of DVB, which would allow the SDTV part of an HDTV signal (or the LDTV part of SDTV) to be received even in fringe reception areas where signal strength is low, E-VSB yields a similar benefit. However, E-VSB places a significant processing overhead on the receiver, as well as a significant transmission overhead on the broadcaster's total bitrate. These are not a problem with DVB-H.
A-VSB is a different and, as of July 2008, unapproved addition to ATSC, which is also designed to send programming to mobile devices, and to allow for single-frequency networks. It is one of several proposals for ATSC-M/H, the as-yet undecided standard for mobile broadcasting via ATSC.Ghosting (television)
In television, a ghost is a replica of the transmitted image, offset in position, that is super-imposed on top of the main image. It is often caused when a TV signal travels by two different paths to a receiving antenna, with a slight difference in timing.KYES-TV
KYES-TV, virtual channel 5 (VHF digital channel 7), is a MyNetworkTV-affiliated television station licensed to Anchorage, Alaska, United States. The station is owned by Gray Television, as part of a duopoly with NBC affiliate KTUU-TV (channel 2). The two stations share studios on East 40th Avenue in Anchorage; KYES-TV's transmitter is located in Knik, Alaska. On cable, the station is available on GCI channel 5. It is also carried on DirecTV and Dish Network in the Anchorage television market.List of ATSC standards
Below are the published ATSC standards for ATSC digital television service, issued by the Advanced Television Systems Committee.
A/49: Ghost Canceling Reference Signal for NTSC (for adjacent-channel interference or co-channel interference with analog NTSC stations nearby)
A/52B: audio data compression (Dolby AC-3 and E-AC-3)
A/53E: "ATSC Digital Television Standard" (the primary document governing the standard)
A/55: "Program Guide for Digital Television" (now deprecated in favor of A/65 PSIP)
A/57A: "Content Identification and Labeling for ATSC Transport" (for assigning a unique digital number to each episode of each TV show, to assist DVRs)
A/63: "Standard for Coding 25/50 Hz Video" (for use with PAL and SECAM-originated programming)
A/64A "Transmission Measurement and Compliance for Digital Television"
A/65C: "Program and System Information Protocol for Terrestrial Broadcast and Cable" (PSIP includes virtual channels, electronic program guides, and content ratings)
A/68: "PSIP Standard for Taiwan" (defines use of Chinese characters via Unicode 3.0)
A/69: recommended practices for implementing PSIP at a TV station
A/70A: "Conditional Access System for Terrestrial Broadcast"
A/71: "ATSC Parameterized Services Standard"
A/72: "Video System Characteristics of AVC in the ATSC Digital Television System" (implementing H.264/MPEG-4 as well as MVC for 3D television)
A/76: "Programming Metadata Communication Protocol" (XML-based PMCP maintains PSIP metadata though a TV station's airchain)
A/79: "Conversion of ATSC Signals for Distribution to NTSC Viewers" (recommended practice, issued February 2009)
A/80: "Modulation and Coding Requirements for Digital TV (DTV) Applications Over Satellite" (ATSC-S)
A/81: "Direct-to-Home Satellite Broadcast Standard" (not yet implemented by any services)
A/82: "Automatic Transmitter Power Control (ATPC) Data Return Link (DRL) Standard"
A/85: "Techniques for Establishing and Maintaining Audio Loudness for Digital Television"
A/90: "Data Broadcast Standard" (for datacasting)
A/92: "Delivery of IP Multicast Sessions over Data Broadcast Standard" (for IP multicasting)
A/93: "Synchronized/Asynchronous Trigger Standard"
A/94: "ATSC Data Application Reference Model"
A/95: "Transport Stream File System Standard" (TSFS is a special file system for downloading computer files)
A/96: "ATSC Interaction Channel Protocols" (interactive TV)
A/97: "Software Data Download Service" (used by UpdateTV for upgrades and software patches in ATSC tuners)
A/98: "System Renewability Message Transport"
A/99: "Carriage Of Legacy TV Data Services" (for former analog supplemental services that used the vertical blanking interval lines, such as closed captioning and teletext)
A/100: "DTV Application Software Environment - Level 1" (DASE-1)
A/101: "Advanced Common Application Platform" (ACAP)
A/103:2014: "Non-Real-Time Delivery"
A/104: "ATSC 3D-TV Terrestrial Broadcasting"
A/105:2015: "Interactive Services Standard"
A/106:2015: "ATSC Security and Service Protection Standard"
A/107:2015: "ATSC 2.0 Standard"
A/110A: "Synchronization Standard for Distributed Transmission" (single-frequency networks)
A/112: E-VSB (Enhanced Vestigal Sideband)
A/153: ATSC-M/HIn 2004, the main ATSC standard was amended to support Enhanced ATSC (A/112); this transmission mode is backwardly compatible with the original 8-Bit Vestigal Sideband modulation scheme, but provides much better error correction.
ATSC-M/H for mobile TV has been approved and added to some stations, though it is known that it uses MPEG-4 instead of MPEG-2 for encoding, and behaves as an MPEG-4-encoded subchannel, inheriting 8VSB from the remainder of the channel.MPH (ATSC)
MPH (Mobile-Pedestrian-Handheld) was a mobile extension of the ATSC television standard jointly developed by Harris Corporation, LG Electronics, Inc. and its U.S. research subsidiary, Zenith Electronics. The MPH platform allowed local TV stations to deliver ATSC-compatible content to mobile and video devices such as mobile phones, portable media players, laptop computers, personal navigation devices and automobile-based "infotainment systems." The service is called "in-band" because local broadcasters are providing mobile TV services as part of their terrestrial transmission within the same, existing 6 MHz channel they use for their ATSC DTV programming.
With the installation of an MPH exciter and signal encoding equipment, existing TV transmission systems would have transmitted a signal which could be received on "MPH-ready" devices. The system allowed the splitting of the 6 MHz, 19.4 Mbit/s of spectrum into a slice for a traditional DTV signal and a slice for MPH use, serving several types of user with a single DTV channel.
The MPH system is a multiple-stream approach, with the main service stream for existing DTV and HDTV services, and the MPH stream for one or more mobile, pedestrian, and/or handheld services. Key attributes of the MPH system were:
Backward compatibility with existing ATSC 8VSB transmission and receiving equipment
Capability to receive broadcast signals at high (mobile) speed with a single antenna
Use of small handheld receivers without the need for multiple antennas
Power savings in handheld receivers
Flexibility in both data rates and robustness
Use of advanced video and audio codingThe MPH standard was later combined with competing mobile ATSC proposals, to become ATSC-M/H, which remains the current standard.Multiplex (television)
A multiplex or mux (called virtual sub-channel in the United States and Canada, and bouquet in France) is a grouping of program services as interleaved data packets for broadcast over a network or modulated multiplexed medium. The program services are split out at the receiving end.
In the United Kingdom, a terrestrial multiplex (usually abbreviated mux) has a fixed bandwidth of 8 MHz CODFM of interleaved H.222 packets containing a number of channels. In the United States, a similar arrangement using 6 MHz 8VSB is often described as a channel with virtual sub-channels.Output power of an analog TV transmitter
Article needs redesign. See Talk.
The output power of a TV transmitter is the electric power applied to antenna system.
There are two definitions: nominal (or peak) and thermal. Analogue television systems put about 70% to 90% of the transmitters power into the sync pulses. The remainder of the transmitter's power goes into transmitting the video's higher frequencies and the FM audio carrier. Digital television modulation systems are about 30% more efficient than analogue modulation systems overall.Pulse-amplitude modulation
Pulse-amplitude modulation (PAM), is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulses are varied according to the sample value of the message signal. Demodulation is performed by detecting the amplitude level of the carrier at every single period.Single-frequency network
A single-frequency network or SFN is a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel.
Analog AM and FM radio broadcast networks as well as digital broadcast networks can operate in this manner. SFNs are not generally compatible with analog television transmission, since the SFN results in ghosting due to echoes of the same signal.
A simplified form of SFN can be achieved by a low power co-channel repeater, booster or broadcast translator, which is utilized as gap filler transmitter.
The aim of SFNs is efficient utilization of the radio spectrum, allowing a higher number of radio and TV programs in comparison to traditional multi-frequency network (MFN) transmission. An SFN may also increase the coverage area and decrease the outage probability in comparison to an MFN, since the total received signal strength may increase to positions midway between the transmitters.
SFN schemes are somewhat analogous to what in non-broadcast wireless communication, for example cellular networks and wireless computer networks, is called transmitter macrodiversity, CDMA soft handoff and Dynamic Single Frequency Networks (DSFN).
SFN transmission can be considered as a severe form of multipath propagation. The radio receiver receives several echoes of the same signal, and the constructive or destructive interference among these echoes (also known as self-interference) may result in fading. This is problematic especially in wideband communication and high-data rate digital communications, since the fading in that case is frequency-selective (as opposed to flat fading), and since the time spreading of the echoes may result in intersymbol interference (ISI). Fading and ISI can be avoided by means of diversity schemes and equalization filters.Television channel
A television channel is a terrestrial frequency or virtual number over which a television station or television network is distributed. For example, in North America, "channel 2" refers to the terrestrial or cable band of 54 to 60 MHz, with carrier frequencies of 55.25 MHz for NTSC analog video (VSB) and 59.75 MHz for analog audio (FM), or 55.31 MHz for digital ATSC (8VSB). Channels may be shared by many different television stations or cable-distributed channels depending on the location and service provider
Depending on the multinational bandplan for a given regional n, analog television channels are typically 6, 7, or 8 MHz in bandwidth, and therefore television channel frequencies vary as well. Channel numbering is also different. Digital terrestrial television channels are the same as their analog predecessors for legacy reasons, however through multiplexing, each physical radio frequency (RF) channel can carry several digital subchannels. On satellites, each transponder normally carries one channel, however multiple small, independent channels can be on one transponder, with some loss of bandwidth due to the need for guard bands between unrelated transmissions. ISDB, used in Japan and Brazil, has a similar segmented mode.
Preventing interference between terrestrial channels in the same area is accomplished by skipping at least one channel between two analog stations' frequency allocations. Where channel numbers are sequential, frequencies are not contiguous, such as channel 6 to 7 skip from VHF low to high band, and channel 13 to 14 jump to UHF. On cable TV, it is possible to use adjacent channels only because they are all at the same power, something which could only be done terrestrially if the two stations were transmitted at the same power and height from the same location. For DTT, selectivity is inherently better, therefore channels adjacent (either to analog or digital stations) can be used even in the same area.
Digital television in North America