Explanations

Digital radio

Short distance (VHF)

Curta distància vol dir la senyal no sol arribar més enllà de l'horitzó. Aquests senyals s'atenueixen menys que la UHF a dins d'edificis però, a diferència de l'ona curta, les senyals es poden bloquejar més fàcilment per afecte de les característiques del terreny.
Dos propagacions no usuals poden fer que les ones arribin més lluny: propagació troposfèrica (250km) i, la més rara, esporàdica-E (més de 1000km).

Digital Audio Broadcasting (DAB), also known as Eureka 147, is a technology for broadcasting of audio using digital radio transmission.

The original objectives of converting to digital transmission were to enable higher fidelity, more stations and more resistance to noise, co-channel interference and multipath than in analogue FM radio. However, in the UK, Denmark, Norway and Switzerland, which are the leading countries with regard to implementing DAB, the vast majority of stereo radio stations on DAB have a lower sound-quality than FM due to the bit rate levels used on DAB being too low. This assumes that the listener has good reception on both DAB and FM, however, but FM can suffer from fading caused by multipath when the receiver is travelling at high speed which DAB is less prone to. For stationary reception, FM can suffer from hiss when the signal is weak, whereas DAB produces a "bubbling mud" sound when the received signal is too weak for proper decoding.

In November 2006, WorldDMB announced that the DAB system was in the process of being upgraded, and it will adopt the AAC+ audio codec to improve the efficiency of the system and stronger error correction coding to improve the robustness of transmissions. This means there are now two different versions of the DAB system: the current one, developed in the late 1980s, and an upgraded version, which has been named "DAB⁺". Existing DAB receivers are incompatible with the new DAB⁺ standard, but receivers that support the new DAB⁺ standard are expected to be in the shops by summer 2007.

DAB⁺ will replace DAB in all countries that use DAB, and countries where DAB sales have not taken off are free to use DAB⁺ from the outset

DAB⁺

WorldDMB, the organisation in charge of the DAB standards, announced in a press release in November 2006, that DAB would be adopting the HE-AAC audio codec, which is also known as AAC+. Also being adopted are the MPEG Surround format, and stronger error correction coding called Reed-Solomon coding. The new standard has been named DAB⁺ and standardized as ETSI TS 102 563.

Receivers that support the new DAB standard will be released in the UK in Spring 2007, and Ofcom has said that services using the old MPEG-1 Audio Layer II audio format used on the old DAB system can be switched off once the vast majority of receivers can support the new HE-AAC audio format. The new HE-AAC audio format is around 3 - 4 times as efficient as the old MPEG-1 Audio Layer II audio format, so this will allow 3 - 4 times as many stations to transmit as is currently possible, or the audio quality can be increased, or a combination of both these things can occur.

DAB and DAB⁺ can't be used for mobile TV because they don't include any video codecs. DAB related standards Digital Multimedia Broadcasting (DMB) and DAB-IP are suitable for mobile radio and TV both because they have MPEG 4 AVC and WMV9 respectively as video codecs. However a DMB video subchannel can easily be added to any DAB transmission -- as DMB was designed from the outset to be carried on a DAB subchannel. DMB broadcasts in Korea carry conventional MPEG 1 Layer II DAB audio services alongside their DMB video services.

DRM Plus

While DRM currently covers the broadcasting bands below 30 MHz, the DRM consortium voted in March 2005 to begin the process of extending the system to the VHF bands up to 120 MHz. DRM Plus (DRM+) will be the name of this technology. Design, development and testing are expected to be completed by 2007-2009.

Wider bandwidth channels will be used, which will allow radio stations to use higher bit rates, thus providing higher audio quality. One likely channel bandwidth is 50 kHz, which will allow DRM Plus to carry radio stations at near CD-quality. A 100 kHz DRM+ channel has sufficient capacity to carry one mobile TV channel: it would be feasible to distribute mobile TV over DRM+ rather than DMB or DVB-H.

HD Radio (FM)

Digital information is transmitted using COFDM, a modulation method that has been used in different digital television and radio systems, including DVB-T. The audio compression algorithm was initially set to be PAC when iBiquity's standard was first approved by the Federal Communications Commission (FCC) in 2002, but the system was changed to the HDC codec in 2003. HD Radio equipped stations must pay royalties each year to iBiquity, plus the costs paid by the manufacturers of the transmitters which are then passed along to the stations that buy them.

In hybrid mode, FM stations can carry information at 96 kbit/s or 128 kbit/s. HD Radio can also be used to carry multiple distinct audio services (by splitting up the bandwidth), called multicasting but actually more like multiplexing. Second and third channels, such as for weather, traffic, or a radio reading service, can be added this way, though it may reduce the audio quality of all channels on a station. Datacasting is also possible, and RDS-like metadata about the program and station are included in the standard. Stations may eventually go all-digital, meaning they could no longer be heard on regular (legacy analog-only) radio receivers. This will allow for as many as seven different multicast channels.

While in hybrid mode, an HD Radio will lock onto an analog signal first in mono, then stereo, then try to find a digital one. If digital signal reception is lost, the radio will revert to analog, the same way a car radio will go into mono operation from stereo when signal strength is insufficient for stereo. Much of the success of this system property relies on proper synchronization of the analog and digital audio signals by broadcast engineers at the transmitter.

On the wider sidebands of FM stations, HD Radio can carry multiple streams of FM and/or AM quality. National Public Radio in particular hopes to be able to carry several different streams through the transmitters of member stations, calling its proposed addition to the FM standard "Tomorrow Radio". Some have also proposed using the system to carry surround sound broadcasts with 5.1 channel audio, though this or other multichannel setups reportedly may prevent the fade-to-analog fallback on "hybrid" analog+digital broadcasts.

Currently, FM stations in the United States and Canada are licensed to occupy approximately 200 kHz of RF spectrum, i.e., the FM band frequency allocations are 200 kHz apart. When a signal modulates the carrier, an infinite number of harmonically-related sidebands are created, thus the actual occupied bandwidth of the signal extends well past the highest modulating frequency (usually ± 100 kHz), because of the non-linear nature of frequency modulation. In order to prevent harmful interference to other stations, the carrier frequencies of stations within individual markets are seldom authorized to be closer than four channels apart, equal to 800 kHz. Occasional exceptions exist with spacing of three channels (600 kHz). In addition, there are constraints on occupied bandwidth as such. Transmitted RF energy in the sidebands more than 120 kHz from the center frequency is required to be significantly attenuated. FM stations have a baseband bandwidth of about 100 kHz, only 15 kHz of which is used by analog monophonic) audio. Analog stereo uses 53 kHz of baseband space, and RBDS is centered at 57 kHz. The "remainder" is currently available for other services, including rental for secondary broadcast services, paging and datacasting, or as a transmitter-studio link for in-house telemetry.

While the various baseband signals all contribute in a complex manner to the total occupied bandwidth (and modulation level) of the FM signal, it is important to consider two factors. First, the modulating signal will generate a principal RF component at the fundamental sideband frequency, e.g., a 67 kHz subcarrier will generate RF components at ± 67 kHz from the FM carrier. Second, any filtering of the modulated signal, or bandwidth limitation in the transmission system, causes a certain amount of distortion in the received signal, due to the formation of intermodulation components from the various modulating signals.

In regular hybrid mode, an HD Radio station has its full ± 100 kHz of RF bandwidth, and adds its digital signals into part of the upper and lower adjacent RF channels beyond that, using about 1% of the main FM power level. In extended hybrid mode, the bandwidth of the FM signal is reduced to make way for additional OFDM carriers carrying more data. Because of this, FM stations may have to discontinue existing subcarrier services (usually at 92 kHz and 67 kHz) in order to carry extended HD Radio, though such services can be restored through the digital subchannels that are then made available. This will require new receiving equipment for subscribers, however. The current analog stereo subcarrier would, in theory, eventually be dropped to make more room for digital, and eventually stations could elect to drop the analog baseband (monophonic audio) completely and go all-digital. However, considering that there are billions of existing analog-only receivers, this is not expected to happen for a very long time, if ever.

There are still some concerns that HD Radio on FM will increase interference between different stations, though it is thought unlikely to make a major difference since HD Radio still fits within the existing spectral mask. An HD Radio station will not generally cause interference to any analog station within its 1 mV/m signal strength contour, the limit above which the FCC protects most stations. Some interference of this type is already permitted by FCC assignment rules, and the amount caused by HD Radio is expected to be accepted as well. A distinct possibility exists of interference between HD stations in nearby markets, which may be assigned frequencies only one or two channels apart.

As with AM, FM stations may use separate exciters to modulate the very different signals. A combiner is often used, either before common amplification or after separate amplification, though stations are also now allowed to use a separate transmitting antenna slightly higher or lower on the radio tower. In each case the ratio of power of the analog signal to the digital signal is standardized at 100:1.

FMeXtra

FMeXtra is an in-band on-channel digital radio broadcasting technology created by Digital Radio Express. Unlike iBiquity's HD Radio system, it uses any FM radio station's existing equipment and transmitter plant to transmit digital audio data on subcarriers instead of sidebands. It also requires no royalties for its use, which run thousands of dollars per year for HD Radio.

The system is run from a single rack unit box called the X1 Encoder, which is actually based upon a personal computer server and digital audio hardware from Lynx Studio Technologies (LST). Control is entirely via software, via gigabit Ethernet, USB, serial port, and SVGA video monitor. All processing is handled internally by a Pentium 4 running Windows XP.

FMeXtra is fully compatible with HD Radio hybrid mode, which uses additional radio spectrum beyond the ±100 kHz signal. It is not compatible with HD Radio in all-digital mode, however this is not expected to be used for a very long time, given that there are already billions of analog FM radios already in use. It is also not compatible with all existing subcarriers. Thus, a public radio station might have to remove its radio reading service for the blind, and replace it (and its dependent listeners' receivers) with a digital one. This would take up much less bandwidth, particularly since voice can be highly compressed, however the changeover could be somewhat disruptive. The signal is partitioned so that RBDS, stereo, or other existing subcarriers can be protected, at the expense of bandwidth.

The codecs used are AAC and aacPlus v1 and v2, at bitrates of 8 to 384 kbit/s, and sample rates of 8 to 96 kHz, for anywhere from monophonic to 7.1-channel surround sound. Multiple audio programs as well as limited multimedia can also be broadcast, as with HD Radio. The available broadcasting bandwidth for digital audio varies from 48 kbit/s (RBDS protection and stereo) to 128 kbit/s (mono).

CAM-D

Compatible Amplitude Modulation - Digital or CAM-D is a proposed hybrid digital radio format for AM broadcasting, put forth by well-known broadcast engineer Leonard R. Kahn.

While little is known about it, it is an in-band on-channel technology that uses the sidebands of any AM radio station. Analog information is still used up to a bandpass of about 7.5kHz, with standard amplitude modulation. The missing treble information that AM normally lacks is then transmitted digitally beyond this. Audio mixing in the receiver then blends them back together.

Unlike other IBOC technologies like Digital Radio Mondiale and iBiquity's HD Radio, Kahn's apparently does not provide a direct path to all-digital transmissions, nor any multichannel capability. The advantage however is that it takes up far less of the sidebands, thereby causing far less interference to adjacent channels, hence the "Compatible" in the name. That problem has plagued HD Radio on AM, along with the fact that it (like CAM-D) is proprietary.

CAM-D is said to add high-frequency program information digitally transmitted and then overlaid onto the existing low- to mid-frequency analog program. This provides enhanced fidelity that under adequate analog signal conditions may allow AM stations to transmit music and other program content with a more life-like sound.

However, a significant disparity between Ibiquity's AM HD Radio and CAM-D is that of time diversity. Namely, HD Radio's AM scheme^[2] broadcasts two copies of the program offset by a few seconds, allowing signal to be briefly lost and maintain uninterrupted program audio (for example, driving under an overpass, multipath in urban areas, lightning, switching transients, etc.).
Analog AM radio, including CAM-D, since CAM-D requires the analog sidebands to transmit most of the program audio, will suffer loss/disruption of main program audio in these and other circumstances.

These outstanding technical issues present a significant limit to the utility of CAM-D, especially in urban areas, where the transmitter site is often located outside the urban area and thus lacks a more direct path to the receivers (and even with a more direct path, these noise sources can still be quite disruptive to program audio without the time diversity).

Llarga o mitja distància

En l'ona curta les ones reboten en l'atmosfera això fa que s'aconsegueixi arribar a distàncies llargues (3000km), fins i tot donar la volta al món.
En l'ona mitjana les ones segueixen la curvatura de la Terra (400km) i a la nit reboten a l'atmosfera (1000km), comunicacions nacionals o regionals.
Les ones llargues segueixen la curvatura de la terra i són ideals per a comunicacions continentals. A diferència de l'ona curta, les ones llargues no es reflecteixen a l'atmosfera, per això sofreixen menys baixades ("fade-out") de senyals (500km).

DRM

El principal avantatge és que la emissió digital dona una qualitat de so comparable a la emissió analògica d’FM, però per les freqüències i amb les distàncies d’ona llarga, mitjana i curta. DRM és robust per combatre els efectes del fàding i les interferències. Com és un medi digital, DRM pot transmetre altres dades digitals a part de la música digitalitzada, incloent text, imatges, i programari informàtic; per això, el tipus de dades RDS o les dades associades al programa s’assemblen al sistema de dades per DAB. DRM ha estat dissenyat especialment per utilitzar vells emissors dissenyats per àudio AM, així, no calen noves inversions per posar els adaptadors. La codificació i descodificació es pot fer amb un processador de la senyal digital. S’utilitza el format MPEG-4 per codificar el so. Les qualitats van dels 8 kbps als 20 kbps per un transmissió per un canal normal (10 Khz d’ample de banda). És possible arribar a qualitats més amunt de 72 kbps utilitzant més ample de banda (més que 10 Khz). S’utilitza la meitat d’ample de banda per emetre simultàniament en digital i en analògic. La qualitat desitjada depèn d’unes altres opcions com la robustesa al errors, la potència necessària, robustesa per a les condicions de propagació. Utilitza la mateixa modulació que el DAB, però utilitza la modulació QAM quan es transmeten per la mateixa freqüència la senyal analògica i la digital. Pot funcionar en simultàniament amb la AM, i està preparada per tenir vincles amb altres alternatives (per exemple: serveis de DAB o FM). DRM ha passat els tests en ona curta, ona mitjana (amb 9 i 10 Khz de separació de canals) i ona llarga.

HD Radio (AM)

In hybrid mode, the AM version can carry 36 kilobits per second of data for the main audio channel.

AM stations in ITU region II are usually considered to have 5 kHz of audio bandwidth. With double sidebands that are standard for most radio broadcasts, this results in a channel 10 kHz wide. In the other ITU regions of the world, the sidebands are 4.5 kHz, with a 9-kHz channel spacing. However, the AM version of HD Radio adds 10 kHz to each side of the center frequency, meaning that the signal extends out from the center frequency by 15 kHz. Again, with double sidebands, this results in an entire signal that is 30 kHz (three full channels) wide. Even so, the power level of the outer band signals is low compared to the main signal, and the COFDM subcarriers fit within a standard AM spectral mask.

Most analog AM radios have electronic filters to remove all signals more than 5 kHz away from the center frequency, but some "wideband" receivers do not filter this way, making the encoded signal audible. Even on radios that do have such a filter, it is possible to hear the digital sidebands by tuning up or down from the desired frequency by 10 kHz. Proposals for AM stereo have produced similar controversies. Because of the limited bandwidth assigned to AM stations, iBiquity's standard is largely incompatible with C-QUAM AM stereo broadcasts.

Digital television

Terrestrial

DVB-T

"revisar i ampliar->"És l’estàndard del consorci europeu de DVB per la emissió de televisió digital terrestre. Aquest sistema transmet en el format MPEG-2 per a àudio i vídeo. Utilitza la modulació COFDM amb 2000 o 8000 subportadores.

DVB-H

DMB

From Wikipedia.