FM stereo
In the late 1950s, several systems to add
stereo to FM radio were considered by the
FCC. Included were systems from 14 proponents including Crosley, Halstead, Electrical and Musical Industries, Ltd (
EMI), Zenith Electronics Corporation and General Electric. The individual systems were evaluated for their strengths and weaknesses during field tests in
Uniontown, Pennsylvania using
KDKA-FM in Pittsburgh as the originating station. The Crosley system was rejected by the FCC because it degraded the
signal-to-noise ratio of the main channel and did not perform well under multipath RF conditions. In addition, it did not allow for
SCA services because of its wide FM sub-carrier bandwidth. The Halstead system was rejected due to lack of high frequency stereo separation and reduction in the main channel signal-to-noise ratio. The GE and Zenith systems, so similar that they were considered theoretically identical, were formally approved by the FCC in April 1961 as the standard stereo FM broadcasting method in the USA and later adopted by most other countries.
[1]
It is important that stereo broadcasts should be compatible with mono receivers. For this reason, the left (L) and right (R) channels are algebraically encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels in the single loudspeaker. A stereo receiver will add the difference signal to the sum signal to recover the left channel, and subtract the difference signal from the sum to recover the right channel.
The (L+R) Main channel signal is transmitted as baseband
audio in the range of 30 Hz to 15 kHz. The (L−R) Sub-channel signal is modulated onto a 38 kHz
double-sideband suppressed carrier (DSBSC) signal occupying the baseband range of 23 to 53 kHz.
A 19 kHz
pilot tone, at exactly half the 38 kHz
sub-carrier frequency and with a precisely defined phase relationship to it, is also generated. This is transmitted at 8–10% of overall
modulation level and used by the receiver to regenerate the 38 kHz
sub-carrier with the correct phase.
The final multiplex signal from the
stereo generator contains the Main Channel (L+R), the pilot tone, and the sub-channel (L−R). This composite signal, along with any other sub-carriers (SCA), modulates the FM transmitter.
Converting the multiplex signal back into left and right audio signals is performed by a stereo decoder, which is built into stereo receivers.
In order to preserve stereo separation and signal-to-noise parameters, it is normal practice to apply pre-emphasis to the left and right channels before encoding, and to apply de-emphasis at the receiver after decoding.
Stereo FM signals are more susceptible to
noise and multipath
distortion than are mono FM signals.
[2]
In addition, for a given RF level at the receiver, the
signal-to-noise ratio for the stereo signal will be worse than for the mono receiver. For this reason many FM stereo receivers include a stereo/mono switch to allow listening in mono when reception conditions are less than ideal, and most car radios are arranged to reduce the separation as the signal-to-noise ratio worsens, eventually going to mono while still indicating a stereo signal is being received.
------------------------------------------ Tirado da wikipedia
So botei isso aqui pra não falar sozinho.
Vejamos, isso é uma típica multiplexação por FREQUENCIA (canal soma e canal diferença ficam em pedaços diferentes da faixa).
SE FOSSE intercalado, então seria ao longo do tempo : canal soma, canal diferença, canal soma, canal diferença...
FDM vs TDM...
Trocando em miudos :
Os 250khz disponíveis para transmissão de um canal FM são separados em "pedaços" (Caracteristica tipica da multiplexação por frequencia).
repetindo novamente : FM Stereo multiplexa os dois canais de audio na forma FDM e não TDM. (Na realidade, o transmissor cria dois canais, em um soma canal direito com esquerdo, e no outro subtrai canal direito e esquerdo, e injeta esses dois canais derivados em partes diferentes do espectro disponivel para a emissora).
Mais uma vez : FM Stereo é um sistema FDM e não TDM...
Bem, não tenho 5 anos de engenharia e 3 de especialização. Isso eu aprendi lendo um livrinho sobre rádio aos 5 anos de idade.
Veja o mapa do uso da banda passante disponivel em um canal de FM Stereo (Ignorem a parte azul):
Anexo 11713
Lembrando que quando se fala em INTERCALAR subentende-se TDM (Multiplexação por tempo).
Na prática, o T Mágico só serviria CASO fossem dois canais diferentes (Realmente diferentes) justamente para ocorrer uma "Multiplexação" (Tenho algumas razões para botar entre aspas) no dominio da frequencia e não do tempo.
Se você intercalar (O que eu acho possível porem inútil) vai dividir a banda por dois. POIS A QUANTIDADE DE SIMBOLOS POR SEGUNDO EM UM CANAL É PROPORCIONAL À BANDA PASSANTE E A RELAÇÃO SINAL RUIDO.
Agora eu não me sentiria muito feliz em fazer 5 anos de engenharia e 3 de especialização, entrar num tópico com um enorme "sujeito suposto saber" (Sou engenheiro logo estou certo), e esquecer essas minucias para se trair logo em seguida.
