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Mono Stereo
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John Rumm
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Posts: 25,191
Mono Stereo
On 16/04/2019 23:24,
wrote:
Does it give a better S/N ratio?
IIUC, for reasons of backward compatibility, a stereo FM transmission
encodes stereo image into a pair of sum and difference signals[1]. So
mono receivers can simply process the sum signal and get both channels
combined into a single mono channel. A stereo receiver will need to
add/subtract the difference signal from the main combined channel to get
the separate L & R channels.
The sum signal is transmitted using FM and the difference signal is
amplitude modulated onto a sub carrier shifted up from the main carrier.
(IIRC there is also a pilot tone included just above the baseband audio
to signal the received that its a stereo transmission). Since the AM
modulation will suffer more in poor reception conditions it can also
introduce hiss. Many radios hence include a Stereo/Mono switch to elect
for mono with no hiss. Some of the posher car radios actually use a
mixer for the stereo decoding, so they can switch the stereo in and out
in gradual way depending on how much noise is being detected.
[1] Conceptually not unlike the way colour was added to mono TV
--
Cheers,
John.
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