M2TECH JOPLIN MKII, Руководство пользователя

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JOPLIN M
K
II
384
k
H
z
/32bit ANALOG-TO-DIGITAL CONVERTER
REVISION P
R
A – FEBRUARY 2015
Copyright © 2015, M2Tech Srl
51
M2Tech
www.m2tech.biz
8.3.9. MPX filter
Stereo is obtained in FM broadcast by carrying the difference (L-R) information
modulated on a 38kHz carrier, while the sum information (L+R, that is the mono
sound) is placed in the channel’s baseband. Tuner are aware of receiving a stereo
broadcast because of a pilot tone of 19kHz which is also used to recreate the 38kHz
frequency necessary to demodulate the difference information to be blended with the
sum information to get the two stereo channels (in fact, (L+R)+(L-R) = 2L, while
(L+R)-(L-R) = 2R). The 19kHz tone is beyond the bandwidth of the FM tuner (which is
limited to 16kHz), but is well into the ADC bandwidth even at 44.1kHz sampling
frequency. A 19kHz tone can be heard by a young ear, and may also generate
intermodulation distortion, so it could be useful to eliminate it. The MPX filter does
this by cutting away a very narrow band around 19kHz (Figure 21). It presence is
quite inaudible, but the effect on the overall sound can be surprisingly good.