M2tech – M2TECH Joplin User Manual

Page 28

JOPLIN
384

/32bit ANALOG-TO-DIGITAL CONVERTER

REVISION P

B – SEPTEMBER 2012

M2Tech

www.m2tech.biz

Figure 20

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.