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FM-100 Page 8
The second neat feature is the voice detection capability. Voice detection is
used with the auto AGC feature of the FM-100. Essentially pin 8 of U7 goes
high when there is a varying signal level seen on the microphone, as compared
to the constant level of background noise. When pin 8 goes high, it turns
JFETS Q5 and Q6 on which act like voltage controlled pots. The more they are
turned on, the less resistance is seen from source to drain. This in turn mutes
the audio from the line level inputs, allowing only the microphone to be heard
when there is voice detection active.
U4, the microcontroller, has the ability to override this feature by setting the
connection marked AUTOAGC high, thereby turning Q7 on, which disallows Q5
and Q6 from turning on. The microcontroller can also mute the microphone
audio by setting the MUTE line high, thereby turning Q4 on, grounding out all
the microphone audio. R21 and C37 smooth out the switching transitions so
that there is very little popping heard that is apparent in most switches. This
MUTE line is also used to turn on the microphone line when the speaker
sounds a tone on the unit. This prevents the tone from being transmitted over
the air.
In our description of the audio we will only be talking about the right channel
circuitry in the FM-100, since the left channel is identical. Line level audio is
applied to J5 or J6, and then on to R62 or R84. R62 and R84 control each line
level’s transmission volume. These levels are then summed in U8:A, so now
both signals are present on pin 1 of U8. This signal is then sent through R63,
which when combined with Q5 gives the voice detector circuitry the ability to
mute the audio. The audio continues to U8:B, where it is summed with the
microphone audio. The microphone audio level is controlled by R26, and is sent
to both the left and the right channels. The microphone audio combined with
the line level audio passes then on to U9, a switched capacitor filter.
U9 is an 8th order Butterworth switched capacitor low pass filter. It doesn’t
require reactive components like capacitors, inductors and resistors to set its
operating frequency, instead its frequency is set by a clock reference. In this
case it is 1.5MHz from the PLL that we discussed earlier. The actual operating
frequency is then 1.5MHz divided by 100, or 15KHz. Anything over this
frequency, like high frequency tones from cheap CD players and people with
squeaky voices, is simply cut out to prevent interference with the stereo
subcarrier in the BA1404.
U9 also has a built in op-amp which we use for our 75uS equalization. This
equalization conforms with the standards for North America FM broadcast. The
audio is then sent to U11:A for further low pass filtering. This filter gets rid of the
1.5MHz clock signal that may be present in the audio from the MAX291 (U9).
Now we have nice clean audio all doctored up for serving across the airwaves.
The audio is then sent to R23, which sets the relative audio level to be
transmitted.
Содержание FM-100
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