The 3D audio enhancement improves the apparent stereo
channel separation whenever the left and right speakers are
too close to one another, due to system size constraints or
equipment limitations.
Application Information (Continued)
The value of Rf is 30kΩ.
The last step in this design example is setting the amplifier’s
−3dB frequency bandwidth. To achieve the desired 0.25dB
pass band magnitude variation limit, the low frequency re-
sponse must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well within the 0.25dB
desired limit. The results are an
An external RC network, Shown in figure 1, is required to
enable the 3D effect. The amount of the 3D effect is set by
the R5 and C7 or C3D ADJ. Decreasing the value of R5 will
increase the 3D effect. Increasing the value of the capacitors
(C7 or C3D) will decrease the low cutoff frequency at which
the 3D effect starts to occur., as shown by Equation 13.
F3D(–3dB) = 1 / 2π(R3D)(C3D
)
(13)
fL = 100Hz/5 = 20Hz
and an
Activating the 3D effect by applying VDD to pin 9 (3D Control)
will cause an increase in gain by a multiplication factor of (1
+ 20kΩ/R5). Setting R5 to 20kΩ will result in a gain increase
by a multiplication factor of (1 + 20kΩ/20kΩ) = 2 or 6dB
whenever the 3D effect is activated. The amount of per-
ceived 3D is also dependent on many other factors such as
speaker placement and the distance to the listener. There-
fore, it is recommended that the user try various values of R5
and C3D to get a feel for how the 3D effect works in the
application. There is not a “right or wrong” for the effect, it is
merely what is most pleasing to the individual user. Take
note that R3 and R4 replace R2, and R7 and R6 replace R8
when 3D mode is enabled.
fH = 20kHz*5 = 100kHz.
As mentioned in the External Components section, R1 and
C1 create a highpass filter that sets the amplifier’s lower
bandpass frequency limit. Find the coupling capacitor’s
value using Equation (12).
C1 ≥ 1/(2πR1fL)
(12)
The result is
1/(2π*20kΩ*20Hz) = 0.398µF.
Use a 0.39µF capacitor, the closest standard value.
The product of the desired high frequency cutoff (100kHz in
this example) and the differential gain, AVD, determines the
upper passband response limit. With AVD = 3 and fH
100kHz, the closed-loop gain bandwidth product (GBWP) is
300kHz. This is less than the LM4888’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance-restricting
bandwidth limitations.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
=
Figures 3 through 6 show the recommended two-layer PC
board layout that is optimized for the 24-pin SQ package.
These circuits are designed for use with an external 5V
supply and 8Ω, 4Ω, 3Ω speakers.
These circuit boards are easy to use. Apply power and
ground to the board’s VDD and GND pads, respectively.
Connect the speakers between the board’s −OUTA and
+OUTA and OUTB and +OUTB pads.
NATIONAL 3D ENHANCEMENT
The LM4888 features a 3D audio enhancement effect that
widens the perceived soundstage from a stereo audio signal.
www.national.com
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