BU9253AS_05 Datasheet, PDF (12/17 Pages) Rohm – Karaoke/Surround Digital Echo

●Description of external components
< BU9253AS/FS > (Refer to Fig. 27.)
・Echo level
Echo signal level is determined by the gains of A/D side operation amplifier and the D/A operation amplifier, since there
is no gain in A/D → SRAM → D/A. Path in applied circuit example,
Gain at A/D side : R4 ÷ R5 = 10K ÷ 20K = 0.5
Gain at D/A side : R11 ÷ R9 = 15K ÷ 10K = 1.5
When the original signal is defined as 1, the echo signal level becomes the feedback ratio.
Echo signal feed back ration = 0.5 x 1.5 = 0.75 = 75%
・Echo level setting method
Echo level can be set by adjusting DC voltage by VR1. It does not change in low voltage range, therefore, R8 is added,
and it can be adjusted by VR pot.
・Clock frequency and delay time
Clock frequency and delay time are as the following equations.
Sample frequency = clock frequency ÷ 6
Delay time = (1 ÷ sample frequency) x 8192 (SRAM 8192 bits)
As an example, when clock frequency is 375kHz, then sample frequency is 375kHz ÷ 6 = 62.5kHz. Therefore, the delay
time is:
Delay time = (1 ÷ 62500) x 8192 = 131ms
Clock frequency is determined by R2, C1 connected to CR terminal (pin 18 in BU9253AS, pin 16 in BU9253FS). When
C, R are changed, oscillation frequency changes, but the delay time is kept fixed.
・LPF frequency characteristic
When the band width is set too narrow, echo voice becomes worse, and when band width is set wide, A/D cannot be
converted, and S/N becomes worse. Band width of applied circuit example is 2kHz.
・A/D, D/A external capacitors C3, C6
Feedback is set by resistance and capacitor so that output follows even when input signal level and frequency of
A/D, D/A change. Variation in the externally attached capacitor can influence the sound quality. A small capacitor can
generate noise, while a capacitor that is too large will attenuate. In applied circuit example, 0.01uF is selected to avoid
influence upon LPF.
・Mute
Can be controlled by input voltage of MUTE pin. It takes more than one SRAM cycle for switching between mute → mute
release (L → M → H). It is necessary for initializing SRAM and insuring stable status before changing operating modes.
MIX OUT output is muted, so original sound and echo sound output is stopped.
MUTE pin
H
M
L
Mode
Mute release (operating)
Mute
Clock stop & mute
In the applied circuit example, MUTE is set by R3, C2. In the specifications, voltage range of M is 1.6V ~ 2.8V, therefore,
it is necessary to change R3,C2, and set the time of 1.6V ~ 2.8V over 131ms. In consideration of fluctuation, C2 = 22uF,
R3 = 22kΩ. When MUTE pin is controlled by the microcontroller, select M range to 1.6V ~ 2.8V and keep M level over
131ms.
・Maximum signal input
When original signal and echo are in phase and mixed, both the signals are added to reach maximum output. In order
not to clip this signal, the maximum output at power source voltage of 5V is about 4Vpp. When echo feedback ratio is
0.75, then maximum signal input becomes about 1.0Vpp.
Vout MAX＝1／(1－A)×Vin
＝1／(1－0.75)×Vin＝4Vpp
∴Vin＝1Vpp
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