LP3950SL datasheet(19/31 Pages) NSC

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Audio Synchronization

The LEDs connected to the RGB outputs can be synchro-

nized to incoming audio signal with Audio Synchronization

feature. Audio Synchronization has two modes. Amplitude

mode synchronizes LEDs based on the peak amplitude of

the input signal. In the amplitude mode the user can select

one of three amplitude mapping options. The frequency

mode synchronizes the LEDs based on bass, middle and

treble amplitudes (= low pass, band pass and high pass

filters). The user can select between two different responses

of frequency for best audio-visual user experience. Both of

the modes provide a control for speed of the mapping with

four different speed configurations. Programmable gain and

AGC (Automatic Gain Control) function are also available for

adjustment of the optimum audio signal mapping. The Audio

Synchronization functionality is described more closely be-

low.

INPUT SIGNAL TYPE

The LP3950 support four types of analog audio input signals

for audio synchronization

Single ended audio

Differential audio

Stereo

Single ended and differential audio.

Figure 15 shows how to wire the LP3950 audio inputs case

by case (NC = Not Connected).

USING A DIGITAL PWM AUDIO SIGNAL AS AN AUDIO

SYNCHRONIZATION SOURCE

If the input signal is a PWM signal, use a first or second

order low pass filter to convert the digital PWM audio signal

into an analog waveform. There are two parameters that

need to be known to get the filter to work successfully:

frequency of the PWM signal and the voltage level of the

PWM signal. Suggested cut-off frequency (-3dB) should be

around 2 kHz to 4 kHz and the stop-band attenuation at

sampling frequency should be around -48dB or better. Use a

resistor divider to reduce the digital signal amplitude to meet

the specification of the analog audio input. Because a low-

order low-pass filter attenuates the high-frequency compo-

nents from audio signal, MODE_CONTROL=[01] selection is

recommended when frequency synchronization mode is en-

abled. Figure 23 shows an example of a second order

RC-filter for 29 kHz PWM signal with 3.3V amplitude. Active

filters, such as a Sallen-Key filter, may also be applied. An

active filter gives better stop-band attenuation and cut-off

frequency can be higher than for a RC-filter.

To make sure that the filter rolls off sufficiently quickly, con-

nect your filter circuit to the audio input(s), turn on the audio

synchronization feature, set manual gain to maximum, apply

the PWM signal to the filter input and keep an eye on LEDs.

If they are blinking without an audio signal (modulation), a

sharper roll-off after the cut-off frequency, more stop-band

attenuation, or smaller amplitude of the PWM signal is

required.

Schematic Diagram

20129327

FIGURE 15. Wiring Diagram for LP3950 Audio Inputs

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