AudioMUX Click Board™

Product Code: MIKROE-3344

Featuring a set of 3.5mm vertical jack connectors onboard, it allows a simple connection to an existing audio chain. Because of its very good sound characteristics, including 0.01% of total harmonic distortion (THD), signal to noise ratio (SNR) of 100dB, 84dB of maximum attenuation, and more, the AudioMUX Click Board™ represents an ideal solution for developing and building audio applications that can be used for various HiFi systems, home stereo components, computer speakers with selectable input, and many other similar audio applications that require a digitally controlled input selection and audio processing. How Does The AudioMUX Click Board™ Work? The AudioMUX Click Board™ is based on the TDA7468D IC from STMicroelectronics, which is a two-band digitally controlled high-quality audio processor with BASS ALC feature and a four-channel input selector. This integrated audio processor is quite popular, and it can be found in several brands of HiFi and home stereo systems. Despite its simplicity, it offers a full set of options required for a realization of the four-channel audio multiplexer with volume, balance, and tone controls.

The TDA7468D IC is digitally controlled over the I2C interface; however, the internal sections are purely analog circuits, with a separate analog GND reference. There are four input channels which can be selected over the I2C interface. The IC accepts up to 2.5V peak to peak at its inputs. The impedance of the input terminals is 50 kΩ, and each input is decoupled by a 440nF capacitor. There are not many registers in this IC, which makes the firmware development very easy. Each channel is routed to a vertically mounted 3.5mm jack, which allows the AudioMUX Click Board™ to be easily interfaced to an existing audio chain. Some registers have a single function, while others are used to control more functions. The datasheet of the TDA7468D clearly explains each register and its functions. After the input is selected by setting the specific bits in the INPUT SELECT register, the audio signal is routed to the first gain stage. It is possible to adjust the gain of the input signal up to +14dB, in 2dB steps. This is very useful when the amplitude of the input signal needs to be adjusted (gain staging) The signal is then fed to the first (pre-EQ) volume control section which is able to attenuate the signal down to -63dB, in 1dB steps. If the signal is too high, a clipping might occur when the equalization is applied in the next section. Therefore, lowering the volume in this section will allow more headroom for the equalizer (EQ) section. The volume in this section is determined by the first 6 bits of the volume control register, referred to as the VOLUME 1 bitfield. The last 4 bits are related to the post EQ volume section and are referred to as the VOLUME 2 bitfield. Both volume sections feature independent volume control registers (VOLUME LEFT and VOLUME RIGHT), allowing balance function to be implemented. The EQ sections can adjust bass and treble frequencies in the range from -14dB to +14dB, in 2dB steps. The frequency response of these sections is determined by external RC elements. The Click board™ has these values optimally selected for the best performance. The low-frequency range is processed by a T type bandpass filter with its center frequency at around 32Hz, while the high-frequency range is processed by a high pass filter with the -3dB attenuation point at about 3kHz. A single register is used to control the entire EQ section (TREBLE & BASS register). After the sound is processed by the EQ section, there is an output stage, which allows attenuation of -24dB in 8dB steps. This post-EQ stage allows the volume to be fine-tuned and adjusted to the input stage of an audio amplifier. The volume of this section is controlled by the VOLUME 2 bitfield, in two independent registers, one for each channel (VOLUME LEFT and VOLUME RIGHT) The BASS ALC is basically a compressor, applied to the low-frequency range. It is a very useful feature, keeping the low-frequency content even when the volume is set to a high value. The low-frequency band will be dynamically attenuated when the programmed threshold value is exceeded, after the programmed attack time. The BASS ALC is automatically turned off when the EQ is set to attenuate low frequencies, as there is no point to use it in that case. There are also some additional settings for the BASS ALC function. For the complete list of features, please refer to the datasheet of the TDA7468D. The BASS ALC function is controlled by bits in the BASS ALC register. The output of the TDA7468D IC can be enabled or disabled by a single control bit of the OUTPUT register. When building audio applications in a purely digital surround, special care should be taken to isolate the audio signal path as much as possible, in order to avoid any noise at the output. The AudioMUX Click Board™ is equipped with two 0 Ω resistors (jumpers) at strategic positio