These days most audio systems come with remote controllers. However, no such facility is provided for normal audio amplifiers. Such audio controllers are not available even in kit form. This article presents an infrared (IR) remote-controlled digital audio processor. It is based on a microcontroller and can be used with any NEC-compatible full-function IR remote control.

This audio processor has enhanced features and can be easily customised to meet individual requirements as it is programmable. Its main features are:

1. Full remote control using any NEC-compatible IR remote control handset
2. Provision for four stereo input channels and one stereo output
3. Individual gain control for each input channel to handle different sources
4. Bass, midrange, treble, mute and attenuation control
5. 80-step control for volume and 15-step control for bass, midrange and treble
6. Settings displayed on two 7-segment light-emitting diode (LED) displays and eight individual LEDs
7. Stereo VU level indication on 10-LED bar display
8. Full- function keys on-board for audio amplifier control
9. All settings stored on the EEPROM
10. Standby mode for amplifier power control

(Click to rotate Fig 2)

Circuit description

9A4_fig-1EC2_parts-listFig. 1 shows the block diagram of the remote-controlled digital audio processor. The system comprises Atmel’s AT89C51 microcontroller (IC1), TDA7439 audio processor from SGS-Thomson (IC4) and I2C bus compatible MC24C02 EEPROM (IC5). The microcontroller chip is programmed to control all the digital processes of the system. The audio processor controls all the audio amplifier functions and is compatible with I2C bus. All the commands from the remote control are received through the IR sensor. The audio amplifier can also be controlled using the on-board keys.

Microcontroller. The function of the microcontroller is to receive commands (through port P3.2) from the remote handset, program audio controls as per the commands and update the EEPROM. A delay in updating the EEPROM is deliberately provided because normally the listener will change the value of a parameter continuously until he is satisfied.

The 40-pin AT89C51 microcontroller has four 8-bit input/ output (I/O) ports.

Port 0 is used for indicating through LEDs the various functions selected via the remote/ on-board keys.

Port 1 drives the 7-segment display using 7-segment latch/ decoder/ driver IC CD4543.

Port 2 is pulled up via resistor network RNW1 and used for manual key control.

Pins P3.0 and P3.1 of the microcontroller are used as serial data (SDA) and serial clock (SCL) lines for the I2C bus for communicating with the audio processor (TDA7439) and EEPROM (MC24C02). These two lines are connected to pull-up resistors, which are required for I2C bus devices. P3.2 receives the remote commands through the IR receiver module. Pin P3.4 is used for flashing LED9 whenever a remote command is received or any key is pressed.

The microcontroller also checks the functioning of the memory (MC24C02) and the audio processor (TDA7439). If it is not communicating with these two ICs on the I2C bus, it flashes the volume level on the 7-segment displays.

Memory. IC MC24C02 is an I2C bus compatible 2k-bit EEPROM organised as 256×8-bit that can retain data for more than ten years. Various parameters can be stored in it.

To obviate the loss of latest settings in the case of power failure, the microcontroller stores all the audio settings of the user in the EEPROM. The memory ensures that the microcontroller will read the last saved settings from the EEPROM when power resumes. Using SCL and SDA lines, the microcontroller can read and write data for all the parameters.

For more details on I2C bus and memory interface, please refer to the MC24C02 datasheet. Audio parameters can be set using the remote control handset or the on-board keys as per the details given under the ‘remote control’ section.

Audio processor. IC TDA7439 is a single chip I2C-bus compatible audio controller that is used to control all the functions of the audio amplifier. The output from any (up to four) stereo preamplifier is fed to the audio processor (TDA7439). The microcontroller can control volume, treble, bass, attenuation, gain and other functions of each channel separately. All these parameters are programmed by the microcontroller using SCL and SDA lines, which it shares with the memory IC and the audio processor.

Data transmission from the microcontroller to the audio processor (IC TDA7439) and the memory (MC24C02) and vice versa takes place through the two wire I2C-bus interface consisting of SDA and SCL, which are connected to P3.0 (RXD) and P3.1 (TXD) of the microcontroller, respectively. Here, the microcontroller unit acts as the master and the audio processor and the memory act as slave devices. Any of these three devices can act as the transmitter or the receiver under the control of the master.


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