TF Card MP3 Decoder Board -- GPD2856A

This TF card MP3 decoder board uses the GPD2856A MP3 decoder IC, and has an on-board 2-watt L/R mixed mono audio amplifier IC (8002). This module is the updated version of the earlier GPD2846A TF card MP3 decoder board, and in this latest version, there is a TF card slot for a micro-SD memory card, and an LED power indicator. The board measures 35 mm × 22 mm, and requires only a power supply, direct speaker connection, and switches. The manufacturer recommends a working voltage of 3.7 V or 5 V.

Review

At some point, all engineers wonder what it would be like to build a small MP3 player using one of the latest ICs coming out. I was doing just that looking for a simple IC and found the GPD8256A single chip MP3 player solution. Just as I was about to buy it, I found a readymade module that potentially saved me hours of soldering work and squinting into my microscope. Long live the People’s Republic of China!

The GDP2856A is a single chip MP3 player solution that was available in version 2.0 form at some time in 2013. It was for use in MP3 player applications with FM radio, and therefore designed to operate with RDA5807, BK1080, and RTC6207E single chip FM radio solution ICs.

Looking at the pinout, I noticed that that this IC has stereo output, and a USB interface, which they were not able to implement on the board due to lack of space; hence, it has great hacking potential for hobbyists. In addition, this IC also has some useful functions that they were not able to provide because there was space for connecting only three switches. For example, it also has an audio equalizer function that can cycle through normal, classic, bass, live, rock, and pop, sound options. We could implement this; however, this mod requires additional resistors and a switch. In the following sections of this article, I show how keyboard decoding works and how the additional functions could be implemented.

Keyboard Decoding/Hack

The keyboard decoding is very simple, and from the pinout, we can see that pin 8 (AD-key) decodes the switch input. Usually microcontrollers have an analogue-to-digital converter (ADC) for this type of input where each switch feeds a uniquely different voltage and the ADC measures it to determine which switch it was. Hence, each switch has a resistor associated with it, and they all form one-half of a potential divider (PD) network with resistor R17.

The centre point of the resistive PD connects to pin 8 (AD-key), which is a port that measures the voltage received. When the user presses S1, then resistor R22 forms a PD with R17, and the voltage formed at the centre goes to pin 8. If the user presses S2, then R24 forms a PD with R17, and so forth.

Looking at the IC documentation, I learned that my Chinese was not very good, however, I do speak the universal computer language of 1’s and 0’s, and therefore it was not difficult to figure out. Without the radio IC, there are two possible options for switch configurations and their functions. Since the switches provided are for Previous, Play/Pause, and Next; we can deduce that this is the default keyboard arrangement. In this arrangement, the equalizer function is through switch 8. Hence, in order to enable this hack we will require a 100 kΩ resistor forming a PD with R17. However, the resistors do not need to be on the board. You just need a 22 kΩ resistor and 100 kΩ resistor forming a PD across VCC and ground, together with a switch. The output of the PD then connects to pin 8 (AD-key).

Stereo Output Hack

Although they mix the left and right channels of the audio by joining the output pins, we can see that the GPD2856C has stereo output. DAC-Output-Left (pin 4), and DAC-Output-Right (pin 6) will have the necessary outputs.

Cost/eBay

This TF card module is usually for sale all over the internet, and you can pay as little as 99p for it and it comes all the way from China. Good for them I say! It is a nice bit of electronics the size of a stamp and can fit into almost anything. No doubt, there will be many versions of this module with varying degrees of quality, but generally, it is machine manufactured so the tolerances will be small.