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May 28, 2013
about 4 years ago
A word of advice on the musical instrument shield speaker connection: The way the board is wired the sleeve connection on the 1/8 TRS stereo jack is connected to a pin called GBUF on the VS1053. According to the datasheet this pin should not be connected to ground. http://www.vlsi.fi/fileadmin/app_notes/vs10XXan_output.pdf
This means if you are plugging the shield into a stereo, another embedded device, or computer etc that has its sleeve connected to ground (which is common) then you may be hurting your musical instrument shield and degrading the audio performance. If you are connecting to headphones or non amplified speakers then there will be no problem because these are isolated from ground.
I made a simple fix to enable connecting the sleeve to ground for my application. There are many different ways to make this fix. This is just one. Cut the PCB trace going from GBUF to the TRS jack. Solder a wire from GND to pin 1 of JP3. Now you have a ground referenced audio output. Be aware that the output pins LEFT and RIGHT now have DC bias relative to the TRS sleeve on them which will need to be handled appropriately depending on what you are connecting your shield to. For example if you want the RIGHT and LEFT signals to have zero bias voltage insert a large capacitor in series with each signal to block the DC. If you are feeding the outputs of the shield into another 3.3V or 5 V device it may be best to make a direct connection because there will be approx 1.8V bias on the pins.
What does GBUF do and why is it bad to connect it to ground? To make a long story short, audio signals are AC and have a zero average value (DC). It is important to drive loudspeakers, some amplifiers, and headphones with zero average waveforms otherwise they will not sound right or may get damaged. Since the VS1053 is powered by monopolar power supplies it will have a DC bias on its audio output pins. The DC bias (average DC) is typically ½ the chip voltage. This is done so that both positive and negative waveforms can be accurately played out of the chip. A clever way to zero out this DC bias when connecting to headphones or speakers is to use a reference voltage that has a value of this bias voltage. GBUF is that reference. If you then measure the voltage across the LEFT and GBUF or RIGHT and GBUF outputs the average voltage will be zero.
GBUF is created inside the chip using some form of amplifier, kind of like an op amp. There is probably a little bit of signal processing inside that sets GBUF to precisely the average voltages of RIGHT and LEFT. If you connect GBUF to ground you short circuit the internal amplifier in the chip to ground. This can cause all sorts of havoc inside the chip. At best it will distort the output amplifiers because there will be excess currents flowing. At worst the chip will overheat and become damaged.
I hope this will help people out with their projects. Fixing this made noticeable improvements to the music quality of my project. Happy tinkering.
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