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 Post subject: Audio volume decreasing
PostPosted: Tue May 14, 2013 8:10 pm 
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Lately I've been playing with the audio circuit of my pc engine cd interface unit (briefcase model). With the help of tiido I discovered that the system generates pcm sounds internally in the ifu (interface unit). Without modifications the pcm sounds too loud and drowns out the cd audio. I found the source of the pcm audio (again thanks to tiido), but I havn't been able to decrease the volume of it. I tried putting a resistor in series up to 100k ohm, connecting a pull down resistor both at the source and after the first resistor in the circuit to create a voltage divider. The voltage divider idea did decrease the volume but sounds like it loses a little audio quality when I do that. I'm open to suggestions. My next idea is to find the cd audio traces and look for resistors to weaken on that part of the circuit instead.


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PostPosted: Wed May 15, 2013 7:48 am 
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If the D-A converter of the ADPCM circuit uses a simple R2R ladder, then putting in high-resistance parts might override the weaker resistors for the LSBits... that's just one guess. How about adding a diode or capacitor as a buffer first?

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PostPosted: Wed May 15, 2013 5:03 pm 
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Drakon wrote:
Lately I've been playing with the audio circuit of my pc engine cd interface unit (briefcase model). With the help of tiido I discovered that the system generates pcm sounds internally in the ifu (interface unit). Without modifications the pcm sounds too loud and drowns out the cd audio. I found the source of the pcm audio (again thanks to tiido), but I havn't been able to decrease the volume of it. I tried putting a resistor in series up to 100k ohm, connecting a pull down resistor both at the source and after the first resistor in the circuit to create a voltage divider. The voltage divider idea did decrease the volume but sounds like it loses a little audio quality when I do that. I'm open to suggestions. My next idea is to find the cd audio traces and look for resistors to weaken on that part of the circuit instead.


Maybe buffer it before the voltage divider?


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PostPosted: Thu May 16, 2013 5:14 am 
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So a cap followed by a voltage divider? Okay I can try that.


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PostPosted: Fri May 17, 2013 3:18 pm 
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The problem is likely loss of right DC bias with your voltage divider.

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