Nov 052014
 

Today I analyzed a PROZ84 board from a 2.3 with some “odd” sounds.
There was audible distortion, varying with the pitch and the partial wave number.

The oscilloscope revealed (sorry, no image here) that in cases of highest distortion levels only every other sample appeared on the DAC outputs, while the others were at ~0 volts. Half of the partial wave samples having a value of zero is hardly a problem of the wavetable playback, but most likely to be caused by wrong data in the wave RAM.

It turned out that one of the 74S257’s multiplexers outputs in the write circuit had a stuck low output on RAM address line A0, so only even samples were actually written into RAM, with the odd locations containing what is left in the DRAM’s capacitors – in this case, 0.

 

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