I am new to this list and got in in the middle of this discussion, but Here are some thoughts on 0 VU and what it means in the digital domain. You probably already know that an A/D converter when fed silence normally produces an output that is exactly halfway between the number 0 and 0xFFFF for a 16-bit converter. For a converter with more or less bits, the maximum reading is all bits on so it is 2^N power. The halfway point makes the D/A converter output a voltage that is halfway between 0 and the maximum peak voltage that particular A/D converter can put out. All that being said, the A/D converter can't go so much as one bit more than maximum or one bit less than minimum so there is absolutely no head room. If you send out a sine wave to the A/D converter, any part of that wave that goes above the maximum input threshold or below the minimum level will cause the output to flatten out. If you graph the levels, you would see a sine wave with a flat spot on top and another on bottom. A little flat spot probably doesn't hurt anything if it happens infrequently, but the more it happens, the worse the distortion. In other words, if you stay below the hard limits, the distortion is next to nothing, but it hits big time when you reach the limits of the sample value the A/D converter was designed to read. As to how you can tell when you are flat-topping, one could run a program designed to read the data for each channel and look for extreme limits such as all 0's or all 1's for one or two samples in succession and register this. In the amateur experimenting I have done with writing level detectors for 8-bit audio, I look for low values between 0 and some arbitrary value such as 1 to 3 and also values between 0xFF and maybe 0xFc to 0xFE. If I see any of those, the alarm goes off because that is darn close to the limit and may actually be the limit. I haven't messed much with stereo 16-bit audio such as a .wav file, but the bits of the left-channel sample and the right-channel sample are interleaved so you'll have to reassemble the 16-bit word for each channel to test it. Cheers. Martin McCormick WB5AGZ Stillwater, OK OSU Information Technology Division Network Operations Group
