Line Level Audio?

[jbaum81]

Hi All,

I have a Mercury II Artix 7 based FPGA interfaced to my 6502 project to drive my video through HDMI, It's a good solution that seems to be working pretty well so far and I'd like to leverage the included ADC and DAC for line in audio to load old programs, as well as use the DAC to build a sound card. First things first I need the circuits to connect my devices to start building the code, but I really don't know where to start.

I'm hoping someone can point me to some circuit examples that would interface between an amplified (headphone jack) and a ADC (What's the reference voltage?), also I'd need to know how to go from the DAC to a line level output.

[GARTHWILSON]

When I worked at TEAC in 1982-83, they considered line level in the semi-pro high-impedance unbalanced stuff to be -10dbV, or 316mV RMS, which is .894Vp-p for a sine wave. If you can swing 5Vp-p, that's +15dB. (Professional studio line level is much higher, at +4dbm into 600Ω balanced, which is 1.23V for 0VU.) Voltages for 8Ω earbuds will be lower. I have not measured any of the modern ones, so you might want to do that and experiment.

I have example circuits (which I'm actually using on my workbench computer) for A/D and D/A converters on the circuit potpourri page of my 6502 primer at http://wilsonminesco.com/6502primer/potpourri.html#ADC and http://wilsonminesco.com/6502primer/potpourri.html#DAC . They're only 8-bit; but 8-bit is capable of sounding better than the cassettes at the end of their reign just before CDs took over which is pretty darned good, and these converters are fast. You don't have to tell the A/D converter to start a conversion and then come back later when it's finally done. It's almost fast enough to just read it as a memory location, and select it, and find the data available by the end of the same cycle. The D/A is even faster, settling to ½lsb is 150ns (right—not µs, but ns).