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--== audio specs and reqs ==--


1. have sound from AY/YM and beeper/PWM-covox
2. have some extra line-ins
3. independent volume regulation for all of them
4. line out/headset output also volume regulated
5. be able to drive 32 Ohm headsets
6. Use only +12v power supply for audio
7. line-out also goes to extra video output pins




--== audio decisions ==--

(YM2149)

voltage on outputs (loaded as per rev.C)
2.26v min, 3.40v max => 1.14 Vp-p [me] (max 1.35Vp-p by datasheet)
2.22v min, 3.36v max => 1.14 Vp-p [dimka_m]
2.29v min, 3.42v max => 1.13 Vp-p [Nik X]

1.5*1.14 Vp-p (A/C + half B) to 2.0 Vp-p is K=1.17;


(AY8910)

Per datasheet, max 2mA current per output, 2 Vp-p @ 1k

(KC89C72)

0.5v min 1.27v max => 0.77 Vp-p [Nik X]
0.5v min 1.29v max => 0.79 Vp-p [Nik X]




(PT2258 related)

1. Use PT2258 to control volume:
  1 channel for AY/YM
  1 channel for beeper/pwm-covox
  1 channel for output volume
  3 channels for line-ins (same number of line-ins as in rev.C)
  total 2 ICs

2. max Vp-p for PT2258 is 2v (1v amplitude or 0.7Vrms, see PT2257 datasheet and THD vs amplitude there).
   

3. Vcc for PT2258 is ≈9v, can't be 12v. So, there's AVCC9 regulated voltage


(headamp related)

1. Use TPA6120 to drive line-one/headset: high power AND high quality

2. Use AVCC12 (non regulated, directly from AT/ATX and passively filtered)
   to get maximum output swing thus power

3. To compensate for inevitable series resistors at the TPA6120 outputs,
   use opamps.

4. Output resistors should be of high thermal power, preferably MELF or 1205


(general)

1. Use metal film feedback resistors (bigger than 0805, best if MELF),
   NP0/C0G capacitors for low pass filters. Footprint for 1206+0805 is ideal for that.

2. DC blocking/high pass filtering can't be done with polypropylene/etc. caps (too big!),
   so we have to use electrolytics. Try to have related cutoff freqs as low as possible (<<20Hz)
   to decrease distortion from them. Typical distortion disappearance happens at *20 cutoff frequency

Gain (or loss) distribution.
1. YM seems to have 1.14 Vp-p (or 0.57 ampl) outputs each. Not yet known what is with AY, but that's for future.
2. for PT2258, supposedly optimal ampl. is 1V, more leads to distortion (guessed from datasheet for PT2257).
   So we'll keep amplitude from each internal source like that. Afterwards it could be decreased before global
   volume control and will be amplified by audio output circuit.


(low-pass filtering)

3-rd order butterworth, 30kHz -3dB.
First stage for AY is 2nd order Q=1, (multiple-feedback). 1st stage is 2nd order to keep input impedance more or less constant for AY/YM.
For beeper there's low impedance buffer, we use single 3rd-order opamp based stage.
Second stage is just 1st-order RC

(high-pass filtering)

"distributed" 2nd-order high-pass filters with electrolytics.
<<20Hz cutoffs

(line output vs. headphones output)

for headphones there's a need for huge output capacitor.
For line output relaxation time would be then too big (like tens of seconds), so need different (smaller) cap.
Also maybe have line output without volume regulation, from summing opamps.