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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 (max 1.35Vp-p by datasheet)

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


(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


(general)

1. Use metal film feedback resistors (bigger than 0805, best if MELF),
   NP0 capacitors for low pass filters

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.


(low-pass filtering)

distributed 3-rd order butterworth, 30kHz -3dB.
First stage for AY and for BEEP is 2nd order Q=1, (multiple-feedback),
Second stage is just 1st-order RC within output amplifier.

Some extra filtering for BEEP at higher freq.


(high-pass filtering)

distributed "synchronous" (or not so) 1st-order high-pass filters with electrolytics.
<<20Hz cutoffs