Rev 1229 | Details | Compare with Previous | Last modification | View Log | RSS feed
| Rev | Author | Line No. | Line |
|---|---|---|---|
| 1229 | lvd | 1 | --== audio specs and reqs ==-- |
| 2 | |||
| 3 | |||
| 4 | 1. have sound from AY/YM and beeper/PWM-covox |
||
| 5 | 2. have some extra line-ins |
||
| 6 | 3. independent volume regulation for all of them |
||
| 7 | 4. line out/headset output also volume regulated |
||
| 8 | 5. be able to drive 32 Ohm headsets |
||
| 9 | 6. Use only +12v power supply for audio |
||
| 10 | 7. line-out also goes to extra video output pins |
||
| 11 | |||
| 12 | |||
| 13 | |||
| 14 | |||
| 15 | --== audio decisions ==-- |
||
| 16 | |||
| 1230 | lvd | 17 | (YM2149) |
| 1229 | lvd | 18 | |
| 1230 | lvd | 19 | voltage on outputs (loaded as per rev.C) |
| 20 | 2.26v min, 3.40v max => 1.14 Vp-p (max 1.35Vp-p by datasheet) |
||
| 21 | |||
| 22 | 1.5*1.14 Vp-p (A/C + half B) to 2.0 Vp-p is K=1.17; |
||
| 23 | |||
| 24 | |||
| 25 | (AY8910) |
||
| 26 | |||
| 27 | Per datasheet, max 2mA current per output, 2 Vp-p @ 1k |
||
| 28 | |||
| 29 | |||
| 1229 | lvd | 30 | (PT2258 related) |
| 31 | |||
| 32 | 1. Use PT2258 to control volume: |
||
| 33 | 1 channel for AY/YM |
||
| 34 | 1 channel for beeper/pwm-covox |
||
| 35 | 1 channel for output volume |
||
| 36 | 3 channels for line-ins (same number of line-ins as in rev.C) |
||
| 37 | total 2 ICs |
||
| 38 | |||
| 39 | 2. max Vp-p for PT2258 is 2v (1v amplitude or 0.7Vrms, see PT2257 datasheet and THD vs amplitude there) |
||
| 40 | |||
| 41 | 3. Vcc for PT2258 is ≈9v, can't be 12v. So, there's AVCC9 regulated voltage |
||
| 42 | |||
| 43 | |||
| 44 | (headamp related) |
||
| 45 | |||
| 46 | 1. Use TPA6120 to drive line-one/headset: high power AND high quality |
||
| 47 | |||
| 48 | 2. Use AVCC12 (non regulated, directly from AT/ATX and passively filtered) |
||
| 49 | to get maximum output swing thus power |
||
| 50 | |||
| 51 | 3. To compensate for inevitable series resistors at the TPA6120 outputs, |
||
| 52 | use opamps. |
||
| 53 | |||
| 54 | 4. Output resistors should be of high thermal power, preferably MELF |
||
| 55 | |||
| 56 | |||
| 57 | (general) |
||
| 58 | |||
| 59 | 1. Use metal film feedback resistors (bigger than 0805, best if MELF), |
||
| 60 | NP0 capacitors for low pass filters |
||
| 61 | |||
| 62 | 2. DC blocking/high pass filtering can't be done with polypropylene/etc. caps (too big!), |
||
| 63 | so we have to use electrolytics. Try to have related cutoff freqs as low as possible (<<20Hz) |
||
| 64 | to decrease distortion from them. |
||
| 65 | |||
| 66 | |||
| 67 | (low-pass filtering) |
||
| 68 | |||
| 69 | distributed 3-rd order butterworth, 30kHz -3dB. |
||
| 70 | First stage for AY and for BEEP is 2nd order Q=1, (multiple-feedback), |
||
| 71 | Second stage is just 1st-order RC within output amplifier. |
||
| 72 | |||
| 73 | Some extra filtering for BEEP at higher freq. |
||
| 74 | |||
| 75 | |||
| 76 | (high-pass filtering) |
||
| 77 | |||
| 78 | distributed "synchronous" (or not so) 1st-order high-pass filters with electrolytics. |
||
| 79 | <<20Hz cutoffs |
||
| 80 | |||
| 81 |