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Thread: GUIDE: Telling apart good Genesis 1s and Genesis 2s from bad ones.

  1. #1981
    Hedgehog-in-Training Hedgehog-in-TrainingSports Talker JamesF's Avatar
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    I understand the reason behind preferring a stock HD Model1 for having no distortion in the audio circuitry when stock, while most Model2 are distorted when stock because of an engineering flaw.
    I believe once the distortion of VA0-1.8 is fixed by correcting the engineering flaws it will sound just as good as VA4, mainly for having the same filtering values.

    Think about this way: If the 'fixed' Model2 was released before the Model1, we would strive for the more filtered sound of the Model2 as the definitive Genesis sound, while the Model1 will sound too sharp, buzzy and especially ugly with PCM sounds.
    The preference for the Model1 filtering is because it was first and sounded good stock, but sega engineers might have made a poor choice with its 1st-orded LPF, a 2nd-order LPF might have suited the YM2612 much better.

    My point is, before replacing the whole audio output topology with custom external circuitry to resemble Model1, maybe try to listen to the 'fixed' (undistorted) Model2 output and decide if you like it.
    Maybe there are no "bad ones" if one can solder a cap and a resistor.


    EDIT:

    Signal to Noise measurements of Model 1 VA6 and Model 2 VA4 with quality shielded cables.

    Model1 VA6:
    -68db RMS
    -3.4db Peak
    SnR = 64.6db


    Model2 VA4:
    -77db RMS
    -1.3db Peak
    SnR = 75.7db

    10db is twice as quiet.
    Last edited by JamesF; 10-31-2017 at 11:07 AM.
    Sega Genesis Model1 VA6, Model2 VA4|VA3, Mega Everdrive X5

  2. #1982
    Outrunner Eep386's Avatar
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    Well, it wouldn't be that simple, the '324s Sega used on the VA0~1.8 also need 3.9~4.7K pull-up resistors placed on their outputs to fix the MASSIVE crossover distortion inherent to those amps. IMO Sega should really have used MC3403s for the VA0~1.8 - it has roughly similar (actually slightly better) performance and an otherwise similar circuit application, but has a class-AB output stage that dramatically reduces crossover distortion. You can literally drop MC3403s into the circuit without adding pull-up resistors, and it will crunch and crackle less on the spot. (Of course it'll still be far from perfect until we address the rest of the issues.)

    On top of all that, the mono mixer they used is totally wrong for the circuit they're using. They tapped resistors from the stereo output amps, which is a shortcut that ensures that the mono output has the same filtering as the stereo and allows SL2/SR2 to be mixed into mono, but unfortunately it causes very distorted, high-frequency artifacts to bleed from one channel into the other, creating what sounds like a really nasty, noisy high-pass filter effect on stereo outputs. They tried to rectify this in the BA6166/315-5684, where they simply added an extra stereo output amplification stage after where the mono resistors are tapped, which greatly reduces the high-pass distortion effect but still doesn't prevent the channels from bleeding between each other. A better mono mixer would have used a dual op-amp set up as two unity-gain buffers, which are tied by resistors on the outputs, not the inputs, to prevent signals from bleeding between channels. (It should be noted that they used a similar mono mixer arrangement on the VA3~6.8 Model 1, and that system too exhibits some degree of channels bleeding between each other, though due to a different output design it doesn't suffer nearly as badly from it.)

    Incidentally Sega did put 4.7K pull-ups on the BA10358 pre-amplifier's outputs (basically a '324 chopped in half) to bias its output into class-A, making it sound a lot better than how it normally would, but then they had to putz the gain of the pre-amp, and also insert a wild series resistor on the PSG where it didn't belong. Changing the BA10358 to an NJM4556 means you can throw out those 4.7K pull-ups and enjoy a reduction in DC offset in the output.
    Last edited by Eep386; 05-30-2018 at 11:31 PM.

  3. #1983
    Raging in the Streets bultje112's Avatar
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    has there been any added information for european mega drive 2?

  4. #1984
    Creator of the Mega Amp Raging in the Streets Ace's Avatar
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    Not yet, I haven't had any European MegaDrive IIs come my way. I would need at least one such system before I can add anything about it. Information from owners of these systems would be helpful as well.
    HATES ATGAMES WITH A PASSION


    Mega Amp: An all-new audio circuit for your Sega Genesis/MegaDrive and clones.

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  5. #1985
    Death Adder's minion
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    Here you can find a lot of information about European SMD (video, photos, so on).

    Despite the fact that I deeply analyze the American consoles, the European consoles were also in my test lab.

    I recommend that you install the plug-in for the browser like S3 to translate from the Russian language))

  6. #1986
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    I'm pretty sure there are only MD2 VA0, VA1, VA1.8 in europe.
    EDIT : seeing Omonim pics, it seems there are euro MD2 VA4. Must be pretty damn rare as I never ever seen an halfboard on an euro console.

    Also, I'm pretty sure there are only MD1 VA4, VA6, maybe some 6.5 or 6.8 are floating around, but also never ever saw a VA7 (which is a good thing lol).
    Last edited by Ryoandr; 11-01-2017 at 03:43 PM.

  7. #1987
    Outrunner Eep386's Avatar
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    There's nothing to be scared of in the VA7 MD1 IMO. The onboard sound quality sucks, sure, but it's fixable, either by tweaking the circuit or by Mega Amp.
    The VA7 has advantages in power consumption (the lowest of any MD1), recapping/ease of maintenance (far fewer capacitors compared to any earlier revision MD1) and oftentimes has slightly superior composite output too (all the VA7s I've seen use the excellent CXA1145M encoder), in the sense that they almost never suffer from rainbow banding. Changing one cap to an 82pF sharpens their composite output up.

  8. #1988
    Master of Shinobi
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    What's the difference between MD1 Japanese VA1 & VA2 ?
    Last edited by chilled; 11-12-2017 at 05:16 PM.
    Lists of MD games: officially licensed (~925) @ cartridge sizes @ Top 5 @ Top 250 (controversial) @ Sonic hacks @ best title screens @ Arcade ports (150) @ best European

  9. #1989
    Hedgehog-in-Training Hedgehog-in-Training bibington's Avatar
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    Megadrive

    Hello all,

    I recently purchased a Japanese Mega Drive and thought i would add it to this list for reference.
    Its a Japanese VA2 model. I believe that was modded in Hong Kong for Pal hence the RF indside when it was released but its just a guess.

    I Would also appreciate some help putting it back to Japanese factory spec.


    Thanks

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    5dZolQG.jpg
    https://i.imgur.com/5dZolQG.jpg

    fLFYpEj.jpg
    https://i.imgur.com/fLFYpEj.jpg
    Last edited by bibington; 11-24-2017 at 05:10 AM.

  10. #1990
    Nameless One BRNexus7's Avatar
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    James, you prefer the sound of a stock VA4 Model 2 vs a VA3-VA6.8 Model 1?

  11. #1991
    Hedgehog-in-Training Hedgehog-in-TrainingSports Talker JamesF's Avatar
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    Yes.

    Besides the cleaner headroom of the Model1 and the fact that is was the original console, its 1st-order lpf sound and distorted YM2612 is overrated.
    The 3rd-order lpf of the Model 2 suits more the ugly PCM quality and buzzy synth of the Genesis, considering that it is working properly like on VA3/4.
    I think the 3rd-order lpf and YM3438 is a very welcome change to the sound of the Genesis, and suits better its audio topology.
    Whats more, the SNES sound output is very filtered and spectrum-wise matches the Model 2 Genesis, even though it uses all PCM sounds.

    Not to say I dislike the Model 1, but its asymmetric 70s audiophile design and extra cable for stereo is unattractive when on the shelve with other slick consoles like the Super Famicom and N64,
    I prefer the smooth symmetric looks of the Model 2 when sitting near other consoles from the same era.
    Sega Genesis Model1 VA6, Model2 VA4|VA3, Mega Everdrive X5

  12. #1992
    Outrunner Eep386's Avatar
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    I for one can't go without that buzzy TDM distortion of the YM2612. It makes songs that are programmed to abuse it sound 'right' to my ears. (Think Streets of Rage 2's opening piano solos, Novotrade music, etc.)
    As for the 1/2/3-order filter, I believe you can have 3rd-order filtered sound that's every bit as good or better than 1st-order filtered audio - in some ways the extra stages are desirable; they do a better job at eliminating NON-harmonious noise (think hiss and bus racket) once their filter corners are set appropriately. Obviously, in the case of the VA0~1.8, there are some circuit layout problems that makes things much worse than need be.

    Fortunately for people like me, there's the VA2 and VA2.3 Model 2's. They have the 3rd order filter AND YM2612, and the 315-5684's filtering corners are relatively easy to set with the help of this tool: http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm R1 and R2 are 10K. Using this calculator, with my original recommendations of 4700pF for C1 and 1000pF for C2, I get a cut-off frequency of 7.3 kHz with a Q factor of 1.08, with a 6.5 kHz oscillation frequency, 81.4 degree phase margin and absolute overshoot peaks of 1.2, 0.96 and 1.01. With the original 5600pF C1 and 1300pF C2 caps (I measured both caps), the cut-off was 5.89 kHz, Q factor 1.037, phase margin 85.9 degrees, oscillation frequency 5.1 kHz and overshoot peaks of 1.18, 0.97 and 1.01. A C1 value of 2200pF and C2 of 470pF would give broadly similar Q factor, phase margin and overshoots to my recommendations, but with a cut-off frequency of 15.5 kHz, which would be much closer to an 'unfiltered' sound profile for those who are into that.

    Please note that C1/C2 and R1/R2 do not match up to any known motherboard, they are simply the 'equivalent' part references for that calculator. On 315-5684 systems, the 10K resistors for R1/R2 are built-in and cannot be changed. On VA2/2.3 systems, the equivalent references for C1 are C20 and C22, and the equivalent references for C2 are C56 and C58.
    Last edited by Eep386; 11-28-2017 at 01:41 AM.

  13. #1993
    Road Rasher Genesis32XCD's Avatar
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    The Genesis 3 section should be updated. You can look at the serial number to determine which revision it is.



    AG=manufacturing plant code
    8=year (1998)
    1 or 2=motherboard revision
    the last six digits are the unique serial number of the unit
    Last edited by Genesis32XCD; 12-24-2017 at 10:16 AM.

  14. #1994
    Outrunner Eep386's Avatar
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    I believe the Fujitsu encoders should be re-evaluated...

    After changing the BPF and delay line circuits to match a CXA1145's plus a 68~82pF cap on C32 (VA0~3 Model 2s), the quality of the Fujitsu encoder increased drastically. It's no longer dark or oversaturated, and while it's slightly blurrier than before, that's IMO overall an acceptable trade for fixing all the other issues with the encoder. It even greatly reduces the appearance of diagonal lines. It looks much like a slightly sharper CXA1145M in the end, with less color fringing/cross-color interference. Color purity is far better too.

    Some pics of Garfield: Caught in the Act, running on a modified Fujitsu setup:
    IMG_0468[1].jpg
    IMG_0469[1].jpg
    (sorry about the dark band, that's from my camera)

    To fix the Fujitsu, you replace R47, R55 and R56 with 1K resistors, and replace C32 with a 68~82pF cap. 68pF is sharper, but 82pF has nearly zero cross-color interference at the expense of some slight blur.
    Last edited by Eep386; 02-03-2018 at 08:03 PM.

  15. #1995

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    Can anyone help me with this one?

    It is a Sega Genesis 1 High Definition Graphics model. No Ext port. Has TMSS.
    Going by the guide I assume this is a BDM5USA VA6 but I don't see VA6 on the board.








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