Audio balance PSG and external audio sources - Are 'faulty' machines correctly emulated?

By Sandy Brand

Master (226)

Sandy Brand's picture

14-02-2021, 20:15

Hey everyone,

I am currently in the process of trying to make some PSG sound-effects when I remembered that I am emulating a Philips NMS 8055, which is one of those machines that has a 'faulty' volume balance between PSG and external audio.
From what I remember this was mostly noticeable for PSG + SCC, but the same issue should be for PSG + FM-PAC and such.

Just looking at the openMSX Catapult launcher there is nothing to indicate that the balance setting might be different though.

Also, I looked at the configuration .xml files, and all machines seem to have the same volume defined. For example:

<PSG id="PSG">
      <type>AY8910
      <sound>
        <volume>21000
      </sound>
      .....
</PSG>

So my question is: Is this 'fault' actually emulated in some way?

(And how does everybody else create your sound effects? You just need to test them on a couple of representative machines I guess?)

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By Manuel

Ascended (17702)

Manuel's picture

19-02-2021, 21:25

It's possible to emulate it, but this isn't done in the openMSX config files. So you could adjust them yourself if you want the same effect.

By tfh

Prophet (2797)

tfh's picture

19-02-2021, 22:04

Were there ever machines produced in that series that didn't have that bug?
If not, I would expect that OpenMSX, if it aims for accuracy, should emulate the "fault" as default for these machines.

By syn

Paragon (2041)

syn's picture

19-02-2021, 22:06

i think the reason for him is emulate any msx machine to see /test how it would sound on different machines.

For example to me im curious as well, wondering how much the difference between msx with the biggest psg vs cart ratio and the one one with the smallest, for music making purposes.

With the many variations in msx machines in existence i would probably think its hard to tune your software to " please " everyone but it would be nice to be able to just test it, so you would have a rough idea on how things sound on different machines.

Of course I know this might be a lot of work and I know for example for the philips 825x there are like many different adjustments/fixes and now you also got the FRS mods for various 2+ and TR models, so I suppose its close to impossible to have a complete database of all possible settings/configurations

By Manuel

Ascended (17702)

Manuel's picture

19-02-2021, 23:38

tfh wrote:

Were there ever machines produced in that series that didn't have that bug?
If not, I would expect that OpenMSX, if it aims for accuracy, should emulate the "fault" as default for these machines.

If someone comes up with the proper values that match the real hardware, I'd be happy to adjust the config file.

By Grauw

Ascended (9690)

Grauw's picture

19-02-2021, 23:54

As I understand it the issues with the Philips NMS8250 / 55 / 80 machines audio goes much further than balance alone. It is to do with faulty mixer circuitry influenced by electrical characteristics and current draws and impedances and such of various audio extensions plugged in, so it is much more inconsistent and unpredictable than just a fixed imbalance. An SCC or FM-PAC (clone) or MegaFlashROM SCC+ SD will give you different results.

By NYYRIKKI

Enlighted (5736)

NYYRIKKI's picture

19-02-2021, 23:51

syn wrote:

For example to me im curious as well, wondering how much the difference between msx with the biggest psg vs cart ratio and the one one with the smallest, for music making purposes.

I can't say what is the "smallest" as I don't really know even what is the definition of "correct balance". If the channel count and wave form would be same on PSG and OPLL then I would say each channel maximum output volume should be about same level... As it is not, I'm not sure, how to define this. Does anyone have some theoretical definition in mind?

The biggest differences are common to be found from Korean MSX computers. These many times do not have wire connected to cartridge sound input pin at all and that causes extremely low volume when ie. FM-PAC is inserted. Next worse computers that I've used are Spectravideo branded machines. There I think the volume is something like 15dB off.

These balance things are extremely hard to get right especially when emulating external sound extensions, because machines have different kind of mixers that have different balances and input impedances and cartridges (even when they have identical sound chip) may have different kind of output impedances. Also ie. FM-PAC has 3 user selectable volume settings on top of cartridge.

By sd_snatcher

Prophet (3436)

sd_snatcher's picture

20-02-2021, 02:18

NYYRIKKI wrote:

I can't say what is the "smallest" as I don't really know even what is the definition of "correct balance".

Electronically speaking, the standard states that all inputs must be done at -5dBm.

In terms of emulation, you your assumption is right: all chips must have (as close as possible to) the same volume at their maximum output. Preferably, setup them up to output a square (as possible) wave, so it's easier to compare and make the adjustments.

By Sandy Brand

Master (226)

Sandy Brand's picture

21-02-2021, 23:59

Thank you for your replies everyone!

I can imagine doing proper emulation of this is definitely hard and is probably something that would be disabled by almost everyone apart from maybe some fanatical purists Smile So it is not worth the time.

I am happy that I don't need to worry about it when making sound effects, makes it just a bit easier Smile