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Re: IBM PC Speaker RC values?

Posted on 2016-12-29, 11:06

in PC Emulation
As mentioned, I think the '60us' figure is pretty nonsensical. Firstly, at best it's a ballpark figure, since there are tons of different PCs out there with tons of different speakers, not to mention different motherboards with different circuitry driving it. I think this figure just popped up …

Re: IBM PC Speaker RC values?

Posted on 2016-12-28, 13:21

in PC Emulation
There is no aliasing if the frequencies that would get aliased do not exist, that's why they are first filtered away with a low pass filter before decimating (throwing away unneeded samples). That feels a bit counter-intuitive... because your filter is not 'synchronized' with the output, and …

Re: CVX-2 : covox adapter

Posted on 2016-12-28, 12:59

So for a simple mono Covox DAC, there is no need for external latching. And buffering will also cause individual delays to each data bit, but this rippling also should not take more than 30ns. That's good to know. That was my point exactly: we have to analyze the signal going out, and then we get …

Re: CVX-2 : covox adapter

Posted on 2016-12-28, 10:31

A latch is not needed on a plain old regular "Covox", because the LPT port data pins are already latched. Therefore if you do have two LPT ports, it's easier to have two Covoxes for stereo output. Yes, but are they latched in a way that we want for a Covox? I mean, the use-case for LPT is to only …

Re: CVX-2 : covox adapter

Posted on 2016-12-28, 08:41

If you wanna go all the way, why not start here? http://cd.textfiles.com/thegreatunsorted/texts/txtfiles_misc/pcstereo.gif Has a latch + proper DAC + output amplifier (shouldn't be too hard to cut the circuit down to mono). Ah, interesting. For me, the latch was merely a theory I had. I didn't know …

Re: CVX-2 : covox adapter

Posted on 2016-12-28, 08:07

Exactly. You want to preserve the data and none of the analog noise. I would probably favor optocouplers as logic gates aren't generally intended for isolation purposes. I don't think you understood the true purpose of my suggestion of the logic gates then. The isolation would be a bit of a side- …

Re: IBM PC Speaker RC values?

Posted on 2016-12-27, 11:19

in PC Emulation
I haven't studied the topic in-depth, but my intuition would say that steps 3 and 4 should be done as a single step. Resampling by skipping samples ('nearest neighbour' sampling) is not a very good method, and will introduce aliasing. I would say that the low-pass filter should be a kind of 'scale …

Re: CVX-2 : covox adapter

Posted on 2016-12-27, 08:24

gdjacobs wrote: Ripple on the voltage rail is going to be a major offender. That's exactly my point: by using flip flops to decouple the signal, you can also put these flipflops on a conditioned power supply (could be external, or some circuit with some capacitors and such to filter out any ripple …

Re: CVX-2 : covox adapter

Posted on 2016-12-27, 08:22

It would be great to have a TSR for the Covox to emulate a Sound Blaster so any game can be heard. I believe there exist one already, Virtual SoundBlaster if I recall. Yes, from the same guy who also did TEMU (which emulates Tandy audio over LPT DAC, or PC speaker): http://web.archive.org/web/ …

Re: CVX-2 : covox adapter

Posted on 2016-12-26, 22:47

I have a few true Covox Speech Things (from Covox, not a clone). Would it help this project if I took one apart and took hi-res pictures of it? If you can do that without destroying them, I would be interested. I wonder how much they did to condition the signal and optimize the output quality. Also …

Re: SBVGM (DOS) VGM Player

Posted on 2016-12-26, 22:43

Most "register dump" formats like VGM, DRO, IMF and RAW only tell what data to write to which register without the delays needed to access hardware, the delays are only for things like tempo (like in a MOD player to play a row of tracked notes from a timer/vblank interrupt). That only works if the …

Re: CVX-2 : covox adapter

Posted on 2016-12-26, 22:12

You will hear "machine noise" either way with LPT DACs, just like a radio that also has some hissing in it - the output by nature is not crystal clear like a real soundcard. Not necessarily. I think there are two factors that introduce 'noise'... Two types of noise that is: 1) The signal is taken …

Re: CVX-2 : covox adapter

Posted on 2016-12-26, 14:44

dreamblaster wrote: okay, I tested with 5nF (2x10nF in series) https://youtu.be/w8gLaZD46ew (I'll add the link to the first post) Is this better tha […] Show full quote okay, I tested with 5nF (2x10nF in series) https://youtu.be/w8gLaZD46ew (I'll add the link to the first post) Is this better than …

Re: CVX-2 : covox adapter

Posted on 2016-12-26, 14:17

Which other demos / games / music software (that support covox adapter) are recommended to try with the CVX adapters ? Crystal Dream 2 also has LPT DAC support I believe, and should have a better quality mixing routine than Crystal Dream 1. Fast Tracker II also has LPT DAC support (select 'Sound …

Re: CVX-2 : covox adapter

Posted on 2016-12-26, 13:41

Yea, 100nF is definitely too muffled. I think 10nF is acceptable. It affects the sound slightly, but it remains bright enough. The difference between 1nF and no-cap seems negligible. I think my vote goes to 10nF.

Re: SBVGM (DOS) VGM Player

Posted on 2016-12-26, 12:41

The program does presently support SN76489 playback on the Tandy 1000, PCJr, and IBM PS/1 Audio. Ah, cool. I downloaded it, and looked at the readme, but it didn't mention this support, so I never tried it. I'll have to see what it does on my machine. I'm wondering how the program was able to …

Re: SBVGM (DOS) VGM Player

Posted on 2016-12-25, 14:06

This thread The Tandy 1000 ISA sound card brainstorm thread talks about an ISA card that uses the SN76489. It seems sold out at the moment though. I will likely add support for the card (add options to select port that it is at) at some point later. I have one of these cards (installed in my IBM …

Re: SBVGM (DOS) VGM Player

Posted on 2016-12-24, 09:20

Instead of using timer interrupts, wouldn't it be better to just poll the timer? That's an approach I've tried for sample playing (the 61h command). It takes all CPU, but it should be the most accurate way to play back such files. Also, you should be able to run the timer interrupt at much more than …

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