CVX-2 : covox adapter

Discussion about old sound cards, MIDI devices and sound related accessories.

Re: CVX-2 : covox adapter

Postby keropi » 2016-12-26 @ 20:12

I have made some recordings with my retrocables.es covox clone:

Image

Crystal Dream demo 44khz output: https://soundcloud.com/keropi666/cd1-re ... les-lptdac
Pinball Fantasies: https://soundcloud.com/keropi666/pbf-retrocables-lptdac

The recodring setup is simple: LPTDAC output fed directly to the AUX-IN of my Auzentech X-Fi Forte sound card , recorded with Audacity and exported as 320kbs MP3.
Crystal Dream sounds better to me with dreamblaster's clone.
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Re: CVX-2 : covox adapter

Postby Beegle » 2016-12-26 @ 20:55

keropi wrote:Crystal Dream sounds better to me with dreamblaster's clone.

It's true, I hear a big difference too.
From what I can make out, the resistor is 10uF in yours, keropi. However it sounds like a 80~100uF would sound in dreamblaster's. Why do you think?
Also I believe I hear machine noise embedded in keropi's. Is that a result of using 44khz output?
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Re: CVX-2 : covox adapter

Postby keropi » 2016-12-26 @ 21:22

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.
But yeah, the retrocables adapter is not as good as dreamblaster's one.
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Re: CVX-2 : covox adapter

Postby Scali » 2016-12-26 @ 22:12

keropi wrote: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 directly from the data pins of the printer port. This port is not designed for audio output, so the signal may not be 'clean'. Also, I'm not sure how long a signal is on the data pins, and what happens when you send a different value.
2) The timing of the data sent to the printer port. This is generally done from a timer interrupt. Depending on how tightly the code is written, there may be less or more temporal jitter.

For 1) you could probably decouple the signal from the data lines by using some flip flops (latches). You can then create a nicely conditioned power supply to these flip flops, so they act as a sort of 'buffer', making the outgoing signal a solid clean one, regardless of the incoming signal.
Then you can make sure the flipflops only change value based on the strobe pin.
That way you'd also eliminate any noise during the 'transition phase' from one value to the next.

For 2) you can minimize jitter by either crafting very tight code (eg making sure the CPU is executing a hlt whenever you expect the interrupt to fire, so there will be no latency when responding to the interrupt), or by not using the timer interrupt at all, but instead using a cycle-counted loop to ensure that the delay between any two samples is always the same.
This is mainly important for slow machines such as 8088s. On a fast machine, the jitter is negligible anyway, because every instruction executes so quickly.

So, I think you can solve both.
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Re: CVX-2 : covox adapter

Postby MobyGamer » 2016-12-26 @ 22:19

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?
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Re: CVX-2 : covox adapter

Postby MobyGamer » 2016-12-26 @ 22:21

keropi wrote:Crystal Dream demo 44khz output: https://soundcloud.com/keropi666/cd1-re ... les-lptdac


This sounds quite muffled...?
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Re: CVX-2 : covox adapter

Postby Scali » 2016-12-26 @ 22:47

MobyGamer wrote: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, I wonder if they used a discrete resistor ladder, or some IC for the DAC.
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Re: CVX-2 : covox adapter

Postby dreamblaster » 2016-12-26 @ 23:56



What song / level is this in pinball fantasies ?
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Re: CVX-2 : covox adapter

Postby keropi » 2016-12-27 @ 00:11

^ it's the title screen song
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Re: CVX-2 : covox adapter

Postby dreamblaster » 2016-12-27 @ 00:12

oh i clicked that away
will try
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Re: CVX-2 : covox adapter

Postby gdjacobs » 2016-12-27 @ 00:31

Scali wrote:
keropi wrote: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 directly from the data pins of the printer port. This port is not designed for audio output, so the signal may not be 'clean'. Also, I'm not sure how long a signal is on the data pins, and what happens when you send a different value.
2) The timing of the data sent to the printer port. This is generally done from a timer interrupt. Depending on how tightly the code is written, there may be less or more temporal jitter.

For 1) you could probably decouple the signal from the data lines by using some flip flops (latches). You can then create a nicely conditioned power supply to these flip flops, so they act as a sort of 'buffer', making the outgoing signal a solid clean one, regardless of the incoming signal.
Then you can make sure the flipflops only change value based on the strobe pin.
That way you'd also eliminate any noise during the 'transition phase' from one value to the next.

For 2) you can minimize jitter by either crafting very tight code (eg making sure the CPU is executing a hlt whenever you expect the interrupt to fire, so there will be no latency when responding to the interrupt), or by not using the timer interrupt at all, but instead using a cycle-counted loop to ensure that the delay between any two samples is always the same.
This is mainly important for slow machines such as 8088s. On a fast machine, the jitter is negligible anyway, because every instruction executes so quickly.

So, I think you can solve both.


Ripple on the voltage rail is going to be a major offender.
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Re: CVX-2 : covox adapter

Postby peklop » 2016-12-27 @ 02:24

dreamblaster wrote:Hi,

Which other demos / games / music software (that support covox adapter) are recommended to try with the CVX adapters ?


Lemris :lol:
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Re: CVX-2 : covox adapter

Postby James-F » 2016-12-27 @ 06:09

Duke Nukem 3D supports the Covox as Disney sound source, the game will fully mix all sounds send it to the LPT, great way to test how sound effects sound besides MOD music.
It would be great to have a TSR for the Covox to emulate a Sound Blaster so any game can be heard.
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Re: CVX-2 : covox adapter

Postby Beegle » 2016-12-27 @ 06:36

James-F wrote: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.
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Re: CVX-2 : covox adapter

Postby Scali » 2016-12-27 @ 08:22

Beegle wrote:
James-F wrote: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/200805140428 ... dldos.html
It does not emulate FM though, or so it says.
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Re: CVX-2 : covox adapter

Postby Scali » 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 from the LPT).
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Re: CVX-2 : covox adapter

Postby keropi » 2016-12-27 @ 10:22

Tried to make some recordings with my Disney Sound Source but I totally forgot it does not work with covox/lptdac stuff... anyone knows a game (preferably) that supports both options so a comparison can be made?
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Re: CVX-2 : covox adapter

Postby gdjacobs » 2016-12-27 @ 23:25

Scali wrote:
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 from the LPT).


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.
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Re: CVX-2 : covox adapter

Postby PhilsComputerLab » 2016-12-28 @ 02:01

Would the sample rate also matter? What I mean is, if you play at a lower sample rate, would a different capacitor value maybe be better suited? Just thinking of someone on a slower machine.
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Re: CVX-2 : covox adapter

Postby gdjacobs » 2016-12-28 @ 03:39

Yeah, it would. You want the cut-off point of the filter to be just above the Nyquist frequency, so RC should be increased for a lower sampling rate.
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