CVX-2 : covox adapter

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

Re: CVX-2 : covox adapter

Postby Jepael » 2017-12-04 @ 21:08

Scali wrote:
Paralel wrote:Yeah, but has anyone tested to see what the actual cut off for the frequency range for the R-2-R covox is? Can it actually output a 22.05 KHz frequency?


The Covox has no filter at all (and the CVX4 also has a no-filter setting). So theoretically there is no cutoff.

I think the easiest way to verify is just to create a test sample, play it back with CvxPlay and record it with a modern sound card that is high-end enough (like a 24-bit 192 kHz card, that should easily pick up 20+ kHz).


But wasn't the Covox a complete system with amplified speaker, so shouldn't it be taken as a whole?
Even if we assume that the resistor network array does not have a filtering capacitor, there could be a low pass filter on the speaker box.
The wire between the plug itself and any input it is connected to has some capacitance. Audio cables can have as high as 500pF per meter, so given the high output impedance of say about 25 kohm as indicated by measurements, even just 4 meters of cable between the plug and sound card input could act as a 3.2kHz lowpass filter.

So the cutoff can happen quite soon. In reality you could have 4 meters of 50pF/m low capacitance cable for 32kHz filter.

The Covox patent also mentions approximately 5nF capacitor and lowpass cutoff of about 3kHz, but that should not be taken as the absolute truth, as even the R2R network is drawn incorrectly. It is missing resistance from data bit 0 summing junction to ground to be a real R2R ladder DAC, and measurements on the real device indicate the custom resistor network has this.

Given the low sampling rates typically used, I would not be surprised it to have a filter approximately at that frequency to filter out the aliasing artifacts, as every DAC should have an output filter to remove these artifacts as frequencies above half the sampling rate should be filtered out. Given that it really is a first-order RC filter, it is not very steep (20dB/decade), so it will attenuate 20dB at 30kHz, while 8-bit data theoretically has 48dB dynamic range.

The high output impedance of the plug makes it difficult to measure the frequency response, as everyone has different input impedance (resistance and capacitance) on their amplifier or sound card line input, and these are typically unknown, so everyone will get a different result.

Well, what could be done, is to build a good op-amp buffer right at the Covox output, so the cable or line input properties do not matter any more.
Or use extremely short wires and a device with known input impedance (such as direct connection without probes to an oscilloscope, the input resistance and capacitance usually reads on the unit).
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Re: CVX-2 : covox adapter

Postby Paralel » 2017-12-04 @ 21:20

Jepael wrote:
Scali wrote:
Paralel wrote:Yeah, but has anyone tested to see what the actual cut off for the frequency range for the R-2-R covox is? Can it actually output a 22.05 KHz frequency?


The Covox has no filter at all (and the CVX4 also has a no-filter setting). So theoretically there is no cutoff.

I think the easiest way to verify is just to create a test sample, play it back with CvxPlay and record it with a modern sound card that is high-end enough (like a 24-bit 192 kHz card, that should easily pick up 20+ kHz).


But wasn't the Covox a complete system with amplified speaker, so shouldn't it be taken as a whole?
Even if we assume that the resistor network array does not have a filtering capacitor, there could be a low pass filter on the speaker box.
The wire between the plug itself and any input it is connected to has some capacitance. Audio cables can have as high as 500pF per meter, so given the high output impedance of say about 25 kohm as indicated by measurements, even just 4 meters of cable between the plug and sound card input could act as a 3.2kHz lowpass filter.

So the cutoff can happen quite soon. In reality you could have 4 meters of 50pF/m low capacitance cable for 32kHz filter.

The Covox patent also mentions approximately 5nF capacitor and lowpass cutoff of about 3kHz, but that should not be taken as the absolute truth, as even the R2R network is drawn incorrectly. It is missing resistance from data bit 0 summing junction to ground to be a real R2R ladder DAC, and measurements on the real device indicate the custom resistor network has this.

Given the low sampling rates typically used, I would not be surprised it to have a filter approximately at that frequency to filter out the aliasing artifacts, as every DAC should have an output filter to remove these artifacts as frequencies above half the sampling rate should be filtered out. Given that it really is a first-order RC filter, it is not very steep (20dB/decade), so it will attenuate 20dB at 30kHz, while 8-bit data theoretically has 48dB dynamic range.

The high output impedance of the plug makes it difficult to measure the frequency response, as everyone has different input impedance (resistance and capacitance) on their amplifier or sound card line input, and these are typically unknown, so everyone will get a different result...

All excellent points I don't think anyone remembered to include in the discussion. Thank you for pointing this out.

No one ever has opened up one of those speakers... I would really like to know what is inside one...

The creator of the SoundJr. specifically said to use it with a 32 Ohm speaker.
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Re: CVX-2 : covox adapter

Postby Scali » 2017-12-04 @ 21:33

Jepael wrote:But wasn't the Covox a complete system with amplified speaker, so shouldn't it be taken as a whole?


I don't see why really. Many early sound cards were sold as complete kits with (cheap) speaker kits. I don't think anyone ever argued that the speakers need to be included for the overall quality of the sound card there. And I don't see why it would have to go for the Covox either.
The Covox unit just has a jack, to which you can connect either the bundled speakers, or whatever other equipment you like (as Trixter demonstrated earlier in his reference video).

Jepael wrote:The high output impedance of the plug makes it difficult to measure the frequency response, as everyone has different input impedance (resistance and capacitance) on their amplifier or sound card line input, and these are typically unknown, so everyone will get a different result.


I believe Trixter used an E-Mu 0404 to record with, which has 'Hi-Z' inputs, so very high impedance to minimize these effects.
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Re: CVX-2 : covox adapter

Postby Jepael » 2017-12-04 @ 22:19

Scali wrote:
Jepael wrote:But wasn't the Covox a complete system with amplified speaker, so shouldn't it be taken as a whole?


I don't see why really. Many early sound cards were sold as complete kits with (cheap) speaker kits. I don't think anyone ever argued that the speakers need to be included for the overall quality of the sound card there. And I don't see why it would have to go for the Covox either.
The Covox unit just has a jack, to which you can connect either the bundled speakers, or whatever other equipment you like (as Trixter demonstrated earlier in his reference video).


Fair enough, I see the point. It's just that I've never gotten speakers bundled with any of my sound cards :)

My reasoning just was that the plug has so high impedance output so it really needs an amplifier and it comes with it so it forms a complete system. I have not seen the adverts so I just don't know if it was ever advertised to be able to connect to other amplifiers such as home stereo amplifiers, so I just assumed it is expected to use the amplifier it comes with. Contrasting that to SB, GUS and other soundcards that have low output impedance or speaker amplifiers, so that they can be connected directly to speakers or home stereo amplifiers, even if they were bundled with lousy speakers, amplified or unamplified.

Scali wrote:I believe Trixter used an E-Mu 0404 to record with, which has 'Hi-Z' inputs, so very high impedance to minimize these effects.


OK, it does sound it should have minimal capacitance, if it's meant to be connected directly to high-z things like electric guitar for example. The capacitance is just not specified though.
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Re: CVX-2 : covox adapter

Postby Scali » 2017-12-04 @ 22:36

Jepael wrote:My reasoning just was that the plug has so high impedance output so it really needs an amplifier and it comes with it so it forms a complete system.


True, you really want a preamp close to the Covox for best results.

Jepael wrote:I have not seen the adverts so I just don't know if it was ever advertised to be able to connect to other amplifiers such as home stereo amplifiers, so I just assumed it is expected to use the amplifier it comes with.


Well, the Covox was used for a lot of things it was never intended for I suppose :)
Judging by the name ('Speech Thing'), the idea was originally mainly to play voice samples. You wouldn't need very high sample rates for that, or very high-end speakers.
When the thing was launched, gaming on PC wasn't such a big thing yet, and PCs were still too slow to have sampling rates that allowed good quality sound, especially if you also wanted to run a game or demo at the same time.

The popularity of the Covox as we know it today is mainly from the demoscene, where it was (re)discovered as a cheap device that allowed you to play Amiga music at acceptable quality. But that mainly happened after 1990, when CPUs were much faster, and 20+ kHz software mixing in the background was not an issue, even without DMA support.
Many of the games that support Covox tend to come from developers that have some ties to the demoscene.
The original designers of the Covox had probably never imagined anything like that.
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Re: CVX-2 : covox adapter

Postby dreamblaster » 2017-12-13 @ 11:19

Hi
here's some news for those who still wanted a CVX4 :
24 pcs of CVX4 available here ! (this was my last stock, it was shipped to 8 bit guy, and now it finally arrived)
http://www.the8bitguy.com/product/cvx4- ... ompatible/
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Re: CVX-2 : covox adapter

Postby dreamblaster » 2018-1-11 @ 18:52

Okay, now also ordered the CVX4 case from 3d hubs : https://www.thingiverse.com/thing:2743947
cvx4_render.jpg


Will post photos here when I have it.
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