VOGONS


CVX-2 : covox adapter

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Reply 100 of 452, by MobyGamer

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Are you saying that there should be a single pin for one probe that, when the other probe touches the other pins, the value should be the same? Maybe I didn't understand, but I wasn't able to find a common pin where all other pins reported the same resistance.

Is there anything else I can do? I guess i could map out all 256 combinations, but gosh that would be a PITA...

Reply 101 of 452, by keropi

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yes, if this is a "common resistor network" then there is a single common pin and when measured against the others you should get the same resistance. Have you tried in pairs? 1+2, 3+4 , 5+6 , etc
Maybe that's a special network with various resistances in it - hard to tell though without having the thing at hand...

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Reply 102 of 452, by Scali

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MobyGamer wrote:
Okay, here are all three of mine disassembled: […]
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Okay, here are all three of mine disassembled:

https://goo.gl/photos/PGaVfpzbvTYyTGdB6

https://goo.gl/photos/NYfmWroJ71xD32e97 # This is the one I recorded the reference video with

https://goo.gl/photos/MZzrVvLaJpD5waAe8

Great, many thanks for that!

I am surprised at Covox #2. Looks like they had all sorts of wild plans for the thing, judging by the room on the PCB for various components. But they aren't assembled here.
The other two seem to have virtually the same PCB, where no extra components can be put on.
I see that they wanted to put a 74HC30 on there. That's a bunch of NAND gates. And a HC05, that's a hex inverter.
Seems like they wanted to include some logic to respond to specific commands sent to the port...? Could this be similar functionality to the Disney Sound Source? That one was also made by Covox, wasn't it? Just branded as Disney?

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Reply 103 of 452, by Great Hierophant

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I think that Trixter's Covoxes correspond to figure #3 in the Covox patent, although figure 5 is also a possibility. Essentially you have a resistor network summed with resistors. Pins 1-8 of that package are connected to D0-D7 of the parallel port, pin 9 & 11 give signal and ground (not sure which is which) to the jack and pin 10 goes to one of the parallel port grounds.

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Reply 104 of 452, by Beegle

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keropi wrote:

yes, if this is a "common resistor network" then there is a single common pin and when measured against the others you should get the same resistance. Have you tried in pairs? 1+2, 3+4 , 5+6 , etc
Maybe that's a special network with various resistances in it - hard to tell though without having the thing at hand...

Hmmm. But would a resistor network have the Covox logo on it? Isn't that a standard thing that we can buy in an electronics store?
My guess is that it's something more custom-made than that. But that's just a guess.

Also funny thing is that I also have two Speech Thing versions. One beige with the two wires (3.5mm and 2.5mm) as seen in the video, and another chrome one with only the 3.5mm.

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Reply 105 of 452, by stamasd

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I feel like Christmas is back again. I just went to get today's mail, and found a package from Belgium. I thought "that's the CVX-2 from Serge" but I was wrong.

It was 2 kits! One SMD, and one through-hole. Wow!

I'm too drunk now to do any SMD soldering, and besides in 2h I'm leaving for a New Year's party. But my soldering station is warming up, and I'll put together the regular through-hole tonight.

Thank you Serge for the kits, and the greeting card. I'll surely test them in the next day or two. I'm not great at making videos (in fact I have only 1 youtube video in the past 10 years: https://www.youtube.com/watch?v=oNz5cLAw4ZM ) but I'll do my best.

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Reply 106 of 452, by keropi

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@Beegle
yes it could be a covox-specific network maybe with some different values

@GH
where are these patents located?

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Reply 107 of 452, by Jepael

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It is custom labeled R2R DAC network, so in theory armed with info how R2R networks should look like, it is possible to determine the resistances.

So between any two data pins, you should find 2R+R+2R.
Between output and most significant data bit (D7, LPT pin #9), there should be 2R.
Between ground and least significant data bit (D0, LPT pin #2), there should be 2R+2R.

Or just buy one of these, with say 10k and 20k resistances :https://www.bourns.com/pdfs/r2r.pdf

Although, the Covox patents say the resistances should be R=100k, 2R=200k, but to attenuate the audio from 5V levels to audio levels, the output to ground resistance is 15k. Capacitance about 0.005 microfarads, so I suspect 4.7nF, resulting into about 3000 Hz low pass filter bandwith.

Edit:
OK, it does not completely add up. 15k in parallel with 100k R2R output impedance is about 13k, and 13k with 4.7nF is already 2600 Hz low pass filter.
Also a slight point to note is that the in the patent schematics, the R2R network's least significant bit's side is not connected to GND through 2R. In theory it should, in practice it most likely makes little difference regarding the sound quality.

Last edited by Jepael on 2016-12-31, 23:50. Edited 1 time in total.

Reply 108 of 452, by Great Hierophant

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keropi wrote:
@Beegle yes it could be a covox-specific network maybe with some different values […]
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@Beegle
yes it could be a covox-specific network maybe with some different values

@GH
where are these patents located?

Trixter linked to it on page 4 of this thread :

https://www.google.com/patents/US4812847

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Reply 109 of 452, by keropi

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ah I missed that... there are even details on the "DETAILED DESCRIPTION OF THE INVENTION" part of the document. Maybe they used these resistor values in the final product?

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Reply 110 of 452, by dreamblaster

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MobyGamer wrote:
Okay, here are all three of mine disassembled: https://goo.gl/photos/PGaVfpzbvTYyTGdB6 https://goo.gl/photos/NYfmWroJ71xD32e97 […]
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Okay, here are all three of mine disassembled:
https://goo.gl/photos/PGaVfpzbvTYyTGdB6
https://goo.gl/photos/NYfmWroJ71xD32e97 # This is the one I recorded the reference video with
https://goo.gl/photos/MZzrVvLaJpD5waAe8

Let me know if the pictures don't show up. While the middle one has a more complicated board, all three appear to use the same resistor package.

THANKS!

on covox nr 1 and 3, the 'black pack' has 11 pins.
on covox nr 2 (which you recorded), the pack has 10 pins!
So they sure are different.
Do they all sound the same ?

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Reply 111 of 452, by MobyGamer

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keropi wrote:

yes, if this is a "common resistor network" then there is a single common pin and when measured against the others you should get the same resistance. Have you tried in pairs? 1+2, 3+4 , 5+6 , etc
Maybe that's a special network with various resistances in it - hard to tell though without having the thing at hand...

I tried, but maybe I was doing something wrong. Here's a quick video of what I was seeing (sorry for poor quality): https://youtu.be/Zl9LL2n6n3I

Let me know if there's anything else I can do. (Should I attempt what Jepael wrote?)

Last edited by MobyGamer on 2017-01-01, 01:52. Edited 1 time in total.

Reply 112 of 452, by MobyGamer

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dreamblaster wrote:

So they sure are different.
Do they all sound the same ?

I was afraid you were going to ask me that 😀 I haven't tried. I will make a recording of Crystal Dream using one of the 11-pin pack versions in the next 24 hours.

Reply 113 of 452, by dreamblaster

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thanks for the sacrifices !!, I have sent you a pm, by the way.

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Reply 114 of 452, by keropi

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I can't really understand atm these readings, what it strikes me as odd is that we are getting reading on the 500k~700k ohm range... isn't that too much?
Maybe someone else has a better idea how to crack this.

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Reply 115 of 452, by gdjacobs

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That seems high unless it's designed for a ridiculous input impedance on your preamp.

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Reply 116 of 452, by MobyGamer

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gdjacobs wrote:

That seems high unless it's designed for a ridiculous input impedance on your preamp.

When I recorded the first reference, the output was low and I had to add +20db gain.

Reply 117 of 452, by Great Hierophant

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With the 10-pin package, the two pins on the end seem to go to the audio jack. If that is so, one pin is used for the audio signal and one pin is used for ground. Using your multimeter in continuity mode, you can determine which pin is which by seeing which pin goes to the tip (signal) and which goes to the sleeve (ground). Ideally you should be putting one end of your multimeter on the signal out and the other end on your data pin.

This article describes the workings of a Covox very well. http://kb.gr8bit.ru/GR8BIT-KB0010-Adding-mult … ce-23102013.pdf

It seems that all Covox type devices use two kinds of resistors, with one set double the resistance value of the other set. Perhaps because a resistor ladder package is used, all the resistive elements within that package share the same tolerances.

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Reply 118 of 452, by James-F

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keropi wrote:

I can't really understand atm these readings, what it strikes me as odd is that we are getting reading on the 500k~700k ohm range... isn't that too much?
Maybe someone else has a better idea how to crack this.

gdjacobs wrote:

That seems high unless it's designed for a ridiculous input impedance on your preamp.

Might be transistors between the pins that add 300k-500k to the measurement.


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Reply 119 of 452, by MobyGamer

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dreamblaster wrote:
on covox nr 1 and 3, the 'black pack' has 11 pins. on covox nr 2 (which you recorded), the pack has 10 pins! So they sure are d […]
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on covox nr 1 and 3, the 'black pack' has 11 pins.
on covox nr 2 (which you recorded), the pack has 10 pins!
So they sure are different.
Do they all sound the same ?

Well, I did some re-recording tonight with Crystal Dream: One on the 10-pin device (same device used as before), and another on the 11-pin version:

ftp://ftp.oldskool.org/pub/drivers/Covox/Spee … al%20Dream.flac
ftp://ftp.oldskool.org/pub/drivers/Covox/Spee … al%20Dream.flac

You will notice one second of silence in the beginning of each file; this was done deliberately to illustrate the noise level. The 486 I was using last night died 😒 so I pulled out a Pentium Pro, and unfortunately there is some noise when I connect to this system, so I included noise for comparison. The 10-pin device has less "ambient" noise, if that makes sense.

They sound similar to me, but I think the 10-pin one has a slight edge in quality because it is less noisy.

I will determine which pin in the 10-pin version is ground and output (thanks GH) and then I will try to use that information to determine what 2R is (thanks Jepael), and that should hopefully solve the mystery of why the 10-pin version sounds so good? But not for another 10-12 hours, because Happy New Year 😉