CVX-2 : covox adapter

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Re: CVX-2 : covox adapter

Postby dreamblaster » 2017-1-01 @ 10:52

here are the schematics for CVX-2 (SMD version) and CVX-1 (through hole version), this may be helpful for the investigation.
Although CVX-2 seems to sound better than the clone versions, the schematic is very similar to the clones and circulating schematics, where I in fact inspired on for this first revision.
Many clones may practically omit some components, use different resistor values, combine resistors to one value, use lower tolerance resistors, or worse pcb layout (or no pcb at all -> may get more noise picked up). There is not much to it. CVX-2 uses all the same resistor values to have a close tolerance. (I also used 0.1% resistors initially, compared to 1% resistors, but most important, all resistors from same batch/strip, so the values are close to eachother). So the original covox still sounds better : CVX-2 may still have a difference or mistake, compared to the original covox, let's find out !


CVX2_SCHEM.PNG
CVX2_SCHEMATIC


CVX1_SCHEM.PNG
CVX1 SCHEMATIC


Maybe it is just a matter of getting the optimal values :
Resistor value : once we determine the value of 2R in covox, I can try using the same here.
C1 = low pass filter --> filters out the high frequencies. --> We played with this value and determined 4.7nF sounded best for the current setup. (actual filter freq. depends on resistor values as wel as cap value, so we may tweak this again if the resistor value changes)
C2 = decoupling cap --> filter out the DC component from the audio signal. ---> the original covox doesn't seem to have this, so we can short C2 as a test.
The pins used (for example for ground connection), may differ from the original covox, not sure if that matters at all.
Other differences you can trace ?
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Re: CVX-2 : covox adapter

Postby keropi » 2017-1-01 @ 12:19

I have made 2 more recordings since the original Covox apparently has no capacitors , one with no capacitors at all installed on the CVX2 and the other with only the 4.7nf capacitor installed

no caps: https://soundcloud.com/keropi666/serdac ... tal-dreams
only 4.7nF: https://soundcloud.com/keropi666/serdac ... tal-dreams
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Re: CVX-2 : covox adapter

Postby Jepael » 2017-1-01 @ 12:20

If the patent diagrams resemble the actual hardware, then one difference is the output voltage. In the original Covox there should be a resistor from output to ground to make output voltages lower.

Dreamblaster's designs don't have this resistor, so the output is 5Vpp, which is quite a lot for line level input of consumer equipment, so it could be useful to add.

Also the 100uF AC decoupling cap is relatively large, as it has a highpass cutoff of 0.1 Hz - It does no harm though, but even a 10uF would be sufficient for audio frequencies.
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Re: CVX-2 : covox adapter

Postby dreamblaster » 2017-1-01 @ 12:39

good find, that may well be the 11th pin on the covox !

the typical R2R pack (see from https://www.bourns.com/pdfs/r2rap.pdf, the 2nd image) seems to have 10 pins.
BOURNS_R2RAP.png
BOURNS R2R Pack


This voltage divider may be the 11th pin.
It does make a lot of sense indeed, to add it, the usual clones don't have it (and the 10 pin covox probably also hasn't)

--> I will test with a 7.5k resistor added.
it is easy to add on the CVX2 :-), just at it parallel at the same place of the C1 high pass filter.

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Re: CVX-2 : covox adapter

Postby gdjacobs » 2017-1-01 @ 13:09

James-F wrote:
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.


Without an external voltage rail, I have no idea what they'd be doing with it. The way to tell would be by identifying ground and performing a load test. This will require two multimeters, some load resistors, and a +5V supply.

Connect the multimeter -ves and supply -ve to ground. Connect a resistor (start with something like 100k) between ground and the output. Monitor the output voltage with the first resistor. Monitor the supply voltage with the second multimeter. Energize one of the data pins with +5V and proceed to vary the load resistance (33k, 10k, 3.3k, 1k, etc).

If the voltage drop across the ladder package varies proportionally with the voltage drop across the load resistance, there's no active components for a DC signal.
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Re: CVX-2 : covox adapter

Postby dreamblaster » 2017-1-01 @ 14:28

I tested with the voltage divider, it sounds the same but more quiet. Still it is a good option, I will add it to the next PCB revision, and we can test again
when other changes are done. What remains to be tested, is different resistor values, I will await the conclusion of the covox pin measurement.

I did hear some strange things running crystal dream, the start of the 2nd part of the crystal dream demo sounds bad, long load time, totally wrong sound timing,very noisy,
this cannot be due to the sound adapter. I changed my freedos boot stick today to have more conventional ram, to allow part 2 to start,(before it did not run due to insufficient memory),
It worked but something definitely is wrong, a probably due to using USB as disk medium. I will change my setup (to real MSDOS, and/or different disk medium) and try this again later.
For now, my pc (it is an athlon XP so also not too retro) is no good reference.

Ideally, for the final test, we should compare the real Covox vs CVX-2 on exactly the same pc, same recording device (settings), to exclude other factors.
I couldn't find a speech thing for sale (let alone an affordable price so I could dissect it).
But I still have a spare CVX-2 I can send to MobyGamer, will await for the measurement conclusion first, maybe I can test more first here, because I am running out of CVX-2 prototypes.
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Re: CVX-2 : covox adapter

Postby Scali » 2017-1-01 @ 14:38

The demo is just two EXE files (CRYSTAL.EXE and DREAM.DAT), where the second file is called after the first one (it actually passes the selected audio device as switches the command line, and the second EXE has its own copy of the whole sound init and replay routines).
So it does not perform any disk I/O during the demo, only between parts. So I think it's unlikely that the disk medium has an effect on this.
The DOS version might. Also, any drivers you load, may have some effect (eg there could be disk caching in the background, or things like mouse drivers, network drivers or whatever causing interrupts).
I would suggest to strip down the DOS config.sys and autoexec.bat down to the bare essentials (also avoid EMM386 and himem.sys if you can).
If you want to be a stickler for accuracy, I'd also remove any hardware devices you don't need, so you can be sure they don't generate any kind of interrupts that would steal away cycles and throw off the timing.
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Re: CVX-2 : covox adapter

Postby stamasd » 2017-1-01 @ 16:11

Dreamblaster CVX-1 finished; took just long enough to warm up my soldering station properly. Starting to work on the CVX-2.

Image

I made sockets for both capacitors from female pin connectors.
I/O, I/O,
It's off to disk I go,
With a bit and a byte
And a read and a write,
I/O, I/O
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Re: CVX-2 : covox adapter

Postby Great Hierophant » 2017-1-01 @ 16:18

Covox's patent indicated that the value of R should be 100 KOhms and the value of 2R should be 200 KOhms in the resistor ladder network. Trixter's tests with the 10-pin package would support those values. When he used his multimedia on each pair of pins next to each other (1 & 2, 2 & 3 etc.), he always got the same result, .486MOhm, or 486KOhm. When he is doing this, he is essentially measuring the resistance of three resistors connected together in series. Each pin is connected to a 2R and the pins are connected by an R, so 200 + 200 + 100 = 500KOhms. If the R2R resistor ladder network package Covox used has a 5% tolerance, these values would be well within what you would expect.
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Re: CVX-2 : covox adapter

Postby stamasd » 2017-1-01 @ 17:12

More pictures of building the CVX-2 for your viewing pleasure.

Image
I found that when dealing with small SMD components like these size 0603 resistors, it's better to pre-tin one set of pads, then place the resistor onto the pad, hold it down with a fine tip and solder that end. This way you avoid getting the resistor stuck onto the soldering tip by capilarity, which may destroy it by overheating.

Image
First row of resistors tacked at one end. It's a lot messier than I would have hoped, but it'll do. These hands and eyes aren't getting any younger.

Image
First row of resistors finushed. Checked with a multimeter that all soldering joints are correct.

Image
And so on for the remaining ones. Good thing I'm not OCD or else that alignment would drive me nuts.

Image
Finished soldering all the parts. Added female pins as sockets for both capacitors. All solder joints checked for continuity.

Image
All finished and with capacitors installed.

Total time spent: about 40 minutes.
I/O, I/O,
It's off to disk I go,
With a bit and a byte
And a read and a write,
I/O, I/O
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Re: CVX-2 : covox adapter

Postby keropi » 2017-1-01 @ 17:23

oh nice idea using these sockets for the caps, they are better than what I used... the fun part is that I do have several rows of sockets like these but I saw the other kind first and used that LOL

@Great Hierophant
you might be on to something, dreamblaster also noticed that the FTL adapter uses 100K resistors: http://dmweb.free.fr/?q=node/1514
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Re: CVX-2 : covox adapter

Postby stamasd » 2017-1-01 @ 17:39

keropi wrote:oh nice idea using these sockets for the caps, they are better than what I used... the fun part is that I do have several rows of sockets like these but I saw the other kind first and used that LOL


Yeah I bought several dozen of these 40-pin breakable female sockets a while ago; they're very useful for all sorts of things. For instance making connectors for external battery packs when I replace the CMOS batteries on old motherboards.
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Re: CVX-2 : covox adapter

Postby Great Hierophant » 2017-1-01 @ 17:45

keropi wrote:@Great Hierophant
you might be on to something, dreamblaster also noticed that the FTL adapter uses 100K resistors: http://dmweb.free.fr/?q=node/1514


I completely forgot about the FTL Sound Adapter. The schematic falls well in line with the Covox patent and what we have been talking about.

If anything, the "ideal Covox", the one using the 10-pin package, may be about as easy as it gets to build. Essentially all you would need to cobble one together is a DB-25 male connector, the resistor network shown on the FTL page, and an audio cable.
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Re: CVX-2 : covox adapter

Postby dreamblaster » 2017-1-01 @ 18:17

Hi,
I tested using 100k resistors on my PC, it sounds similar to my other CVX no cap recordings, but much lower audio level (had to set my Zoom Q4 recording sensitivity to high instead of normal).
I removed the freedos EMM driver, copied the demo on the harddrive, and ran it from there, but still severe timing issues, especially in part 2 of the demo.
So my current PC setup is not good for this this demo. Anyway here is the recorded video : https://youtu.be/mHumhIcgDPk
I'd guess the covox is nothing more than this (and I think we can improve it a bit ourselves by adding a small cap) but my PC isnt running the demo well.
I guess the best way to compare would be to have it run on the same pc as the covoxes, and recorded using the same method.
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Re: CVX-2 : covox adapter

Postby Scali » 2017-1-01 @ 18:55

dreamblaster wrote:So my current PC setup is not good for this this demo. Anyway here is the recorded video : https://youtu.be/mHumhIcgDPk


To my ears, this sounds cleaner, especially in the 'softer' parts. The synths at the start of the second part of the demo now sound much less noisy than before (comparing to your earlier captures, which I assume is the same machine), much closer to the real Covox.
I guess the DAC response with these resistors is more linear than it was before. It's certainly an improvement.

dreamblaster wrote:I guess the best way to compare would be to have it run on the same pc as the covoxes, and recorded using the same method.


Yes, I'd say that's a requirement. Both the timing and the output voltage of the LPT could affect the sound quality. So you can't really do 1:1 comparisons across different machines.
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Re: CVX-2 : covox adapter

Postby James-F » 2017-1-01 @ 19:39

I recorded and volume balanced the three variations at the right moment when the noise floor is most significant.
Clearly the 100k version is much quieter than the 7.5k version, but the real Covox is just slightly quieter than the 100k version.

The downside is the larger the resistors the quieter is the output, nothing that a BJT or Op-Amp amplifier at the end can't solve.
Attachments
100k+Divider+500pF.mp3
(43.97 KiB) Downloaded 14 times
100k+Divider.mp3
(44.12 KiB) Downloaded 17 times
Covox.mp3
(34.12 KiB) Downloaded 24 times
CVX2 100k.mp3
(43.25 KiB) Downloaded 20 times
CVX2 7.5k.mp3
(31.07 KiB) Downloaded 22 times
Last edited by James-F on 2017-1-02 @ 04:49, edited 1 time in total.
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Re: CVX-2 : covox adapter

Postby dreamblaster » 2017-1-01 @ 20:07

did you compare with the 11 pin covox recording ?
I added the 100k voltage divider resistor (the '11th pin') (in a socket) and will record again :
CVX2_100K_SOCKET.JPG
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Re: CVX-2 : covox adapter

Postby James-F » 2017-1-01 @ 20:17

The covox recording I used is from the first MobyGamers youtube video.
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Re: CVX-2 : covox adapter

Postby MobyGamer » 2017-1-01 @ 20:20

James-F wrote:The downside is the larger the resistors the quieter is the output, nothing that a BJT or Op-Amp amplifier at the end can't solve.


The quieter output was done intentionally because the Speech Thing was intended to be used with the amplified speaker it came with (which uses, or acts as, a lowpass filter; it sounds slightly "muffled"). Super-quick video: https://twitter.com/MobyGamer/status/815452144354652160

The speaker's filtering was an intentional design choice for rounding the harshness off of speech signals and speech compression (it is, after all, called the Speech Thing). I don't like using the amplified speaker very much because of the filtering.
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Re: CVX-2 : covox adapter

Postby Scali » 2017-1-01 @ 20:20

James-F wrote:I recorded and volume balanced the three variations at the right moment when the noise floor is most significant.
Clearly the 100k version is much quieter than the 7.5k version, but the real Covox is just slightly quieter than the 100k version.


Thanks, that allows for a really nice comparison.
And I agree with your assessment... The 7.5k is very noisy in that part, the 100k is quite clean, but the Covox is just a smidge better still.
Interesting how just varying the resistor values can have such an impact. It seems like Covox people put a lot of effort into fine-tuning it for maximum quality.
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