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Mikael Abdellah 9th December 2006 09:35 AM

Bringing the covox into the 21 century
A couple of weeks ago I took a long walk down memory lane and installed DOS on an old computer I had laying around. Back in the days (1992 or something) I had a computer with a sound device normally known as a Covox. Actually the Covox Speech Thing was a commercial product but since it was so easy to build you could pick one up for a couple of bucks almost anywhere.

Some explenation to what a covox basically is. In the early pc days sound cards were fairly expensive for a 13 year old kid so someone designed a rather nifty little DAC you could connect to the printer port of almost any computer at the time. There are a few different versions but the main idea is to make use of the 8bit resolution of the printer port output and connect them to a resistor ladder and the to the amplifier.

I thought I would like to try it with a ready DAC chip so I ordered a couple of AD558 samples from Analog Device. (Thank you very much),2877,AD558,00.html

I assembled everything on a pcb one late night a few days ago but there is a problem. When I tried it out I got sound but it sounds HORRIBLE. Some kind of extreme distorsion. The DAC has 2 voltage output ranges but I am 100% sure I am using the 2.56V range. I have also connected a potentiometer between mu covox and amplifier just to make sure its not input distorsion in my amp.

I think the problem might be that the DAC does not have a symmetrical voltage output but wouldnt that be the case also with just resistors on the printer port??? Any good ideas where I should start?

Mikael Abdellah 1st March 2008 01:01 PM

What's wrong with this picture
2 Attachment(s)
Did some more measurements today and something is VERY weird. I tried to reverse all 8 bits bit that made it even worse :S Any ideas what I should do?

Mikael Abdellah 1st March 2008 01:05 PM

Forgot to give you some info. The test signal is a 440Hz wave file 22.050kHz 8bit mono, and the scope is set at 1V/div and 0.5msec.

Bernhard 1st March 2008 02:29 PM

invert only msb

Mikael Abdellah 1st March 2008 03:36 PM

Great idea. I'll give it a try as soon as I can. Thanks.

Mikael Abdellah 3rd March 2008 10:33 AM

Solved the problem late last night. As it turns out I had connected the 7th bit to the input of the 8th bit, and the 8th bit was not connected at all. Discovered the problem with a software that allowed be to turn on and of every bit individually. Now it works great and plays music the way it should. Ahhhh how I miss the good old days.

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