Bringing the covox into the 21 century

Mikael Abdellah · 9th December 2006, 09:35 AM

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)

http://www.analog.com/en/prod/0,2877,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

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.

9th December 2006, 09:35 AM	#1
Mikael Abdellah diyAudio Member Join Date: Jan 2003 Location: Sweden	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) http://www.analog.com/en/prod/0,2877,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?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Should I bother bringing these back to life?	lowpoke	Multi-Way	4	15th March 2009 12:37 PM
Bringing Marantz PM55 up to SE spec	harrygrey382	Solid State	3	16th June 2007 06:39 PM
Bringing the truth...	roibm	The Lounge	34	19th April 2004 06:15 PM
Dantax century	Luckyme	Multi-Way	0	14th March 2004 03:51 PM
Help bringing up Aleph2. (Smoke)	harvardian	Pass Labs	24	3rd February 2003 09:28 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

1st March 2008, 01:05 PM	#3
Mikael Abdellah diyAudio Member Join Date: Jan 2003 Location: Sweden	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.

1st March 2008, 02:29 PM	#4
Bernhard diyAudio Member Join Date: Apr 2002 Location: Munich	invert only msb

1st March 2008, 03:36 PM	#5
Mikael Abdellah diyAudio Member Join Date: Jan 2003 Location: Sweden	Great idea. I'll give it a try as soon as I can. Thanks.

3rd March 2008, 10:33 AM	#6
Mikael Abdellah diyAudio Member Join Date: Jan 2003 Location: Sweden	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.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)