CDROM - No Digital Audio Outpt -- Totally Stumped

x-pogo

Member
2007-12-15 6:15 pm
Trying some DIY stuff with the digital audio output of a CD-ROM drive and am stumped.

Been using a yard sale DVD-ROM for some fun. It works fine under Win98SE and Windows Media Player but it is way too noisy. The two pin digital audio output works and for testing have it monitored via a scope -- no other connection -- no sound card. Nice square waves.

Decide to change to a Teac CD-540E (got two). Both were pulls from operating equipment. Got the first one installed, Windows media player likes it. No problems.

Except -- no signal out of the two pin digital audio connector on the back of the drive. Flat line on the scope. Figuring that the drive might need some enable signal from Windows, I went into the Microsoft MCE and then to "CD Audio Device" and clicked on "digital audio." Rebooted and confirmed. Dunno what that does but still no signal.

Downloaded Teac's latest 3.0 firmware and confirmed it was responding with the right firmware update. Still no signal. Installed the second Teac drive with the earlier firmware and same result.

Then installed my yard sale drive and again the scope showed nice square waves. Not my setup.

Called Teac's tech line and got a guy who was equally clueless. The output SHOULD be working -- no special enable required.

What am I missing??? The output should be regular TTL, right? No special pull up or load resistors needed?

Any suggestions?
 
Pulse-R is right. Open the unit up and trace the connection from the SPDIF output backwards. It will end up at a big QFP chip with 80 pins or so. But in between will probably be an empty spot for an SMT resistor. If you stuff this resistor (probably an 0603 or an 0805) with a 100 ohms or so, it should give an output. Good luck
 

x-pogo

Member
2007-12-15 6:15 pm
Thanks for the info.

I took the bottom cover off and found that the trace to the digital output connector disappears into a thru-hole to the other side of the board. More disassembly is required.

The output presently sits at about 100 millivolts above ground. I tied a 4.7K ohm pull-up resistor to 5volts and the output went to about 750 millivolts above ground. . My guess is that the output is actively driven and sitting at the low level -- not an open circuit.

Good design practice would suggest sticking a TTL buffer in to drive off-board signals so maybe the missing link is farther up the signal path.

More tests later.
 
x-pogo said:
Good design practice would suggest sticking a TTL buffer in to drive off-board signals so maybe the missing link is farther up the signal path.

Yes, but they don't care about "good design practice". They only care about keeping the cost low. I have looked at many CD-ROM drives and they all drive the S/PDIF output directly from a large specialized chip that controls many functions of the drive. The spec for S/PDIF output is 1 V p-p, but all of the CD-ROM drives use CMOS levels, but with a high output impedance (> 75 ohms). They just need to get a usable signal to the sound card.