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Old 27th October 2008, 01:55 AM   #1
x-pogo is offline x-pogo  United States
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Join Date: Dec 2007
Default CD ROM Digital Out Problems.....

Just for fun I fired up an Asus CD-S520/A5 CD ROM, which plays fine off the IDE cable. Took a look at the digital output (two pins) on the back of the unit. They were active TTL at 5 volts.

I connected a Toshiba TOTX177 optical transmitter to see if I could connect it to my system. Picked up the 5 volts and ground from a splice in the power cable. The output LED is nice and bright. This is sort of a no-brainer transmitter -- straight 5 volts power and ground plus the TTL input.

But when connected to my Sony there is no sound output. A Panasonic player with digital output works just fine through the Sony so it has to be related to the Toslink or the the CD-ROM.

Any suggestions?
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Old 27th October 2008, 01:05 PM   #2
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Location: Johannesburg, South Africa
It could be an impedance mismatch, that the panasonic is more tolerant of. Try putting a 50-ohm (more or less, i think 47, or 51 will be close enuff) resistor on the end of the SPDIF data cable where it connects to the toslink device.
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Old 27th October 2008, 04:29 PM   #3
x-pogo is offline x-pogo  United States
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Sorry I wasn't clear.

As I understand it, the two pin digital audio output on the back of the CD ROM is TTL which is not directly compatible with the coax inputs on regular digital audio gear. I happened to have a brand new Toshiba TOTX177 optical transmitter in my parts box and decided to use it to convert from the CD ROM's TTL output to Toslink optical. Then run an optical cable to my audio gear.

I checked the CD ROM output and it was active TTL with highs around 4 volts and lows at less than 1/4 volt. All looks OK.

I pulled power for the Toslink adapter from the CD ROM drive cable and used a two pin connector from the CD ROM drive's digital audio output pins to the Toslink's data input. The LED is nice and bright. I then put a CD in the loader on play for testing. In short -- there is no coax spdif cable involved so no impedance or level issues.

I connected the optical cable to my DEQ2496 and it reported the bit rate correctly but no action on the level meters. So the signal appears to be getting through the optical cable and being properly detected as present. At the moment I figure that the CD ROM drive's digital DATA output is disabled in software but the SIGNAL, with no actual data, is still present -- that is, silence is being transmitted.

I'll try to test this by installing the drive in a computer, and use Windows to enable the digital /analog outputs. If the default mode of the CD ROM is no digital output, then the drive is useless for standalone use.
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Old 27th October 2008, 05:19 PM   #4
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Oh kay - i follow your setup correctly now.
Sounds like an enable / disable thing, though I've never heard of the SPDIF being switchable - except via maybe some service mode. Usually a soundcard accepting SPDIF, if not required, would just ignore it.
But the SPDIF is evidently putting out a word clock signal (as does my sandalone audio CD player, even when not actially playing a disc.) so your hardware must be correct.
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Old 27th October 2008, 06:18 PM   #5
x-pogo is offline x-pogo  United States
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I've sort of been through this before trying to find a "standalone" CD-ROM to be used as a CD music player.

In Windows there is an option to enable/disable the "analog outputs" of the drive which also affects the digital audio output. I've tested about a dozen drives -- most have the digital audio output disabled by default -- the two pins are totally dead on power up. However, all that I tested came to life after installing them in my computer and setting them to be enabled in Windows.

Without the appropriate software command they were dead. Except for the Asus which was active on boot up.

Now I suspect the line is active but with no data.

I'm about done trying to find a standalone drive that will have the digital audio outputs active and working on boot without external commands.

Maybe somebody else has had better luck?
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Old 27th October 2008, 06:47 PM   #6
kf_tam is offline kf_tam  Hong Kong
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In fear of stating the obvious , have you included at least a resistor between the CDROM SPIF out and the TOTX?

The TOTX is simply LED anyway, and therefore you should limit the current according to datasheet.
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Old 27th October 2008, 07:22 PM   #7
x-pogo is offline x-pogo  United States
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Thanks for the suggestion.

Got the TOTX177 datasheet including the "application circuit."

The datasheet's Maximum Ratings for power calls for Vcc of -.5 volts to 7 volts at 25 deg C. I measured Vcc at 5.05 volts. There is no mention of maximum rating for input current. I imagine there are derating curves somewhere but not on the Toshiba datasheet. The transmitter is simply hanging loose from some wires in open air so temperature should not be an issue.

The "application circuit" shows no current limiting resistor, only a .1 uF bypass cap between Vcc and ground, which I tack soldered directly to the transmitter's pins.

I also seem to remember early transmitter modules required current limiting but from the diagrams in the datasheets I've seen lately from Toshiba and Sharp they have what they call a "driver IC" ahead of the LED. I assume this IC does current management and maybe even some temperature compensation to hold specs to the maximum 70 deg C.
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Old 27th October 2008, 07:54 PM   #8
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Recently my Marantz CD player started acting up, showing signs of its age. I decided to try the CD-ROM drive/DAC approach instead of buying another CD player. Using a controller module bought from member bbp I have successfully played different CD/DVD-ROM drives using their S/PDIF output. Since bbp did not publish the controllerís source code I canít tell you about the initialization commands he used, but you may be able to ask him about it.
How are issuing commands to the CD-ROM drive (play, stop, etc.)?
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Old 27th October 2008, 09:35 PM   #9
x-pogo is offline x-pogo  United States
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This is a just-for-fun project -- a full featured CD ROM based player for less than $50.00 that doesn't require a lot of work. Here is how:

Some early DVD players used IDE DVD drives with the standard computer type ribbon cable and power supply plug.

If you take out the DVD drive and replace it with a CD ROM drive the DVD player doesn't know the difference. All the normal functions including the front panel buttons, display, and remote operate normally as if you popped in a CD into the DVD player. Except, of course it will no longer play DVDs. This gives you a CD-ROM based player with all the features. The digital output plays normal SPDIF audio derived from the IDE bus but, of course, no Dolby Digital. You will need a TV/monitor to do the setup but after that no video is required.

I thought it might be interesting to use the digital audio output directly from the CD ROM instead of the digital output derived from the IDE bus. Some think this is better than the output through the IDE bus and all the downstream circuits. Still, the IDE is parallel and buffered for DVD densities so I figure it might be good enough. Others of course disagree so it was worth a shot. Anyway, the DVD player's command set does not include the command to enable the digital audio output of the CD ROM. Therefore, to use the digital audio output the CD-ROM needs to have it enabled as the default. So far, after testing about a dozen likely drives, I have not found any that will work.

Yes, the DVD ROM based players with a custom controller can and indeed do solve this problem as well as sending the command to slow down the drive to cut noise. But I am lazy and would prefer to do a simple drive swap without all the related construction headaches and expense.

At the moment I will use the digital output derived from the IDE bus instead of the digital audio output and call it good.

I'll do a post on the project in due course.

However, here are a couple tips if you want to give it a try.

Both APEX and SAMPO made early DVD players where a CD ROM will work. Later models used a non-standard drive and cable. In the U.S. on Craigslist they go for about $15.00 or so. $25.00 on eBay including shipping. Both used and new CD ROMs are likewise available and cheap but selecting one is not easy.

Test to see the current draw for the DVD drive and make sure the CD ROM does not exceed it. Current ratings on the drive's label are not accurate - often off by 100%. I upgrade the power supply caps anyway just as a precaution - reduces overall noise levels, especially on the 12 volts. Early CD ROMs consume more power and don't play later CD formats well (CD-R.) But they are quiet. Later units consume less power, likely play more formats, but can be noisy -- stay below 40X or so unless the drive automatically cuts speed on a CD. Requires some testing.

Mechanically the CD-ROM will need "feet" to get to open correctly through the front panel. I used three rubber bottle stoppers trimmed to the correct height and glued them to the drive and chassis. This also provides some vibration damping. The glue joint is strong enough for normal handling but probably not good if you are going to ship the unit. A razor blade will cut through the glue joint for removal, and, after a good cleanup can be glued again.

After the drive is mounted connect the power, IDE cable, and coax/toslink to your system and you are done.
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