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Old 11th September 2005, 06:21 PM   #1
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Default Need Help with USB device Cmedia CM108

Hi

I need help on how to buffer spdif output signal to coax from the usb chip.

Last week i got one of usb sound card (around $10US) and when with my curiousity I opened it up. What if found is it use CM108 from cmedia as usb-dac chip.
I trace the datasheet of the chip and be able to find out the pin for spdif out; however, I don't know how to build a buffer of its output, or can I simply solder them to a RCA plug. Searching through this site, I saw some thread show diagram for PCM2704 using C 1uf seried with R220K thread 62537. Can I use the same technique with this CM108?


Reference
CM108 datasheet
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Old 4th December 2005, 09:21 AM   #2
Bentso is offline Bentso  Finland
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I would also like to do spdif out to my usb sound card with CM108.

Any luck how to do it?
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Old 4th December 2005, 09:47 PM   #3
JohnW is online now JohnW  Hong Kong
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Hi Guys,

Be very careful when adding a non-isolated SPDIF output to a USB bus device - I've just destroyed my Sony VGN A170 motherboard (taken out the Southbridge device). Apparently there’s insignificant grounding on the Intel 82801 - high currents can flow though the USB data lines due to earth leakage currents flowing from the "Isolated" laptop supply though the USB - DAC - Amplifier "Ground" connection.

The CM108 can provide a bit accurate SPDIF output (Pin 1), but you must disable the internal mixer Pin 38 Low) – then set the Windows mixer to flat setting, no effects and Max Volume.

I confirmed Bit accuracy by replaying a HDCD WaveFile with no decoding problems.

Use Optical or isolation transformer otherwise you could be in the same boat as my Sony Motherboard.

The only problem with the CM108 SPDIF / Windows USB Audio streaming is that it only outputs SPDIF (Data) while “Playing” so the DAC will be going in and out of “Lock” continually – can be annoying to hear the DAC’s Mute relays click with the inevitable Loudspeaker “thump” just to replay a half missing “Mouse Click” while surfing… However I’m sure I was one of the few to have a USB connected Pink Triangle Dacapo DAC with HDCD filter on Skype!!! – ER just moments before I blew my Southbridge…..

Not to mention Sony does not provide replacement motherboards (A8068822A) for my 14 month old machine

John
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Old 8th December 2005, 06:44 AM   #4
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Thank for the info JohnW

Sorry for the problem you got there, may be I am lucky with the direct wiring from the chip.

I've been using CM108 for month now with my NOS DAC TDA1541 (spdif use cs8414) and Cal Lab GAMMA. I connect spdif directly from the chip pin cause I cant find schematic on how to buffer it.

It is good to know that CM108 has bit perfect output. Currently, I play music through foobar2000 with kernal streaming mode and TTA plugin and you are correct that it will output signal when play.

For the other, please beware of the problem JohnW has suggested.
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Old 8th December 2005, 11:22 AM   #5
JohnW is online now JohnW  Hong Kong
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goda_takeshi,

Apparently the insignificant grounding issue is known to effect the Intel 82801- which is used in Laptop computers only…

I bought my Laptop as a ‘Gray import’ from HK – and I’m currently in Prague, the VGN series is apparently for the domestic US market only - So it’s not serviced in Europe – so for anybody who is unlikely enough to face similar problems with Sony servicing Nexttronics (speak with Chris) where able to help – with first class service.

http://www.nexttronics.com/index.html

For 'Bit accuracy' with the CM108, Pin 38 must be tied Low (disable the internal mixer)

USB has very poor Phase Noise performance, and is not helped by the large pulsing Earth leakage currents and high levels of RF introduced by the PC – also the absolute sampling rate is determined by the PC’s internal clock – so expect large PPM errors, which drift about like a drunken sailor, and vary wildly between computers.

John
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Old 8th December 2005, 04:45 PM   #6
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Hi

I checked the data sheet and it say the pin 38 (MSEL) when 0 it is without mixer. And from what you suggested to set it to low, does it means tied it to ground or simply cut any connection to it.

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Old 8th December 2005, 05:49 PM   #7
JohnW is online now JohnW  Hong Kong
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Yes, you have to connect to Ground, if left unconnected it will be High - Mixer enabled.

You will most likely have to lift pin 38 from the PCB, and connect to ground - unless you’re lucky and can cut the trace (it might be connected underneath the chip).

John
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Old 12th December 2005, 02:39 PM   #8
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I did pull pin 38 to ground (very hard to solder) and the result is better than I expected.

Thanks for the suggestion, JohnW
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Old 10th January 2007, 06:27 AM   #9
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I'm using one of these CM108 USB adapters but I don't have SPDIF on my amp. Will disabling the mixer as you describe improve the sound quality in that case? I'm getting a significant amount of hiss and some digital clicking sounds.

I saw another suggestion elsewhere to try adding a 47uF capacitor on the power supply lines so I might try that as well...
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Old 3rd April 2015, 03:26 AM   #10
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You can get the full usb2.0 drivers from here: Cmedia CM108AH Audio Drivers Download
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