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Old 27th June 2005, 05:01 PM   #1
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Question PCM2906 PCB Digital Input Problem

I have attached the PCB layour for my PCM2606 USB sound card. If you want a schematic look at the PCM2906 datasheet, I have used the basic connection without a voltage regulator. The unit works perfectly well playing audio from the PC and also when I use the onboard ADC with the analog inputs (although the input volume seems rather low...but it measures as flat as possible with SpectraLab). The problem I have is that when I connect a digital source such as my NEC3XP+ cd player I get no sound. As I am new to digitla I was wondering if any of you digital gurus out there could help me out. Do I need some kind on filter on the input or something to change the signal level etc? Any help would be much appreciated.
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Old 27th June 2005, 05:06 PM   #2
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Here is a photo of the board itself. I have since swapped out the ceramic SMD caps on the output for some polypropylene units of the same value...sounds much better. Anyways any help getting the digital in to work would be appreciated. By the way I have only tested it with one source so far, but I assumed that the digital out on the CD player would be a perfet source...maybe I am wrong?
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Old 27th June 2005, 06:56 PM   #3
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Hello Gcollier,
it's normal that you don't get any sound connecting a digital input to the PCM2906 because it is not a DAC!
With the digital input normally you can only record.

Anyway, to listen what is on the digital input you can use any software that offers a direct monitoring of what is in input.
Usually this is a function offered in software that use ASIO drivers. For this purpose check the ASIO4ALL free driver, that works perfectly with the PCM2906.

Bye
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Old 27th June 2005, 08:13 PM   #4
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Quote:
Originally posted by paologatto
Hello Gcollier,
it's normal that you don't get any sound connecting a digital input to the PCM2906 because it is not a DAC!
With the digital input normally you can only record.

Anyway, to listen what is on the digital input you can use any software that offers a direct monitoring of what is in input.
Usually this is a function offered in software that use ASIO drivers. For this purpose check the ASIO4ALL free driver, that works perfectly with the PCM2906.

Bye

Paologatto,

Thanks for the reply. I guess that could be my problem. I was under the assumption that if I were using the digital in that the built in windows drivers would automatically output the sound, as with the analog in . I will give ASIO4ALL a try. By the way I hve seen your products on the Purebits website...VERY NICE! I am actually intending on using this unit for measuring speaker parameters with the analog in...but wanted to try out the digital section...just because.

Thanks
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Old 27th June 2005, 11:58 PM   #5
kostix is offline kostix  Mexico
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Hello!

I am almost 100% sure that your problem resides in the digital levels

As you can read from the PCM2906 datasheet, the SPDIF digital in and outs, are TTL level (0v-5v), and MOST of the professional audio equipment have their SPDIF inputs and outputs at a much lower level (0.5V peak to peak), so thats why when you connect your cd player to the digital inputs, you dont hear anything.

I had exactly same problem when i designed a PCB for this nice chip. After looking thru the web, i came along this little level converter circuit which suited perfectly these needs.
Its a simple circuit based on a HIGH speed 7404 inverter (74HCT04)
I attach the Eagles files for this converter circuit.
and the source for this information is at

http://sound.westhost.com/project85.htm

scrolling down the page, at the end of it, you'll find 2 circuits managing TTL - SPDIF coaxial conversion and viceversa.


Good luck
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File Type: zip coax-ttl-coax converter.zip (21.9 KB, 124 views)
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Old 28th June 2005, 03:26 AM   #6
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Quote:
Originally posted by kostix
Hello!

I am almost 100% sure that your problem resides in the digital levels

As you can read from the PCM2906 datasheet, the SPDIF digital in and outs, are TTL level (0v-5v), and MOST of the professional audio equipment have their SPDIF inputs and outputs at a much lower level (0.5V peak to peak), so thats why when you connect your cd player to the digital inputs, you dont hear anything.

I had exactly same problem when i designed a PCB for this nice chip. After looking thru the web, i came along this little level converter circuit which suited perfectly these needs.
Its a simple circuit based on a HIGH speed 7404 inverter (74HCT04)
I attach the Eagles files for this converter circuit.
and the source for this information is at

http://sound.westhost.com/project85.htm

scrolling down the page, at the end of it, you'll find 2 circuits managing TTL - SPDIF coaxial conversion and viceversa.


Good luck

Thanks I am pretty sure that this is my problem. I will give the Hex invertor a try...I just happen to have a few of these I can desloder from an old junk board. Hope it works!

G.
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Old 28th June 2005, 11:18 AM   #7
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Gcollier,
maybe it's an hardware problem, but also try the trick about recording..
I (re)found a little nice utility that allows you to use the PCM2906 as a DAC. You find it here:

Simtel link

or search on simtel website for the file wdsp01b2.zip

It is a software that records what is in soundcard input and plays it in real-time on output (also with optional equalization). Just set 44100 Hz sample rate and 16 bit IN/16 bit OUT and give a SPDIF signal to the PCM2906 digital input.
Note that it uses the preferred sound device in windows, so be sure to have set the USB CODEC as preferred device in windows multimedia control panel.

Let me know..
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Old 28th June 2005, 06:20 PM   #8
diyAudio Member
 
Join Date: Mar 2003
Location: Windsor
Quote:
Originally posted by paologatto
Gcollier,
maybe it's an hardware problem, but also try the trick about recording..
I (re)found a little nice utility that allows you to use the PCM2906 as a DAC. You find it here:

Simtel link

or search on simtel website for the file wdsp01b2.zip

It is a software that records what is in soundcard input and plays it in real-time on output (also with optional equalization). Just set 44100 Hz sample rate and 16 bit IN/16 bit OUT and give a SPDIF signal to the PCM2906 digital input.
Note that it uses the preferred sound device in windows, so be sure to have set the USB CODEC as preferred device in windows multimedia control panel.


Let me know..

Thanks Palogatto I'll give it a try. I tinkered around with ASIO to see if I could get it going and I am almost certain now that the problem resides in the input level as suggested kostix. When tinkering last night I could clearly see the the connection is good, I just need to do the SPDIF to TTL level switching. I'll let you know how it turns out.

G.
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