Go Back   Home > Forums > Source & Line > Digital Line Level

Digital Line Level DACs, Digital Crossovers, Equalizers, etc.

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
Reply
 
Thread Tools Search this Thread
Old 14th April 2014, 01:17 PM   #1
esgigt is offline esgigt  Netherlands
diyAudio Member
 
Join Date: Dec 2013
Default SPDIF voltage-level

Some time ago I built this DAC and connected it via SPDIF (coax) to my CD player. Sounds great... no problems.

Recently I tried the same DAC on the SPDIF output of my PC. 44.1Khz and 48Khz worked fine in both 16 and 24 bits. 96Khz and 192Khz failed, both in 16 and 24 bits..

To find out what caused this failure, I connected a scoop to the cable. The waveforms @ 44.1 and 48 Khz looked nice (a somewhat rounded square-wave) and at good level.

96 and 192 Khz waveforms looked more like a sawtooth/sharp-sinus and at a considerably lower level... not good (unloaded spdif line!). 96Khz level was a bit better than the 192Khz level...

So, now I need to find out what causes this deterioration of the waveform and level..
The options I came up with are:
- the PC card (Asus Xonar D2X) puts out a crappy SPDIF signal.
- the cable (common RCA) I used, ruins the signal.
But how to determine the real cause?

Or can I use a (cheap) fix to improve the wave-form quality and the signal-level?

Last edited by esgigt; 14th April 2014 at 01:23 PM.
  Reply With Quote
Old 14th April 2014, 01:31 PM   #2
diyAudio Member
 
s3tup's Avatar
 
Join Date: Dec 2004
Location: Israel
- scope, probes, probe attenuator settings?
__________________
The missing link between lead and gold in alchemist's world was BS and commerce.
  Reply With Quote
Old 14th April 2014, 02:02 PM   #3
esgigt is offline esgigt  Netherlands
diyAudio Member
 
Join Date: Dec 2013
Quote:
Originally Posted by s3tup View Post
- scope, probes, probe attenuator settings?
Do you want to know which scoop etc.??
- Tek 2215
- probe 1x
- .2V/div

Probably you want to know something else.. please be more specific. Currently I'm not able to record/photograph the screen..

44 and 48 gave a bit over 3 div's (ca. 0.6Volts square-wave with rounded leading/falling edge)
96Khz : a bit under 3 div's (ca. 0.55V but more triangular)
192khz : 2 div's (0.4 V and even more triangular)
  Reply With Quote
Old 14th April 2014, 02:39 PM   #4
diyAudio Member
 
s3tup's Avatar
 
Join Date: Dec 2004
Location: Israel
Tek 2215, 60meg, seems okay (spdif@192=12MHz). To see it as more-or-less sinewave.

- probe 1x
^^ might be that's the problem. You need 10x probe, with more-or-less known bandwidth, at least 50meg.

The reason - the probe has some input capacitance, stated for 10x mode operation. If you use it as 1x, you get lot more capacitance.
Tek P2100 has 14.5 pF-17.5 pF@ 10X and 80 pF-110 pF@1X.
If your probes are more like 50meg, you get something closer to 2x of the capacitance of P2100, thus 160-220pF.
Such capacitance at 12mhz is R=1/2pi*f*c=1/(2*3.14*12000000*220/1000000000000)=60ohm.

There is a series resistor on every SPDIF output, 75ohm. When connected to 60ohm impedance of capacitor, it should give you less than half of real unloaded voltage.

So it is OK, if your probe is 1x.
From your voltage levels it looks you've got 100mhz probes.
Try with 10x probe
__________________
The missing link between lead and gold in alchemist's world was BS and commerce.
  Reply With Quote
Old 14th April 2014, 03:14 PM   #5
esgigt is offline esgigt  Netherlands
diyAudio Member
 
Join Date: Dec 2013
S3tup, thanks for the directions how to use the probes in this case. Voltage levels now look better. Wave-shapes however do not improve towards an acceptable level...
44.1 and 48 still do look acceptable (but too much rounded imho)
96Khz shows a bad wave-shape (DAC reports "error")
192Khz is as bad as it was...
  Reply With Quote
Old 15th April 2014, 07:44 AM   #6
marce is offline marce  United Kingdom
diyAudio Member
 
Join Date: Jun 2007
Location: Blackburn, Lancs
Sketch the waveforms...
Show us your DAC input, PCB traces if possible, cable used etc
There is no correlation between the digital wave appearance and the audio out, apart from it getting so bad that you get no sound out.
it sounds like some signal integrity problem (I presume the DAC can handle these higher rate signals) as the sample rate goes up so does the data rate, and it seems it is this increased data rate that is causing the problem.
Things to try and measure, SPDIF output with a simple 75R resistor across the output of the sound card, look at data rates and RISE TIME for the various data rates. This should determine whether the output is getting corrupted before the cable etc.
Then repeat with the resistor at the end of your cable.
That should isolate some causes and parts of the signal chain.
  Reply With Quote
Old 15th April 2014, 11:23 AM   #7
esgigt is offline esgigt  Netherlands
diyAudio Member
 
Join Date: Dec 2013
Marce, thanks for your reaction.

- Yes, The receiving DAC is capable of 24/192.
- I'll try to measure the SPDIF output. But that requires some diy... and I don't know if I have the right parts in store...
- The wave-shape shows increasing deterioration due to bandwidth-limitations... so checking the SPDIF-out is a very good suggestion.

I'll keep you posted.
  Reply With Quote
Old 15th April 2014, 07:09 PM   #8
esgigt is offline esgigt  Netherlands
diyAudio Member
 
Join Date: Dec 2013
I've been thinking about the symptoms.. Decreasing wave-form with increasing frequency.. First point of interest is the cable, for I do not think that it's likely that electronics are THAT limited in bandwidth...

EDIT: The cable i've been using was meant for composite-video. which might explain the limited bandwidth and crippling the rather HF spdif signal.

My first action (before starting further investigation into the characteristics of the spdif output on the PC) is to try an other cable with better bandwidth-specs...

Let you know ...

Last edited by esgigt; 15th April 2014 at 07:18 PM.
  Reply With Quote
Old 15th April 2014, 07:40 PM   #9
diyAudio Member
 
s3tup's Avatar
 
Join Date: Dec 2004
Location: Israel
Do you have RCA connector to plug directly to Xonar? Without a cable, just the connector.
Try measuring the thing directly on that plugged connector.

Do tou have resistors in your drawer? 75ohm, 100 in paralel with 300ohm, other resistor combos will work.

This way you eliminate the cable altogether, and see what's going on on Xonar's output.
__________________
The missing link between lead and gold in alchemist's world was BS and commerce.
  Reply With Quote
Old 16th April 2014, 08:32 AM   #10
sesebe is offline sesebe  Romania
diyAudio Member
 
Join Date: Jan 2011
Location: Iasi
Which is the length of cable you used?

Be sure that it is 75Ω cable. You can use a standard TV antena cable. This cable it is guaranteed to work until 500-800MHz and it is cheap and easy available. Check if the input of DAC it is 75Ω. If not put a 75Ω resistor to maintain the adaptation. Must be 75Ω to work on high sample rate.

Check if the output of your sound card it is 75Ω. Put the scope directly on SPDIF out without any resistor (output opened), note the voltage level, put a 75Ω resistor (can be made from 2-3 resistors to obtain 75Ω) and now the voltage level must be half if the output impedance it is 75Ω. Similar you can check the input impedance of the DAC, but you probably need a signal generator.

Because you have a scope it is easy to check the cables but be sure that you maintain the adaptation to 75Ω from output of sound card until input of DAC.

Anyway some pictures of the signal will help to identify the problem. So please post some photos.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Voltage level shifter akis Solid State 4 3rd May 2011 07:21 PM
DIY 24bit Mic level A/D -> SPDIF out? scott wurcer Digital Source 1 14th May 2007 09:15 AM
reducing spdif signal level Sjef Digital Source 10 29th May 2005 04:24 PM
what's the best way to convert SPDIF into CMOS level? banana Digital Source 16 23rd June 2004 05:57 PM
Line Level, Balanced Voltage Level... dkemppai Solid State 3 10th December 2002 11:04 PM


New To Site? Need Help?

All times are GMT. The time now is 03:15 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2