AVS Forum banner
Status
Not open for further replies.
1 - 3 of 3 Posts

·
Registered
Joined
·
5,897 Posts
Discussion Starter · #1 ·
In preparation for a serious calibration attempt in the soon to be new theater, I downloaded ETF (5.983 I believe) and began the process of setup. I assume that the setup part should function fine before they send my license file.


Hardware:


dbx 386 mic preamp and ADC. It has a tube gain section, but for this purpose I have the tube drive essentially turned off, with all the gain in the SS section.


The dbx 386 is sending S/PDIF into an M-Audio Delta Dio 2496 soundcard, using coaxial connection.


For the soundcard setup in ETF, I have the Delta Dio analog outs connected to the line inputs on the dbx 386, which then sends the signal back to the Dio after ADC. I have a Behringer ECM8000 to use for measurements.



Current settings:


No dithering or noise shaping currently on the 386, and at the moment it is set to 48kHz sampling and 24bit word. Delta Dio is set to sync to the incoming S/PDIF signal, and expects a 48kHz sampling rate. Sound card buffering in ETF is set to 48kHz.


I have adjusted gain on the 386 to reach a -2dB signal using the ETF level check.



Problem: I get a strong impulse at 0ms where it should be with a very flat response elsewhere, except for a series of weaker and decaying impulses spaced roughly every 1.6ms or so. On the linear frequency response chart I see a horrid 10db frequency response variation with a very regular sine shaped frequency dependancy for the raw soundcard device channel, but the compensated system response looks quite flat still.


When I try other sampling rates (other than matched 44.1/44.1) or try using the internal Delta Dio clock for incoming S/PDIF sync (instead of the clock in the dbx), I get the resampling variation expected.


Latency settings don't change anything. I get the same results on both channels of the dbx, and noise shaping and dithering don't change things either.


My hunch is that this regular error is caused by the slight mismatch in the Delta and dbx clocks (with the Delta generating ETF's test tone and the dbx doing the A/D conversion back into the software).




Any suggestions or ideas for how to "correct" this? I know that ETF claims the soundcard's inherent frequency response won't matter due to using one channel as a feedback correction (which does seem to work, as evidenced by the still flat system freq. response), but I'm uneasy about this for three reasons: (1) the Delta/dbx combination together have a nearly ruler flat frequency response normally, and (2) the "horrible" frequency response soundcard example they show in the help files has only about a +/- 2dB swing, and (3) the variation is large and regular enough to manifest itself as the quite noticeable series of ~1.6ms spaced measured impulses. Relatively speaking, what I'm measuring would probably be considered about the worst of the worst situations for a soundcard.


Anything I can do to correct this without purchasing still more hardware? I do have an SBLive! 5.1 card with the requisite line in/out, which would require me to purchase a cheap mic pre for the ECM8000. The Delta Dio card doesn't have analog inputs at all (it doesn't have an ADC internally at all), so an outboard ADC is required for that card. The dbx has a wordclock input, so in theory it could sync to the Delta's output, except that the Delta doesn't have a dedicated clock output. Any ideas on making use of one of the unused digital outputs on the Delta to eliminate what I can only assume is a clock sync problem?


And I suppose the most important question is will this repeating impulse response series cause errors in my measurement attempts (which will eventually include raw driver, enclosure, and room measurements)? I measure a muted and/or open loop input level of -75dB in ETF, which according to them would be fabulous (they quote "better than -50dB" for "high end" cards). If I lose the ability to make useable distortion measurements because of the described problem, I would be most disappointed.


And last but not least, if there is a better place to ask these questions, by all means point me that direction. I'd rather exhaust the online community help before going directly to ETF with the problem... generally because I learn more in the process this way.
 

·
Registered
Joined
·
1,028 Posts
Bigus-


I've never encountered your problem so unfortunately I have no ability to help you.


Try emailing ETF acoustic directly, I bet they can address your problem, and they would probably like to hear about your issues.


Also try posting at the tweaks/diy section of Audio Asylum. Lots of people there with extensive experience with the product.
 

·
Registered
Joined
·
5,897 Posts
Discussion Starter · #3 ·
Thanks, I'll try posting at AA with the same question. When I encounter problems like this, I typically like to get input from the online world. Not all of the input is useful or correct, but I usually learn a lot more than I would from the manufacturer. And, if in the end I do go directly to ETF (which might be just a matter of days), I'll be much more knowledgeable about the problem, what possible solutions don't work, and be able to more quickly understand whatever suggestions they might have.


Here's to hoping that there's some ETF experts at AA! :D
 
1 - 3 of 3 Posts
Status
Not open for further replies.
Top