Rational Acoustics

Joseph Pearce
September 3rd, 2012, 04:38 PM
Hey Guys,

A quick question: when I switch to "reference to generator" I get incorrect delay times- stupid times although the freq response measurement is good.

I am using an Edirol UE-25EX and have been using it for a while and haven't encountered this problem before- so I guess I have changed something or the unit has developed a fault.

Has anyone come across this before?



September 5th, 2012, 09:57 AM
From the seldom-read Help files ;-)

"Transfer Function measurements can also be referenced internally to Smaart's signal generator by enabling "Allow Multi-Device Transfer Function" option on the Transfer Function page of the main Options dialog and selecting the Generator as your reference signal source. In practice there are a few caveats to doing this, again mainly associated with delay times. For one thing, the latency of the measurement system from the output to the input becomes a factor in measuring absolute delay times. (This is generally not a factor when actively actively recording both input signals.) Another is that the latency through the measurement system may not be consistent from one measurement to the next, meaning you may need to measure the delay again each time you stop and restart the measurement and/or the signal generator. And finally if the measurement signal input and the output used to stimulate the system under test are not identically clocked, then the reference delay time will be subject to drift while measuring, and not just when restarting the measurement."

Joseph Pearce
September 7th, 2012, 06:23 AM
Thanks Johnny,

Yes, I think it might be the latency of the device. When I do an analog "ref-to-gen" with a cable from output to input, all is good. It is only when choosing the "ref-to-gen" loopback in the software do I get incorrect delay times, hence the question on Smaart support.

The UA-25ex is not an expensive pre and for what it is, has worked quite well for me for several years. I thought more people would be using it, perhaps some people who read this forum...maybe not.

September 10th, 2012, 09:36 AM
You will never get an accurate delay time when using ref to gen, even if the input and output devices are on the same clock. At the very least there will be some additional delay as the reference signal has little propagation delay and is virtually instantaneous. Driver type will also impact the delay. As you are using a 2 channel device I suspect that you are using WAV drivers. Which is the worst case scenario. The delta between the signals will likely be considerably larger than expected and may vary wildly from measurement to measurement. This is because of how the WAV API is structured and is beyond my control. If there are ASIO drivers available, you will likely get a consistent delta that is much smaller.