Confused measuring Thiele Small Parameters

[rstcologne]

Hi all,

likely my question is related to a user error but I can't figure it out.

I'm running 5.20.4 and I'm trying to measure Thiele Small parameters following a mix of the audio judgement tutorial and the manual. I have built the cable and measured it thorugh a couple of times, no short circuits, near zero ohms where expected and the 100 ohm also where expected.

When trying to run the short circuit calibration, I get the following error:

Measurement input should be much lower than the reference input for the short circuit calibration but it is not - the measurement peak is -32,8 dBFS, the reference peak is -21,2 dBFS.

If I switch left and right on the input side (hardware), I get the same error but the values change, measurement is -21, reference is -32.

As I do see different values for the channels, it looks to me like my cable is ok and that my soundcard is indeed picking up two separate channels. However I'm really confused that I get the error in both cases. Is this a bug or am I missing something?

Thanks,

Richard

 

[Antonio Di Motta]

You certainly have problems with how the impedance is measured. Impedance calibration involves three tests, the first with an open circuit, the second with the circuit shorted and the last with a reference resistance.
The measurement performed by rew is perfect if the suggested circuit is made correctly and if an external amplifier is used that is able to deliver an adequate level of output power.
With the open circuit we have that the same voltage level is present on both input channels.
With the closed circuit we have the voltage level generated on one channel and on the other a voltage close to zero volts. After these two tests, the third with the reference resistance is necessary to have an accurate reading of the impedance value.

Greetings Antonio

 

[rstcologne]

Hey Antonio,

thanks for your reply which summarizes well what I have tried based on videos, articles and the manual.

I have tried a few more things and eventually deinstalled the 5.20.4 version of REW and installed 5.20.1. I then used the same cable and settings and went through all three steps and at least the process worked. Still some messages about raising levels but that's tweaking I guess. Apologies for not providing an update earlier and saving you the reply.

I now believe 5.20.4 has a bug in the calibration routine as the error message shows up when there is in fact a significant delta in measurement's peaks as expected. It's even stated like that in the message.

Thanks again for trying to help,

Richard

 

[John Mulcahy]

The difference is only 11.6 dB, which is a factor of 3.8. It is not unreasonable to expect a short circuit (so the input on that channel is actually connected to ground) to measure a lot lower than an input which is connected directly to the output. The shorted input level is still more than 2% of full scale, it really should not be. You don't say what interface you are using, but perhaps it struggles with the 100 ohm load it sees on the output when running the short circuit calibration. If so a higher sense resistor could be tried, but a more powerful output would be better. Failing that there may be some error in the cable.

 

[Antonio Di Motta]

when you do the open circuit test, you must verify that the input signal of the two channels is identical, otherwise the software is not able to make the correction for a certain difference in height. Probably the signal height threshold in version 5.20.4 has been decreased and therefore gives you an error.

As you can see from the diagram I have deliberately specified that the divider constituted by the resistances must be with an accuracy of at least 1% precisely to avoid excessive differences in level measurements between the right and left channel.

Then there is also a possible problem of imbalance of the input channels that can lead to this problem. I advise you to check in open circuit with the generator set to 1Khz 0dbfs and the level meter and check the value of the signal on the two channels.

Greetings Antonio