Sample rate and end frequency

[sm52]

John, hi. You had to make two corrections in a couple of days. But that's not all. When in the preferences window I change the sampling rate and then start the calibration of the sound card, the calibration is performed with the set frequency. But when I go to the measurement window, there is always a final measurement frequency of 22050 Hz. I switch the sampling rate to any in the measurement window, but the final frequency remains 22050 Hz. If you go to the preferences window and switch to any frequency, and then to the measurement window, the final frequency remains 22050 Hz. Although, when you hover the mouse over the end frequency in the context menu, the changed frequency. If you start a measurement, it ends at 22050 Hz. When you press the generator button, the final frequency is 22050 Hz. One video file is too large. Therefore, there are 4 of them.

 

[John Mulcahy]

Changing the sample rate does not change the measurement end frequency unless the end is beyond the valid range for the chosen rate. If you want a higher end frequency, enter it.

 

[sm52]

Changing the sample rate does not change the measurement end frequency

Yes. This is normal. I wrote about the default behavior. If I changed the sampling rate, then the final frequency should automatically change. If I need a different end frequency, I will change it after changing the sample rate. This behavior seems normal to me by default. In addition, there are always beginners who want everything to work out right away.

 

[sm52]

Hi. I opened the generator and RTA. Turned on the memorization of peaks. It turned out that the generator does not work up to the end of the range. Somewhere 2 kHz before the end of the range, the generator level begins to fall non-linearly. This happens at any sample rate. Could it be possible to make the generator work to the end of the range that I specify? If I indicated up to 24 kHz, then up to 24 kHz its level should be flat (decrease linearly). If this is a feature and it cannot be changed, then let it work a few kHz above the specified range, so that a linearly falling level remains throughout the specified range. If I specify to work with 12 kHz, it starts from 6 kHz, and by 12 kHz the level goes to the working value. There is no such thing at the end of the range.

 

[John Mulcahy]

If you want sweeps without fade-in or fade-out choose a logarithmic sweep and set the fade in and out you want. The measurement sweep is for making measurements.

 

[sm52]

choose a logarithmic sweep and set the fade in and out you want

I did this. The generator began to operate at the specified frequency, but nothing changed at the end of the range. I cannot set 26 kHz. The generator only works up to 24 kHz. The level decreases linearly up to 22 kHz, non-linearly from 22 kHz.

 

[John Mulcahy]

If your sample rate is 48 kHz there are no frequencies beyond 24 kHz. In your screenshot the generator is set to fade out over the last 1/12th octave of the span. If you measure to the limits of the sample rate the DAC reconstruction filter and ADC antialias filter will also affect the response.

 

[sm52]

In your screenshot the generator is set to fade out over the last 1/12th octave of the span

Made not to fade. The generator starts abruptly at the specified frequency, but at the end it still fades out smoothly. Do you think this is a feature of the sound card?
Data cannot be received above half the sample rate. Is it possible to force the generator to run to half the sampling rate + 2 kHz?

 

[John Mulcahy]

The generator starts abruptly at the specified frequency, but at the end it still fades out smoothly. Do you think this is a feature of the sound card?

It is a feature of every system that converts sampled data to and from analogue signals.

Is it possible to force the generator to run to half the sampling rate + 2 kHz?

It is not possible to represent any frequency higher than half the sample rate. At half the sample rate there are only two samples in each cycle of the waveform. Above half the sample rate there are fewer than two samples, and the result is the same as sampling a much lower frequency, which is what would be produced. That is called aliasing.

 

[sm52]

Above half the sample rate there are fewer than two samples, and the result is the same as sampling a much lower frequency, which is what would be produced.

I understand it. But I am saying that you do not need to receive data above half the sample rate. We get only the data that is possible - up to half the sample rate. But the generator will run a little longer to ensure data linearity at the end of the range - the last 2 kHz. Data above half the sample rate will be discarded. For a sampling rate of 48 kHz, the generator will run up to 26 kHz. But we will take the latest data at 24 kHz. It's impossible?

 

[jtalden]

Hear is one of many sources if info.
Anti-aliasing filter - Wikipedia

 

[John Mulcahy]

The generator data also consists of samples! Sampled data cannot contain frequencies above half the sample rate. You really need to spend some time to understand what sampling means.

 

[sm52]

Ok John. I read all the links you give. Thank you very much. I am also very glad that you are always ready to answer questions. To obtain 'linear measurements', it remains to use a higher sample rate.
Merry Christmas! I wish you health, many happy users, and every success in improving REW and in other endeavors!