Optimal REW settings for measuring amplifiers

[reallystate]

Hello everyone,

I'm starting to use REW to measure the performance of my headphone amplifiers and I have several questions about the optimal configuration. I've done some reading, but I would appreciate some expert advice to ensure my measurements are accurate. My main questions are:

• Should I measure at the maximum possible sample rate, or should I limit it to 48 kHz?
• Should I use low-pass and high-pass filters? If so, at what frequency should I set them?
• What FFT value and window type should I choose? Should these values be changed for different types of tests (THD vs level, IMD vs level, etc.)?
• What sweep length value should I choose when measuring the frequency response of a device, since this choice affects the phase measurement result?
• What buffer size should I set in the sound card settings section?
• Should I use the Overlap option?
  
  Thank you in advance for your guidance.

 

[John Mulcahy]

Should I measure at the maximum possible sample rate, or should I limit it to 48 kHz?

Generally best measuring at the lowest rate that covers the bandwidth you are interested in.

Should I use low-pass and high-pass filters?

Can't think why you would.

What FFT value and window type should I choose?

For RTA measurements longer FFTs would give greater frequency resolution and lower noise floor. 64k is generally sufficient for many purposes. Blackman-Harris 7 is a good window choice for most measurements, though you could use a Rectangular window if your input and output are on the same interface.

What sweep length value should I choose when measuring the frequency response of a device, since this choice affects the phase measurement result?

No, sweep length doesn't affect phase. Longer sweeps give greater signal-to-noise and a lower distortion noise floor.

What buffer size should I set in the sound card settings section?

16k is usually sufficient.

Should I use the Overlap option?

Using maximum overlap will produce faster RTA display updates, each update needs to wait for sufficient samples to be acquired.

 

[reallystate]

Thank you for your responses.

Can't think why you would.

When i use low-pass and high-pass filters, the noise is lower

Generally best measuring at the lowest rate that covers the bandwidth you are interested in.

I don't know what sampling frequency is worth looking at for measuring amplifiers. At a sampling frequency of 48 kHz, the noise is lower than at 192 kHz. And if you also enable filters at 192 kHz, the noise will be even lower. On the other hand, the use of filters can negatively affect the calculation of the distortion level.

No, sweep length doesn't affect phase

Is this a bug or a feature?

 

[John Mulcahy]

The distortion limits for most published specifications are 20 Hz to 20 kHz. Sample rate will have an effect if your limits exceed half the sample rate.

For sweep length differences attach the mdat file with the measurements rather than images.

 

[reallystate]

For sweep length differences attach the mdat file with the measurements rather than images.

 

[reallystate]

Sample rate will have an effect if your limits exceed half the sample rate.

When I measure a certain device at -20 dBFS using 192 kHz with 20-20k Hz filters, the noise level is -81.5 dBr, but when I use 48 kHz with 20-20k Hz filters, the noise level is -75.9 dBr. This can be seen in the screenshots above. What sampling rate should I use in this case?

 

[John Mulcahy]

Looks like an issue with clock rate differences between the input and output devices. If you add a loopback connection as a timing reference REW can use it to correct the clock difference.

 

[John Mulcahy]

Interface behaviour can also change with sample rate, best check that. For headphone amps 48k should be fine.