To test this feature, visit your live site.

Resample recordings

Is it possible to resample a recording (as opposed to downsampling). If for instance I record at a rate of 250000 thinking there may be some high frequency bats, but in fact there are none, but there are some bird calls of interest. Then to save space and to listen to the recordings at correct pitch I would like to convert file to 48000 (just as if I had recorded at that rate in the first place). My go-to software for listening / processing audio is Adobe Audition and that does not support playing files with sample rates above 192000 - so I can't just listen to these high sample files with that. Well I am sure it must be possible, but I am not sure what software to use. I have Adobe Audition, but that will covert to a lower rate by stretching out the file. And the AM tool reduces pitch.

13 comments

13 Comments

davidlee_malvern

Aug 11, 2022

@j.j.lloyd

Hi Jim,

It is straightforward to resample a recording using Audacity - and an anti-aliasing filter is automatically applied before down-sampling.

The procedure is a bit counter-intuitive, though, and you control the sample rate of an exported file by setting the "Project Rate" at the bottom left of the Audacity window.

The "Rate" option on the track drop-down just changes the playback rate without resampling the data - so it changes the pitch and speed, as you have observed. Whilst the "Resample" option on the Tracks menu does what it says, it only resamples the selected track within Audacity and is used when you are mixing tracks recorded with different sample rates.

Whatever the sample rate of an individual track within the program, it will always be resampled if necessary and exported with the selected Project Rate.

Hope that may be helpful.

David

j.j.lloyd

Aug 11, 2022

Replying to

Thanks David

i can do it in Audition which is what I normally use.

The issue is that that antialiasing filter will only work if you have sampled at a high enough rate in the first place (2x highest f in the environment) - otherwise the aliasing is ‘baked in’ and cannot be removed.

So if as I my case there are lots of bats around but I am only interested in birds I have to sample a lot higher than I need and then downsample - (which takes more power and storage) or put up with interference.

This is why the proposed analog filter is such a good idea.

Jim

davidlee_malvern

Aug 11, 2022

Replying to

@j.j.lloyd

Jim,

Apologies! I misread your original post and thought you were using Audacity.

I also use Audition most of the time (although still using ver 3). It looks like you have sorted re-sampling in Audition, but, for anyone else who may be struggling, you need to use "Convert Sample Type..." (F11) rather than "Adjust Sample Rate" - on the Edit menu.

j.j.lloyd

Aug 11, 2022

Replying to

Thanks David I have a subscription to Adobe so have the lastest version. I have the options of 'Interpret Sample Rate' - or 'Convert Sample Type' (both under the Edit menu). The former allows you to hear the sound at a lower rate, whereas the later make a permanent change with more options (including filter).

Jim

j.j.lloyd

Jul 24, 2022

Thanks Alex

I don’t think that does what I want ?

So I recorded at 250000 to see if there were high frequency bats in the area. In fact there was not much of interest at high frequency but there was some bird calls at a few kHz. So I want to just discard the high frequencies and keep the lower frequencies- just as if I had sampled at 48000 to start with. So making a smaller more manageable file, without any pitch shift.

is that possible?

sorry if I have misunderstood

jim

Alex Rogers

Jul 25, 2022

Replying to

Yes, the next generation of AudioMoth will likely have configurable analog anti-alias filters to avoid this. The software oversampling that we do functions as an anti-alias filter but you can do much better in hardware. One option is to sample at a high frequency and apply a filter afterwards. But then you still end up with a large file size.

j.j.lloyd

Jul 25, 2022

Replying to

Thanks Alex

In practice I suspect the number of occasions where a bird and bat call will overlap will be pretty small so for long overnight recordings I will use 32 kHz I think and use the higher frequencies for shorter recordings around dusk where both bats and birds will be of interest.

Thanks for the earlier demonstrations of noise reduction with downsampling. Would you get the same effect just by using a low pass filter? (of course you would have a file size bigger than it needs to be in this case) Jim

j.j.lloyd

Jul 26, 2022

Replying to

Alex

just a quick addition to my earlier answer. I think it would be a useful addition to the downsampling tool to add the low pass filter set at the nyquist limit of the new rate - as a User option.

I understand what you say though as to why you don’t have that at the moment.

Jim

Alex Rogers

Jul 24, 2022

Yes, you can downsample from 250kHz to 48kHz in the AudioMoth Config App. It will handle the necessary resampling.

You can also view the 250kHz recording in the AudioMoth Filter Playground (https://playground.openacousticdevices.info/) and see the effect of downsampling immediately with the sample rate controls in the top right.