# What Autonomous Recording Units (ARU) allow on-board compression?

In an attempt to spare storage, I'm interested in reducing the size of continuous recordings of soundscapes. Are there some Autonomous Recording Units that are able to do this on-board? More specifically, I'm thinking about:

• compression algorithms
• ARUs that allow choosing the bit depth of the recorded data
• other techniques I might not know

For the Wildlife Acoustics SongMeter series, their firmware

"adds a WAV compression option called W4V that will record using various levels of compression to save card space. Resulting W4V files can be opened and/or converted back to uncompressed WAV files using the free tier of Kaleidoscope software."

From the user guide (this is for the SM4 though, note that the Micro does not have this option; I'm not sure about the Mini):

SM4 WAV file compression can be selected to record in a proprietary compressed W4V format. The W4V format is a WAV file compression developed specifically for minimizing loss of useful information in bioacoustics audio recordings while maximizing compression to save on valuable card space. W4V records the same rich meta-data as when recording to WAV. There are three compression levels which result in the following compression: ■ W4V-8: 50% (allows recording twice as long on a given card as compared to uncompressed WAV) ■ W4V-6: 62.5% (allows recording almost three times as long on a given card as compared to uncompressed WAV) ■ W4V-4: 75% (allows recording four times as long on a given card as compared to uncompressed WAV). The compression algorithm does increase the noise floor of the recording but for most bioacoustics uses, the difference between an uncompressed WAV and W4V-8 and W4V-6 will be undetectable since the noise floor in the recorded environment will be higher than the increased recording noise floor. W4V-4 will likely cause a slight increase in broadband noise in the background of the recording. It is best to experiment with your particular environment to make sure the increased noise is either undetectable or tolerable. Wildlife Acoustics’ Kaleidoscope Viewer and Pro software can natively open the compression formats. The free Kaleidoscope Converter can be used to convert the files to standard WAV format for use in other software packages. Additionally, the W4V algorithms are open-source under GPLv3 license. Please contact us if you are interested in a commercial license. Values (Hz): none, W4V-8, W4V-6, or W4V-4 Default: none

The Audiomoth folks have tried lossless compression options but found that the compression rate ended up not being that good (i.e., didn't reduce the data a ton) for terrestrial recordings and therefore didn't pursue it further.

https://www.openacousticdevices.info/support/sd-card-support/does-the-level-of-activity-of-a-soundscape-affect-a-recording-s-file-size

Froniter Lab's BAR-LT supports lossless FLAC compression: https://www.frontierlabs.com.au/bar-lt

EDIT

I'll note we regularly see about 50% compression using FLAC. Not the best compression format (lossy is better) but it is free, open, and a standard.

I think you can use a .flac compression with the soundmeters from wildlife acoustics. They also allow you to choose bit depth. I don't really know about other recorders.

Also, .flac is a lossless compression algorythm. I wouldn't recommend using the .mp3 format as it will result in significant loss of data (certain frequencies attenuated).

• SongMeters from Wildlife Acoustics do not compress to flac format. The modern Songmeters can do W4V format (proprietary?) and the older units used wac format and they provided a c header files for decompressing the files outside their software (Kaleidoscope, SongScope). Jul 28, 2022 at 17:40

The soundtrap models from Ocean Instruments have build in loss-less compression.

Compression is X3 algorithm that is discussed here

For the underwater environment, the WISPR system by Embedded Ocean Systems records directly to FLAC.

WISPR is just the internal board so it’s not really off the shelf available to purchase, but it’s flexible so can be integrated into whatever platform you are wanting to use.