It may be necessary for the Collector to compress audio data in order to reduce its file size and facilitate its preservation on a server, and/or its digital transfer to third-party recipients over the internet or across a virtual network. Yet, file size compression can lower the quality of the audio data file–such that pitch, tones, or other audio details cannot be heard. The compression of an audio data file must therefore be undertaken in a manner that ensures the audio is not fundamentally altered and remains reliable and probative in the event it is presented as evidence in court.1 While the Collector should strive to maintain the audio data in its non-compressed form, this obligation is to be balanced with the practicality of the volume of data collected.
If the quality of the audio data is affected by the compression to such an extent that its content becomes less intelligible, it may be considered less reliable and probative.
Tech Specs & Resources
The two types of audio compression are ‘lossy’ and ‘lossless’:
- with lossy compression, data is lost and cannot be retrieved in its original form;
- with lossless compression, no data is lost but the compressed file uses fewer bits to represent the information. When the file is reopened, the original data is then reconstructed. The displayed image is identical to the original source image. For a discussion of different compressed audio file formats, see e.g., HigherHZ, Lossless vs. lossy audio: FLAC, WAV, MP3, and other formats, (2021), which notes that ‘MP3 or MPEG Audio Layer III is the most popular lossy audio format and still one of the most used overall’.
For an overview of video and audio compression, see e.g., RGB Spectrum, Digital Video and Compression.
For a description of the potential disadvantages of compression and resulting distortion for evidentiary value, see e.g., A. Koenig and Freeman, Cutting-Edge Evidence: Strengths and Weaknesses of New Digital Investigation Methods in Litigation (2022), pages 1246-1248.
Note: There are no known publicly available tools that permit for the transfer of audio without compression at the time of this Protocol’s completion, although there is at least one such tool known to be in development.
Legal Framework
See section 5.3. on establishing reliability, and therefore the probative value, of audio data. The legal framework does not discuss compression specifically, but an analogy can be drawn to case law dealing with enhancement of audio evidence and particularly the requirement that enhanced data should not be materially different from non-enhanced copies.
Applicable Ethical Principles Do No Harm; Accuracy, Impartiality, and Objectivity.
Footnotes
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Analogical application of Prosecutor v Ongwen (ICC), Trial Judgment, paras. 654-655. ↩