Enhancements must only be made to duplicates of the collected audio data and not to the original copy.1 Enhancements in this context include, inter alia, any anonymisation of the data, per BP 5. The preservation of the original copy is imperative to the audio data’s probative value as potential evidence.
It is important to maintain an unprocessed original copy against which duplicates may be compared. Any duplicate copy must at first be identical to the original copy, such that it would share the same cryptographic hash value as the original copy, until the duplicate is enhanced. Any duplicate should be labelled as a duplicate and should be linked back to the original copy (BP 11).
If there are observable, material differences between the contents of the original copy and the enhanced copy, this may affect the admissibility of the enhanced copy in the event it is tendered as evidence. Consequently, the audio data could fail to be admitted into evidence, or be granted less weight at the evaluation stage of an adjudicative proceeding.
If the audio data has been obtained from a third party and it is not established to be an original copy, the Collector should take steps to obtain an original copy and its efforts to do so should be duly documented (see BP 9).
Tech Specs & Resources
Non-material enhancements of audio data may include:
- cleaning audio from excess noise/static (i.e. ‘denoising’ via compression or echo cancellation);
- normalising the volume level of the audio;
- isolating or highlighting the human voices or other desired sounds (for example, via frequency equalisation).
See e.g., H. Fayyad-Kazan et al, ‘Verifying the Audio Evidence to Assist Forensic Investigation’ (2021) pages 29-30.
On the desirability to obtain the original copy of the audio data, see e.g., SWGDE Best Practices for the Enhancement of Digital Audio (2020), para. 2.4.
For steps to take when a hash comparison between the original and a duplicate fails, see e.g., NIST IR 8387 (2022), page 7.
Legal Framework
See section 5.3. on the need to work with duplicates when enhancing audio data in order to bolster the reliability, and therefore probative value, of this data.
See also Prosecutor v. Mladić (ICTY), Decision on Prosecution Motion for Admission of Documents from the Bar Table, paras. 11-12, wherein the Chamber noted that tendering original audio recordings alongside enhanced versions contributed to finding the recordings to be relevant and probative.
See section 5.3. on the importance of detailed record keeping when enhancing audio data for establishing the reliability, and therefore probative value, of this data.
Applicable Ethical Principles Competency; Accountability.
Footnotes
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Prosecutor v Ongwen (ICC), Trial Judgment, paras. 654-655. ↩