Effects of iTunSMPB metadata on decoded PCM stream hash reproducibility in iOS AAC audio recordin

Abstract

Decoded pulse‐code modulation (PCM) stream hashing is used for forensic content verification, yet its reliability depends on decoder output stability. This study examines whether the iTunes Seamless Playback (iTunSMPB) metadata field affects decoded PCM alignment and stream hash reproducibility in Advanced Audio Coding–Low Complexity (AAC‐LC) recordings generated by iOS devices. A controlled dataset of 280 recordings was collected from seven iPhone models across multiple iOS versions. Eleven AAC‐LC file pairs containing clean and tagged versions were analyzed to assess tag insertion conditions, decoded PCM alignment, signal integrity, system database behavior, and the effectiveness of metadata‐only versus container‐rewrite remediation workflows. The presence of the iTunSMPB field corresponded to reproducible reductions in leading zero‐amplitude sample padding and associated SHA‐256 decoded PCM stream hash mismatches in 8 of 11 AAC‐LC pairs. Waveform, spectral, and quantitative integrity measures (LTASS, PCC, MQD) remained unchanged. Apple Lossless Audio Codec (ALAC) recordings were unaffected regardless of tag presence. Metadata‐only tag removal restored stream hash alignment for all affected files without altering container structure, whereas FFmpeg‐based container rewriting restored 9 of 11 files and introduced structural changes in all cases. These findings demonstrate that decoded PCM stream hash mismatches can occur without audio editing and should not be interpreted as evidence of tampering without considering metadata state, container branding, and decoder behavior. Future work should expand testing to non‐iOS AAC encoders, alternative decoders, and additional container variants, and develop decoder‐independent PCM hashing methods robust to metadata‐associated alignment effects.