Ses Dosyalarının ENF Tabanlı Adli Analizine Kısa Zamanlı Fourier Dönüşümü Parametre Seçimlerinin Etkisi

Hızla gelişmekte olan bilgisayar teknolojisi sayesinde çeşitli tekniklerle dijital ses, görüntü ve video dosyaları üzerinde modifikasyonlar yapılabilmektedir. Bu modifikasyonlar doğrudan dosya içeriğinde olabileceği gibi bazen de meta data üzerinde olabilmektedir. Bu bağlamda dijital medyaların adli analizi büyük önem arz etmektedir. Elektrik şebeke frekansı (ENF) tabanlı adli analiz yaklaşımı dosya bütünlük kontrolünde ve dosyaların kayıt zamanı tespitinde kullanılabilen önemli bir araçtır. ENF sinyali kestiriminde en çok tercih edilen yöntemlerden biri, kısa zamanlı Fourier dönüşümü (Short-Time Fourier Transform - STFT) temelli yaklaşımdır. STFT yönteminde, pencere boyutu ve kaydırma miktarı parametrelerinin seçimi büyük öneme sahip olup, kestirimi yapılan ENF sinyali doğruluğunu, dolayısıyla da ENF tabanlı adli analiz uygulamalarının performansını doğrudan etkileyebilmektedir. Bu çalışmada, STFT parametreleri seçiminin, ENF tabanlı dosya kayıt zamanı doğrulamada performansa ne derece etki ettiği araştırılmıştır. Farklı uzunluktaki ses dosyaları, çeşitli STFT pencere boyutu ve STFT kaydırma miktarlarına göre ayrı ayrı test edilerek karşılaştırmalı bir analiz yapılmıştır.

Anahtar Kelimeler:

ENF, Elektrik Şebeke Frekansı, Ses ve Videoların Adli Analizi, Kayıt Zamanı, Zaman Damgası

The Effect of Short-Time Fourier Transform Parameters Choice on ENF-based Forensic Analysis of Audio

Due to the rapidly developing computer technology, digital audio, image, and video can be modified with various techniques. These modifications may be on the media content or the metadata. In this context, forensic analysis of digital media is of great importance. Electrical network frequency (ENF) based forensic analysis is a significant tool that can be used for checking file integrity and detecting time-of-recording. One of the most preferred methods for ENF signal estimation is the Short-Time Fourier Transform (STFT) based approach. The choice of STFT window size and STFT hop size can directly affect the accuracy of ENF signal to be estimated from media, and hence the performance of the ENF-related forensic applications. This work investigates how the STFT parameters choice affects the performance in time-of-recording verification. A comparative analysis is made for various STFT window sizes and STFT hope sizes by experimenting with different audio lengths.

Keywords:

ENF, Electric Network Frequency, Audio and Video Forensics, Time-of-Recording, Time-Stamping,

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