Transcoding detection

Question

Is it possible to detect whether a digital audio file has been transcoded?

For example, if I take a 96kbps MP3 file, decode it to WAV, and then re-encode it at 320kbps, it'll sound terrible, right? But would there be specific artifacts or patterns in the audio file to show that this horrible deed has taken place?

Answer 1

The main artifacts you can expect to see:

• A complete rolloff of actual signal on the low-end and high-end (especially above 15KHz)
• Occasional "chirps" introduced across the spectrum (especially towards the high-end) during sections which are poorly-represented in low-bitrate MP3s (such as broad-spectrum noise such as from percussion)
• Very regular quantized "peaks" of frequency, due to the DCT algorithm used by MP3 encoding

This paper shows several frequency-domain cross sections in a few representative songs, although its feature detection focuses on high-end rolloff rather than psychoacoustic factors (and annoyingly enough they never show a frequency plot of the original source).