A "lossless" file that was transcoded from MP3 or AAC carries the lossy codec’s signature: a frequency ceiling around 16-20 kHz where the encoder low-passed. Genuine CD rips carry content up to 22.05 kHz. This tool decodes the file locally, FFTs its loudest windows, and reports the ceiling.
Be fair to the music: some genuine masters genuinely have little energy up high (old recordings, quiet acoustic genres). The report always shows the raw cutoff frequencies — judge with them, not only the verdict.
Frequently asked questions
Which cutoff means fake?
A hard stop at 16 kHz smells like 128 kbps MP3; ~19.5-20.5 kHz matches 256-320 kbps. Content right up to 22 kHz is consistent with true lossless — but quiet masters can blur every line.
What about hi-res (96/24)?
The ceiling test still works (Nyquist is just higher); a 48 kHz ceiling on a 96 kHz file is a loud tell that the source was 44.1 kHz.
中文说明
从 MP3/AAC 转码而来的"无损"文件会带上原编码器的签名：16–20 kHz 附近的频率天花板。真 CD 抓轨的内容应延伸到 22.05 kHz。本工具在本地解码文件、对最响片段做 FFT，报告截止频率。
也要对音乐公平：部分真无损母带的高频确实本来就少（老录音、安静的原声曲风）。报告始终给出各窗口的原始截止频率——请结合它们判断，而非只看结论。
常见问题
截止频率多少算假？
16 kHz 整齐截断像 128 kbps MP3；19.5–20.5 kHz 对应 256–320 kbps。能量直达 22 kHz 与真无损相符——但安静的母带可能让界线模糊。
Hi-res（96/24）呢？
判定方法依然适用（奈奎斯特频率更高）；96 kHz 文件的频谱止步 48 kHz，就是源是 44.1 kHz 的响亮证据。