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Comment by vachina | Hacker News Reader

There was a time when Blu-ray players had to implement such audio watermark decoder, to detect and stop playback unauthorized Blu-ray copies.

> Cinavia's in-band signaling introduces intentional spread spectrum phase distortion in the frequency domain of each individual audio channel separately, giving a per-channel digital signal that can yield up to around 0.2 bits per second.

> Cinavia is designed to stay within the audio signal and to survive all common forms of audio transfer, including lossy data compression using discrete cosine transform, MP3, DTS, or Ogg Vorbis. It is designed to survive digital and analog sound recording and reproduction via microphones, direct audio connections and broadcasting, and does so by using audio frequencies within the hearing range.

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So the watermark is audible to listeners.

no, because it's very small phase modulations. The main tradeoff is you need a relatively long segment of audio to detect it.

I recently bought a new synth...i wonder if it has a signature?

*#>Synth manufacturers start licking their lips.

And, for the amateur muso's that still don't get it: imagine having to pay/ have had paid for every time a dx7 was used in a tune, or an ms20?

It's audible in the sense that it exists completely in an audio modality.

But it's not audible in the sense that human ears can't discern it.

The aim of lossy compression is to discard any data that isn't discernible to the human auditory system.

If an audio signal survives a lossy compression algorithm, then there are two possibilities:

- the compression algorithm should be improved, as it preserves some data that isn't important to the human ear

- the signal is audible

That's not exactly the aim of lossy compression. Its aim is to reduce data size while introducing as little discernible effect as realistically possible. That usually means optimizing the algorithm such that most of the loss is indiscernible to us, such as in a darkest regions of an image, or the extremely high frequencies in audio -- both areas we don't perceive with too much granularity. Something like spread-spectrum phase distortion may survive compression just fine but still be indiscernible to us. The two are not mutually exclusive.

Suppose you could encode 0.2 bits per second of watermark in a recording without humans being able to discern it. Suppose the compression algorithm did the same thing to encode additional information which is part of the recording, allowing it to achieve higher quality at the same bitrate or the same quality at a lower bitrate.

It's information theory. Either you can encode additional information without impacting the result, in which case the compression algorithm could use it to be more efficient, or you can't. TANSTAAFL.

And complexity theory says we can't reach the information theoretical limits with generic algorithms.

If there is a known way to cram more data into the same space, compression algorithms are going to want to use it, or something like it which makes use of the same space. Even minor improvements in compression algorithms are extremely valuable at the scale of the internet.

Audio compression algorithms are not trying to be 99.9999999% efficient.

> the compression algorithm should be improved

There is an obvious third choice: the gain in compression isn't worth the effort of writing code to detect these non-discernible artifacts.

People are throwing around 0.2 bits/second in this thread. I doubt a company or individual would write code for such a trivial gain, never mind the slowdown in encoding it might cause.

> isn't worth the effort of writing code to detect these non-discernible artifacts

Why wouldn’t it be worth it? You just have to write the code once and it will affect terabytes of music being compressed from that point forth.

Because it's bad business. How much will people spend to save 0.2bps? My guess is $0. Why spend anything on technology with no return?

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