What is this odd sound spike Spek is showing me at 16kHz?

[Darkmatter]

Hi, I wanted to update my ver of Toradora to 1080p FLAC but all of the FLAC audio uploads have this odd line going through the program Spek at about 16kHz.

Is this a problem? Is this affecting sound quality? I've never seen a graph with something like this in it, but I've only recently been trying to update my anime with FLAC audio and only found Spek recently.


[Attachment 45239 - Click to enlarge]

Thanks.

[KarMa]

I forget what causes that ~16khz signal but it's extremely common, especially when it comes off tape audio sources like HiFi VHS and cassette. Along with other audio sources. I would not worry too much about it.

[Darkmatter]

OK Thanks. This is from a BD but it's not the newest show so who knows what they used when they recorded the original audio.

I'm just happy to know that it's not a big deal.

[pandy]

Use higher resolution for FFT and check if this is 15625Hz or 15750Hz - if this is one of those two then it means that you have TV Horizontal deflection frequency (probably crosstalk from cabling or poor PCB layout) - small differences from nominal frequency usually mean sampling clock shift/drift.

[Darkmatter]

Sorry I don't do encoding so I don't know what FFT stands for and doubt it would matter even if I did. At best I can use MKV Tool Nix GUI to mux in or out parts of a file.

Here is all the info on the file from a program called Media Info.

Sorry you may have to right click and select view image. When I look at it it's scaled down. It only shows its true size if you view the image directly.

https://ibb.co/heUPXS

I have no idea if the info you need is in there. Even if it was I wouldn't be qualified to change the actual encoding.

Thanks.

[dannyboy48888]

Ive wondered this myself as a lot of my rips have it, never "heard" it though so never looked into it

[dannyboy48888]

@darkmatter if you full screen spek it will give a bit more precision and you can save a PNG of it

[Darkmatter]

Ok I have no idea if this has been downsized or not but it was still a 2MB file after I made it an 8bit sRGB PNG file. I even downsized it to a length of 1920px wide...

https://ibb.co/b0Shmn

The line is just above 16kHz. You would think it would cause sound issues but it doesn't seem to. *shrug*

I think this falls into the category of, no harm, no foul.

[raffriff42]

Audacity can determine the frequency with good precision - if the tone is a single frequency and not too distorted.
Select audio, Analyze menu, Plot spectrum.


[Attachment 45258 - Click to enlarge]
- note the frequency under the mouse cursor is shown, and the freq. of the nearest "peak"
- note "size" option is set to largest possible (65K) for best freq. resolution; this is the "FFT resolution" mentioned by pandy above.
- note "function" = "rectangular window"; best for detecting narrow peaks

You can remove the tone in Audacity with a notch filter (download).
Select audio, Effect menu, Nyquist submenu (maybe), Notch filter.


[Attachment 45259 - Click to enlarge]
- enter the frequency you detected in the previous step.
- note this example uses a very high Q (16.0) which creates a narrow notch;
- if the noise is not a pure tone you will need a wider notch, therefore a lower value of Q (Q calculator).

The result after notch filter.


[Attachment 45260 - Click to enlarge]


Pandy is right about the horizontal frequency as a likely cause. The stereo pilot tone is another one:

In the NTSC television system, a pilot tone of 15.7342657 kHz is used to indicate the presence of MTS stereo.

[pandy]

Pandy is right about the horizontal frequency as a likely cause. The stereo pilot tone is another one:

In the NTSC television system, a pilot tone of 15.7342657 kHz is used to indicate the presence of MTS stereo.

Usually pilot is very low level - around -50 - -60dBFS and it should be attenuated by notch filter (a.k.a trap) however frequently it is suppressed by low pass filter, FM radio broadcast or TV usually has bandwidth way bellow 15kHz - thus why for TV 32kHz sampling frequency was chosen - original spectrogram imply that source is rather digital 48kHz (strong presence of signal above 22kHz).

[dannyboy48888]

Good to know, does it "have" to be removed or is it just a visual nuisance? I haven't had it interrupt my movie enjoyment, just curious Is all now that I know what it is lol.

[jagabo]

If you're over 50 years old you probably can't hear over 15 KHz. If you don't have decent speakers they probably don't output over 15 KHz.

[pandy]

There is one argument to remove such signal - this interferer stealing bits from useful signal so you can improve lossy coding by removing unwanted signal thus more bits will be allocated to useful signal (doubt about serious quality improvement but depend on codec and overall bitrate).