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Topic: Get rid of NTSC "flyback" frequency (Read 6002 times) previous topic - next topic
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Get rid of NTSC "flyback" frequency

I have many DVDs with an anoying ~15Hz high-pitched noise whenever someone taks, that I discovered that can be the "flyback" sound of TVs in the studio. I can hear and see it in spectrum analysers. but I don't know how to cut off entirely this noise, w/o damaging too much the rest. What program and what function I use?

If there is some way to eliminate it in the playback, w/o reencoding, better. I just can't watch some of my dvds with headphones and this noise.

Get rid of NTSC "flyback" frequency

Reply #1 –
I assume you mean 15KHz, as 15Hz is certainly not “high-pitched.”

As for a solution, I would try using an equalizer to decrease the volume at ~15KHz. It would affect everything in that range, but I doubt it would be very noticeable, and should greatly reduce the annoyance.

Get rid of NTSC "flyback" frequency

Reply #2 –
Quote from: Caroliano on
If there is some way to eliminate it in the playback, w/o reencoding, better. I just can't watch some of my dvds with headphones and this noise.


Consider yourself lucky.  I now have tinnitus in my right ear which appeared suddenly after a severe ear infection several years ago.  I can never escape.

More seriously, if you can find a parametric equalizer for your playback system, that'll probably do the trick best.

-brendan

Get rid of NTSC "flyback" frequency

Reply #3 –
Third and best way to remove this pitch, is to use the new "drawing"-tools in Audition 3. You can then cut out VERY specifically that frequency. Although you might not want to remove it completely, but probably match it's intensity somewhat close to the other frequencies.

Also you are able to export the frequency analysis to a BMP file, you can edit in an image editor, if you prefer that.

I did this a few times already with great success (although I would prefer a DSP to do the task for me without tampering my files)

The hardest attempt I've discovered is "Groove Armada: My Friend" that not just has a very high pitched sine wave - The pitch itself is sweeped and echoed over a larger frequency width... Hard to describe, but i'll try to post a screenshot.
Can't wait for a HD-AAC encoder :P

Get rid of NTSC "flyback" frequency

Reply #4 –
@odyssey - That doesn't meet the OP's requirement of not reencoding.

Get rid of NTSC "flyback" frequency

Reply #5 –
ffdshow audio decoder (windows) has an option of either an FIR filter (bandstop type in your case) or a generic convolver (where you could supply a designed 15kHz notch filter). Note that the latter involves some DSP knowledge and it is sampling-frequency dependent (you would have to supply different impulse response files for different audio sampling frequencies) if I'm not mistaken.

Get rid of NTSC "flyback" frequency

Reply #6 –
Quote from: odyssey on
Third and best way to remove this pitch, is to use the new "drawing"-tools in Audition 3. You can then cut out VERY specifically that frequency. Although you might not want to remove it completely, but probably match it's intensity somewhat close to the other frequencies.

Also you are able to export the frequency analysis to a BMP file, you can edit in an image editor, if you prefer that.


Actually, both methods are not good for removing a constant tone. (And the BMP method is complete BS).

I would recommend Audion's notch filter, which is designed to handle such issues. A very narrow band FFT filter also could be used. Cavetas: the notch filter is an IIR filter, so it may introduce phase shifts, and the FFT filter is a FIR filter, so it may introduce pre-ringing especially for large filter sizes. Alternatively, noise reduction with just the NTSC tone used as the noise sample could yield good results.

As already mentioned in the thread, all these methods require re-encoding.
Ceterum censeo, there should be an "%is_stop_after_current%".

Get rid of NTSC "flyback" frequency

Reply #7 –
Right!  The "technical answer" is a notch filter.  A notch filter knocks-out one frequency with a minimal affect on the rest of the audio.

Any good audio editing program should include a notch filter.

As far as hardware options, a parametric equalizer[/u] can be adjusted as a notch filter.  Parametric equalizers are usually intended for professional use, and they tend to be expensive  (examples).

A "regular" graphic equalizer[/u] is usually more affordable, but it won't be as effective.  The affected frequency-range will be wider and the noise won't be reduced as much.  (You can't adjust the center-frequency, and "notch" will be wider and less-deep.)

A stereo parametric equalizer with stereo headphones should work fine.  But, with a surround-sound system, you can't easily use a hardware-equalizer.  First, you need 5 channels of equalization, so you'd need three stereo equalizers!  Second, you need "access" to all 5 line-level signals, which you don't have in a typical home theater set-up. 

The software solutions are not so straight-forward either, if we're talking about commercial DVDs with copy protection and surround sound.

Get rid of NTSC "flyback" frequency

Reply #8 –
Well, you could mod the decoder to throw away lines from 15650 to 15850 Hz


Not recommending it, though.
-----
J. D. (jj) Johnston

Get rid of NTSC "flyback" frequency

Reply #9 –
Thanks for all replies. At the moment I'm more concerned with playback time solutions, but re-encode solutions are also welcome, to definitely get rid of this problem in dvd backups.

I will see what I can do tweaking ffdshow's FIR bandstop filter.

EDIT: I solved the problem with configuring the FIR filter in the folowing way:



I also reached good results with an low (32) tap value and an Width of 5000, but with some impact in the vividness of the audio. This latter arrangement with an very high tap value (552 is an random high value that I used and gave good results) and lower width had the same result in the flyback noise w/o any significant side efect (is within the "placebo range". It is subtle, and I'm not doing blind tests). This was an relatively fast tweak and I don't know if I could have done much better, of if the ffdshow filter is partialy broken. But seems sufficiently good for me.

Get rid of NTSC "flyback" frequency

Reply #10 –
ffdshow has some very useful features! Very clever tricks for real time video processing too.

The exact line frequencies (if "faithfully" recorded and reproduced on the soundtrack) are:

NTSC=15734.265734Hz (recurring decimal!)
PAL = 15625Hz

Cheers,
David.

Get rid of NTSC "flyback" frequency

Reply #11 –
Quote from: pdq on
@odyssey - That doesn't meet the OP's requirement of not reencoding.

If you read correctly it's merely a preference rather than a requirement. Still my solution might be the *best* way to remove such noise (which were also stated as a preference) - not the easy way
Quote from: pawelq on
Quote from: odyssey on

Third and best way to remove this pitch, is to use the new "drawing"-tools in Audition 3. You can then cut out VERY specifically that frequency. Although you might not want to remove it completely, but probably match it's intensity somewhat close to the other frequencies.

Also you are able to export the frequency analysis to a BMP file, you can edit in an image editor, if you prefer that.


Actually, both methods are not good for removing a constant tone. (And the BMP method is complete BS).

I would recommend Audion's notch filter, which is designed to handle such issues. A very narrow band FFT filter also could be used. Cavetas: the notch filter is an IIR filter, so it may introduce phase shifts, and the FFT filter is a FIR filter, so it may introduce pre-ringing especially for large filter sizes. Alternatively, noise reduction with just the NTSC tone used as the noise sample could yield good results.

I have not tried the BMP method myself yet, so I can't say anything about that.

I would definitely not recommend Audition notch filter. I tried this method myself, and were not satisfied with either quality or the handling. The drawing-tools give you very accurate precision with ease and I'm even sure that it has no sideeffects like the notch-filter.

Another thing, why don't you think it's good for removing a constant tone? It's been a while since I did this, but it's possible even directly in Audition to create a square just around the exact constant tone. This should also be possible in any image editor.
Can't wait for a HD-AAC encoder :P
 

Get rid of NTSC "flyback" frequency

Reply #12 –
Is the graphical method any different from a FFT filter?