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Topic: Encoding old material at 96KHz question (Read 2043 times) previous topic - next topic
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Re: Encoding old material at 96KHz question

Reply #50
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I would bet that there's significant amount of noise in your recordings and losing it just above 12kHz could be audible...

That's right. When I do an ABX compare of just the noise, it's better on 32KHz (same as source) than 24KHz.


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Also, at 32kHz you keep the NTSC horizontal refresh frequency which you may hear.

I don't particularly hear it on the original files tbh (I don't really hear something above 17KHz, so maybe it's just too quiet)

You talked about near nquist frequencies for DAC, and it's interesting. I guess it's the same issue for digital resampling then. You may have to let some headroom.

Re: Encoding old material at 96KHz question

Reply #51
> NTSC horizontal refresh frequency which you may hear
> I don't particularly hear it on the original files tbh (I don't really hear something above 17KHz, so maybe it's just too quiet)

it's between 15kHz and 16kHz IIRC. 15625 Hz or something like that

Re: Encoding old material at 96KHz question

Reply #52
> NTSC horizontal refresh frequency which you may hear
> I don't particularly hear it on the original files tbh (I don't really hear something above 17KHz, so maybe it's just too quiet)

it's between 15kHz and 16kHz IIRC. 15625 Hz or something like that

Yes I can see that horizontal line above 15KHz in the spectrum. And ther's another one above 30KHz.
But it's not an issue, I guess only animal would be bothered by that :p

Re: Encoding old material at 96KHz question

Reply #53
No, as a child, near CRTs I definitely could hear PAL H refresh which is just a lil bit lower. You may more like sense it than hear, but it definitely can affect audibility. Do yourselve a test - take your original file and lowpass filter it so there's nothing above 15kHz and ABX it... Or highpass filter it so there's nothing under 15kHz and ABX it with digital silence  :))

Re: Encoding old material at 96KHz question

Reply #54
No, as a child, near CRTs I definitely could hear PAL H refresh which is just a lil bit lower. You may more like sense it than hear, but it definitely can affect audibility. Do yourselve a test - take your original file and lowpass filter it so there's nothing above 15kHz and ABX it... Or highpass filter it so there's nothing under 15kHz and ABX it with digital silence  :))

I tried what you suggested with adobe audition : I listened to a frequency between 14500 and 15000 (below NTSC line) and I didn't heard anything disturbing apart from some remaining noise of the track.

Then I listened to the frequency of the NTSC line alone (just above 15KHz) with some dB gain and max volume. It's very high-pitched and annoying.

Is there a way convert to 30KHz instead of 32KHz to make that line dissapear ? or is it good to lowpass to 15KHz on a 32KHz track or not ?

I don't wanna make anything wrong.

Re: Encoding old material at 96KHz question

Reply #55
Lowpass original file so there's nothing above 15kHz and compare it to a file resampled to 24kHz. If you can't ABX them then I'd say that what you ABXed before, was NTSC interference and I would stay with 24kHz files. If you still hear the difference then I'd stay with 32kHz but lowpass filter the NTSC interference. Still there's that noise in 12-16kHz range which, well, is noise - you can hear it in 32kHz files and you may sense lack of it in 24kHz but is it worth keeping when original content is known to end at 12kHz?

And due to masking effects it's not a good idea to judge by listening to extracted noise or NTSC interference alone at insane levels - judge by listening at normal levels with all the lower original content.

Re: Encoding old material at 96KHz question

Reply #56
Lowpass original file so there's nothing above 15kHz and compare it to a file resampled to 24kHz. If you can't ABX them then I'd say that what you ABXed before, was NTSC interference and I would stay with 24kHz files. If you still hear the difference then I'd stay with 32kHz but lowpass filter the NTSC interference. Still there's that noise in 12-16kHz range which, well, is noise - you can hear it in 32kHz files and you may sense lack of it in 24kHz but is it worth keeping when original content is known to end at 12kHz?

And due to masking effects it's not a good idea to judge by listening to extracted noise or NTSC interference alone at insane levels - judge by listening at normal levels with all the lower original content.

Good idea I'll try that.

Re: Encoding old material at 96KHz question

Reply #57
Lowpass original file so there's nothing above 15kHz and compare it to a file resampled to 24kHz. If you can't ABX them then I'd say that what you ABXed before, was NTSC interference and I would stay with 24kHz files. If you still hear the difference then I'd stay with 32kHz but lowpass filter the NTSC interference. Still there's that noise in 12-16kHz range which, well, is noise - you can hear it in 32kHz files and you may sense lack of it in 24kHz but is it worth keeping when original content is known to end at 12kHz?

And due to masking effects it's not a good idea to judge by listening to extracted noise or NTSC interference alone at insane levels - judge by listening at normal levels with all the lower original content.

Ok so I tried what you suggested, and I still can ABX the 96KHz lowpass 15KHz and the 24KHz.
So it's definitely not the NTSC line.

I also think that 32KHz + lowpass 15KHz is a good compromise.

But why when you record in high frequencies, it seems that above the source frequency limit, there is a duplication of the relevant frequencies below ?

For example here, the frequencies squared on the screenshot (>12KHz) are supposed to be TV broadcasting and/or DAC recording noise, but it looks like the source frequencies spread above (the vertical lines). When there's is a voice for example, I can hear some sort of ultrasounds reproducing the pace of that voice. (Here, someone is laughing, and the HA HA HA HA HA is spreading as ultrasounds, I think you understood though)



It's exactly what I was wondering when creating this topic : does recording with a sample rate above two times the frequencies of the source change it, so that relevant data go in higher frequencies and it's "too late" to lowpass ? Or are they just useless "cloned" frequencies, like chroma shifting with VHS (colors that spread all around), and I can ignore them since it's irrelevant ?

Re: Encoding old material at 96KHz question

Reply #58
But why when you record in high frequencies, it seems that above the source frequency limit, there is a duplication of the relevant frequencies below ?

Since the signal doesn't vary much (or really at all with) with frequency, that is probably distortion and not actual signal.   Real audio tends to vary with frequency. 

It's exactly what I was wondering when creating this topic : does recording with a sample rate above two times the frequencies of the source change it, so that relevant data go in higher frequencies and it's "too late" to lowpass ?

Internally your ADC most likely runs at several MHz and then downsamples to whatever rate you pick.  If you pick 96k and then downsample a second time to get to 48k doesn't make any difference. 

Re: Encoding old material at 96KHz question

Reply #59
Such duplication may be due to aliasing, maybe clipping, or other distortion somewhere in playback/recording chain. If you know that the content is up to 12kHz, anything above is noise or some distortion. 96/24 is waaay overshot for VHS.

 

Re: Encoding old material at 96KHz question

Reply #60
Thank you very much to both of you. Now I know what to do.

 
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