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Thursday, May 19, 2011

Hires Audio - Treasure Island

Introduction
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Lets dig up some treasures. It's time to run another little study.
Writing and preparing all these articles gets you nicely into all kind of
subjects. Quite some work. But worth it.

A recent  post at Audio Asylum (by good old Dr. mercman)  referring to
an article about Hires-Audio  and the industry ethics behind it finally
inspired me to dig into another "blind side" on my side.
Usually I read all kind of stuff discussed in those forums and put it
aside with a little more than a slight touch of ignorance.
Probably it's not just me acting like that.

Since I spent quite an amount of money recently on  hirez audio-material
I thought why not spent a little time on the subject.
It shouldn't be that difficult to have a little closer look at my "HiRes treasure".

You never know, what's in your treasure box, if you keep it closed. Although sometimes it's better to keep it closed.




Many of us who are around for a while should still be able to recall
the HiRes "scandal" last year.

HD-Tracks was selling HiRes fakes - They sold upsampled material as native
24/96 HiRes material. Of course they claimed not to be aware of it as soon
as this mess became public. They blamed the providers.

No idea what happened afterwards. I think those customers who
contacted HD-Tracks could choose an alternative album.
HD-Tracks never contacted me (and probably a lot more customers)
proactively. I'd guess that many people still listen to upsampled material
believing they'd listen to great master material.

Anyhow. That's the past. Lets look forward.

Why fakes? Fakes because resampled and especially 44.1khz
upsampled material can not be considered master or original quality.
In the beginning there's a RAW file recorded at a certain sample rate.
And that's the one and only reference.
The post processing resp. mastering of that file obviously will introduce
losses and will add the mastering-engineer-sound-signature (tools and taste)
to those raw data.
Anyhow. That's the master. Every deviation from that original is a
"re-master". Every other sample-rate than the raw-master
is a "resampled master". Both  characteristics should be clearly stated
on every single official HiRes release. Linn e.g. sells "Master Quality"
tracks at different sample rates. There obviously must be at least one
raw data manipulation in the loop. For sure we don't talk about the original
raw-master. Watch out: There is another term floating around -- "transfers".
There are e.g. DSD transfers or tape transfers. Those do not have anything to do with the term "master" either.

Back to the fakes.

44.1 material comes with a bandwidth of not more than 22,05khz ( sampling frequency/2). The resampling process introduced losses (e.g. ringing/phaseshifts). You can't avoid that. If your 44.1khz material wouldn't be based on your raw-master you'd probably face those resampling losses twice.

If we're talking about upsampled 16bit base material the situation even gets worse.
On 16bit base material we usually  have to face artificial dither noise which is typically added to the signal as last step of the mastering process.
Not to forget with 16 bits we're limited to 96db as the maximum dynamic range.
And that's where the artificial dither noise is gonna sit on you 144db wide 24 bit spectrum. Somewhere in the spectrum.
Further you need to digitally attenuate the signal prior to resampling to avoid clipping.  All this nasty stuff affects the quality of the data and wouldn't be necessary with 24bit raw master material.

Selling an upsampled track as HiRes Master Quality is absolutely
unacceptable to me. I'm not even sure  if the term HiRes Audio
or Master Quality or similar has been specified by AES or someone
else.

Apologies from the vendor side are not acceptable if those fakes
make it more then once into the shop and further to the customer.
Especially after being made aware about it from the public resp.
customer base outside.



I thought it's time to have a look at my own files.






Study
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First I'd like to see how to identify resampled material. I used the spectrogram feature of sox to generate below images.

Image 1. shows a native pinknoise spectrogram sampled at 24/96. As you can see the frequency content is shown up to sampling-frequency/2=48khz all over the place.



 Image1: Native 24/96 pink noise spectrum


The 2nd image shows a 24/96 spectrum based on upsampled 44.1/16 pink noise.



Image2: Upsampled 44.1/16 pink noise

As you can see. The original frequency range of a native 44.1khz file remains the same 44.1/2=22.05 if resampled to 96khz. You can't really hide the base data origin.



This example shows pretty clear the effect of upsampling in the HiRes spectrum. That's what we need to look for as the "fake-indicator", when analyzing real live cases.

To identify downsampling from 192khz to 96 khz should be much more difficult!!!

Obviously 48khz upsampling should also rather easy to identify. The  line in the spectrum of Image 2 would have been drawn at 24khz instead of 22.05khz.
And folks  - there are also 48khz based fakes out there.

But what happens if we face analog tape transfers, with quite a low base bandwidth. How to identify those?

Let see how things are developing. I hope my learning curve stays as steep as it currently is for a little while.




Real Live Cases
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I have to admit that I'm be no means a specialist in audio file spectrum analysis.
Please tell me if my below interpretations or conclusions are incorrect or misleading. You're of course invited to support my investigations. Advise is highly appreciated by me and I guess the community reading this article.



I start with analyzing two examples of very recent HD-Tracks downloads.

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Case 1
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HD Tracks - Deep Rumba - Track 01 -  24bits 88,2khz
purchased and downloaded 05-2011


Image 3: Deep Rumba - Track 1 - Audacity plot


You can clearly see the lowpass behavior towards 22khz, which would be  characteristic for 16/44.1 material.
The dip at the end of the spectrum could be some kind of dither added later  after the resampling.

Hmmh. First try first hit!?1? What do yo think?


The 2nd spectrogram doesn't look that obvious anymore



Image 4: Deep Rumba - Track 1 - Sox plot

Here I see artifacts above 22khz.  And shaped dither at the top end. Hmmh.

What would you guys say?? Fake Yes/No?

Looking at the Audacity plot I'd say a clear "YES".  Sox makes me wonder if it is really that clear.

I need to figure out what's about those lines (harmonics?) in the plot. 

To me it's also more then unclear why 24/96 data is being dithered.

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Case 2:
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HD-Tracks Paul Simon - So Beautiful So What - Track 01 - 24bits 96khz
purchased 05/2011




Image 5: Paul Simon - So Beautiful So What - Track 1 - Audacity plot


You see the much wider spectrum towards the end, which is characteristic for that higher sample rate. It still won't tell you if that track is downsampled from an even higher frequency or transfered from DSD. For sure it is not upsampled from 44.1/16.

To confirm the Audacity view - here is the corresponding Sox plot:


Image 6: Paul Simon - Track 01 - Sox plot

This spectrum looks somewhat different then the Case 1 spectrum.
Energy all over the place. That makes me even more suspicious about Case 1.

Though I need to figure out why there's so much going on above 20khz on HiRes material. Is this real data or are these some kind of other garbage artefacts.
This I'd like to find out.

So far the good .

There are very obvious differences between those files. Both are sold as HiRes files - just to remind you. I still need to get those characteristics properly interpreted.

More treasures to be lifted. See Appendix 2.  You'll find all kind of spectra for different files sold at Linn, iTrax and HD-tracks. .

I at least - find this a real interesting exercise. ;)

Enjoy.

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Appendix 1: Generating Spectrograms with sox and audacity
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I do all my analysis on an Ubuntu Linux.

I can very easily write scripts that look for my 2496 files and
generate all plots automatically. 

What you need to do first:

1.
First open a terminal and install the required programs:

sudo apt-get install sox imagemagick audacity

2.
Copy your HiresFile of choice to e.g. /tmp
You don't want to mess around with the original file.

Case 1: Sox
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Copy/Paste below line into a terminal.


FILEX="/tmp/yourfilename.flac"; sox $FILEX -n remix 2 trim 0 30 spectrogram -x 600 -y 200 -z 100 -t "$FILEX" -o $FILEX.png ; display $FILEX.png &

Just swap out "yourfilename.flac" with your Hires-filename and then press return.



4. Done



Case 2: Audacity
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If you want to analyze your own 24/96 tracks you can do it also by using  Audacity. It's freeware and available under Linux as well as Windows.


You just load your track into audacity.
Then you select "Plot Spectrum" from the "Analyse" menu .
And that'll be it.


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Appendix 2:  HiRes - File Spectra
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ALBUM 1
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Looks OK to me.

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ALBUM2
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Hmmh. Could it be based on  a 48khz master?

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ALBUM 3
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Looks OK. This time an example with shaped dither applied.

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ALBUM4
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Hmmh. Pretty flat spectrum. I'm not sure what do do with that one.

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ALBUM5
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Looks rather Ok to me. Though I'm wondering about that distortion at around 16khz. You can even see its harmonic at 32khz. That can't be right.

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ALBUM 6
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 Looks OK - doesn't it.

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ALBUM 7
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Should be OK. There is content above 24khz.

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ALBUM 8
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The spikes go clearly above 24khz. You can also clearly see the added
dither at the top. Still I'm gonna load it into Audacity to verify it.

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ALBUM 9
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This is a native 24/48 file.

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ALBUM 10
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Hmmh. Looks like native 24/96. The spectrum looks pretty distorted though.

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ALBUM 11
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Looks OK to me.

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ALBUM12
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Looks OK to me.

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ALBUM 13
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Looks Ok. There seems to be some dithering done


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ALBUM 14
...........................


I need to have a closer look at this one.

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ALBUM 15
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No idea what to say about that one. I'd say it looks rather OK.

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ALBUM X
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To be Continued


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END OF ARTICLE



4 comments :

  1. Great job, please keep up the good work. Any feedback from producers on suspicious cases?

    ReplyDelete
  2. On some tracks you state that there is dither aplied when spectrum shows rising noise around 40kHz. This is symptom of DSD mastering&noise shaping

    ReplyDelete
  3. I did enjoy reading this post!I am looking forward for your next post.Good job


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    ReplyDelete