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Old 13th April 2012, 04:01 PM   #1
percy is offline percy  United States
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Default Near-Bit-Perfect ?

Just out of curiosity to see what is the quality of the digital output on my Panasonic DMP-BD85K bluray player I burnt a few test signals to an audio cd (cdda) and recorded them on my pc at cdda rate (44.1Khz/16bit). The player does output 44.1Khz. I am not sure if it outputs 16 bits or something else. Anyway to check that ?

Anyway, first I just looked at the spectrum of the recorded files. If things didn’t look good here then there was no point in going further. But they did look good. The spectrum was clean and as good as the original file that was produced by WaveGene, except I see a very small (-110db) 7hz train buried under the noise floor. When I expand the fft vertically to go -120db or lower and zoom horizontally I see it clearly. Its 7,14,21,28 …all across the entire spectrum. What would this be ? A PLL ? The recording was done using a usb soundcard so could it be the pll of the soundcard ? If there was anything about the transport or the transmission (optical), what could it be ?

Another fact that may or may not be related to this is that when I opened both the wav files to look at the actual sample values I see that the samples that are different have a difference of exactly “1”. More specifically they are greater by exactly "1". (hence the title of the thread). I have attached a screen shot of the file compare in hex mode that demonstrates this fact. The different sample values are highlighted. I can’t see any pattern here. It occurs at what appears to be random intervals. It occurs for any values (high, low, positive, negative).

It’s a -0.5db 12.5Khz sine, right side is the reference file and the left is the recorded version.

How would you explain this ? What might be going on ?
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Last edited by percy; 14th April 2012 at 01:04 AM.
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Old 14th April 2012, 12:20 AM   #2
percy is offline percy  United States
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And here are a couple of screen shots of the fft that show the 7-8hz harmonics. The first one is the original file. The second is the recorded file. And the third is a zoom of the recorded.
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File Type: png nbp_fft_orig.PNG (59.2 KB, 195 views)
File Type: png nbp_fft_rec.PNG (70.6 KB, 191 views)
File Type: png nbp_fft_zoom.PNG (43.9 KB, 187 views)
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Old 14th April 2012, 12:49 AM   #3
Teo73 is offline Teo73  Poland
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My guess would be that your recording device is not bit-perfect. It is not easy to keep the bit accuracy while using computer equipment, for hundreds of reasons. Good idea would be to run the same test using another digital source, preferably just a simple CD player. If you get the same kind of differences between the original and recorded .wav, you will know that it not the player that is causing that.
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Old 14th April 2012, 01:07 AM   #4
percy is offline percy  United States
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apparently a -1 is registered every so often (7hz?) even with no signal. This is a portion of the file where there is silence before the signal started. You see those FFFF (-1) embedded between all those 0 value samples ?
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Old 14th April 2012, 01:23 AM   #5
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Hold on a second. So you're generating a digital file, using something like Audacity, then burning this to an optical medium. You then place this inside your bluray player and play it, whilst recording the output on your computer.

The question I've got is this. Are you recording the analogue output of the bluray player or a digital output say S/PDIF?
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Old 14th April 2012, 01:31 AM   #6
percy is offline percy  United States
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Correct, over optical/toslink spdif. I kinda lazily mentioned that in my first post.
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Old 14th April 2012, 01:34 AM   #7
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The error is from your SPDIF receiver - PLL (while recovering the embedded clock).
Toslink is not that great as SPDIF medium.
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Old 14th April 2012, 01:42 AM   #8
percy is offline percy  United States
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are you saying this pll error while recovering the clock wouldn't happen with coax ? Can you please explain in a little more detail ?
Thanks.
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Old 14th April 2012, 01:52 AM   #9
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Try coax, it might work better. That's a general case, because you have two less conversions on the digital path.
Depends also of what SPDIF receiver are you using.
Error is probably not audible, but it would ruin something like the HDCD embedded encoding.
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Old 14th April 2012, 01:54 AM   #10
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I don't think this is errors caused by the SPDIF receiver. If it were they'd be more randomly distributed than this - they'd not only affect the LSB. This looks like low frequency dither is being applied to the output. It could be some kind of watermarking system for copy protection.

Its a weird one for sure.
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