Digital Audio Speed Messed Up

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fathomstory
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I recorded a symposium and the audio playback is really slow, like Rocky after the 40th round. I have no idea why playback would be slow. I need to change the speed so it is normal. I have used this digital recorder, the Zoom H2, for years and this never happened before. Where do I go from here?

mhartzel
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Check the audiofile metadata with mediainfo (especially sample rate). It might give you a clue of what's wrong.

You can force sample rate, bit depth etc in Audacity when you use File / Import / Raw data for importing the file. Experiment changing parameters until the sound is fixed. Mediainfo might give you the correct parameters,

There is also Effect / Change speed in Audacity but use that only as the last resort because that adjustment is not exact.

seablade
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Most likely cause is you are playing back material recorded at a higher sample rate at a much lower sample rate. If you imported existing material into Ardour, but did not let it copy the file to it's session folder, it did not resample to match your session sample rate, and that would cause this. It obviously can also be caused if you import and tell it not to resample the material.

Seablade

alexmitchellmus
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@seablade:

fathomstory and I talked on IRC for a while, the header of the wav is definitely corrupt, (using command line to read it out). The Zoom H2 was disconnected before being able to write out the wav as well, so added corruption within the stream most likely.

I do think that Audacity import raw could help- or otherwise more advanced utilities such as SoX, ffmpeg etc.

Worse case would be to manually re-time the material in Ardour - aligning it with other 2nd system sound (from camera), which should also work. Lucky there is any usable data really- as corrupt wav's can spit out static quite easily.

seablade
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@alexmitchellmus

I would strongly suggest doing the route of SoX first if possible for repair then, after than ffmpeg then Audacity. I only put Sox first because it is the easiest to import and spit out a new file without modifying the audio data (Via resampling etc.) but obviously being commandline based, as is ffmpeg, it can be tricky for many people to wrap their head around how to use, so Audacity can be an ok compromise if needed.

I would not recommend retiming in Ardour, fix it before you import it into Ardour, you will get a better result.

Seablade

mhartzel
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@fathomstory It might be a simple matter of starting up your H2 and checking the sample rate and bit depth used when last recording and then opening the file in Audacity (File / Import / Raw data) and selecting the values you saw in the H2 (Audacity asks for these values while doing the import). You also need to know if the inputfile has one or two channels.

If there is a wav header in the file it is translated as audio data and it causes a click right at the beginning of the file. If my memory servers me well it is exatly 44 bytes long. You can just cut it out.

I have fixed several files with corrupted / missing headers this way.

If you prefer the commandline, then go seablades route.

fathomstory
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Hello, I did chat with alexmitchellmus.

He suggested I use something called qwaveheaderdump. Here is the output. We also tried sox, but that did not get us far.
Here is the output:
https://pastebin.com/R9KsxaDz

mhartzel
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@fathomstory we posted exactly at the same time, check my previous post.

fathomstory
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Just an update.

I have a folder with four wav files. The first two have the slow playback issue, the second two have good playback. Here is an output for each using "qwavheaderdump":

https://pastebin.com/LUKh0gKU

I tried audacity, SoX, and qwavheaderdump to no avail. Now on a lark, I sped up audio files 000 and 001 and at faster and faster speeds, I still get that evil voice effect when I should start getting the 'chipmunk' effect.

Originally, I thought the audio files had slow/bad playback because the audio wav file hit the two gig mark (the size limit for wav's), but the second file is shorter. So something else is causing the issue. Also, qwavdump reports errors for the latter two files, but those play back fine. Am not sure what the issue could be and how to repair.

fathomstory
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Oh yeah, I recorded at 16 bit, 44.1, stereo signal off a mixing desk.

mhartzel
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I checked the pastebin. All those wav headers are corrupted or there is no header at all.

Bad SD - card ?

There are now cards (Kingston Canvas) that have 5 year warranty (in Europe). They must be more reliable than other cards, the manufacturer wouldn't promise such a long warranty without being sure about that. I bought a couple of these for my new Zoom R24 recorder. I shouldn't need to buy more during the lifetime of the device.

If you can put one of these files online I can take a look at it.

fathomstory
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@mhartzel

This begs the question, why is it that the last two files play back with no issue? I also formatted the SD card and tested it and the files play back fine.
As for manufacturer warranty, I have a Kingston SD card too. A manufacturer can always replace an SD card, it will cost them almost nothing. But we cannot replace our precious data!

I would like to post the files, but perhaps the ones that actually play back properly *might* give a clue as to how to get the ones not playing back properly to play back properly. It's just four gigs all together...no biggie... ; - )

mhartzel
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I would copy the SD card full of files and compare them to the originals. The SD - card might have only some defective regions (bad cells).

I trash any SD - card at the first sign of trouble. Usually the trouble only gets worse and as you say any trouble means losing data. I've been bitten too many times by faulty / failing SD - cards.

paul
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fathomstory, email me (paul@linuxaudiosystems.com) a link to where i can find one of the bad files (the original versions).

seablade
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@mhartzel

As fathomstory mentioned the SD card is pretty cheap honestly, the data on it can be much more valuable. It cost me $30 to replace a 64GB SD card I use to record 4k video and shoot photos on. It costs me upwards of $1000+ to replace the time and effort it took to recreate some of those, if they can be done at all. You may never recreate the recording from a console of a mix you did for instance.

SD card warranties are almost pointless. The shipping almost costs me more than the value of the card, the time to go through the process certainly does.

I would love to take a crack at helping with the files themselves as well, but right now I am kinda limited on time sorry.

Seablade

alexmitchellmus
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Once SD cards begin to show errors I replace them as fast as possible, it just isn't worth it. Especially as many recording devices (audio) are unable to recover from bad sectors. Even deep formatting will not help the situation. But as @seablade mentioned - what they are recording is much more valuable than the cost. At least with audio - one does not need to worry about exotic & expensive cards: USHII etc.

alexmitchellmus
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*UHSII

seablade
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Actually with audio you really don't need to worry about UHS, we don't have the track count really to worry to much about that. That being said I use UHS1 for 4k video so I just get the same for audio.

These days I just keep my original files on the SD card and don't reuse if I can avoid it honestly, they are cheap enough. Acts as a backup, though not complete solution for a backup.

Seablade

mhartzel
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@seablade That is an interesting way to think about the fragility of the SD - cards. Buy lots of cheap ones, test them before using by writing them full of data and comparing against original files. Then use a card for real recording and never erase it, just use a new one.

SD - cards have been as unreliable as floppy disks used to be. I remember having windows 95 or 98 installer on about 25 floppies and after installing 18 disks (taking a very long time) the next disk was faulty :) I was hoping that these 5 year warranty cards had some wear levelling hardware on them, but if Kingston is just going to send a new one when one fails then the warranty means nothing. On the other hand they would not get any profit by selling these cards if they needed to send replacements for everybody once a year, so I am hopeful that these ones will last longer. Only time will tell.

fathomstory
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Paul found a fix, "i just told SoX to rewrite the file, pretending that the original was recorded at 88200 and to resample to 44100." The audio sounds good and now syncs perfectly with my shotgun mic audio source. I have no idea how the files got messed with in the first place, but Paul got it working again. Another SD card is on order. Thanks so much Paul!!!