Playback is at uneven rate

[SaberCat]

Hi,
I am using a Blue Snowball mic on Ubuntu 10.04 and Windows 7 systems and running Audacity 1.3.12-beta on Dell Inspiron 570 desktop and 1501 laptop machines. I have noticed variation in the playback of recorded audio. It seems to speed up occasionally (without an increase in pitch) and then return to normal. I am recording piano audio so I ran a test in which I played each note on the keyboard at a constant rate starting at the base end. I also played middle C three times to mark it. I have attached a .wav file of the results. Notice that middle C is to the right of the middle of the time line and that the output rate increases as the upper notes are played back (I played them all at the same rate -- as best I could), but the pitches of those notes are correct. I tried this experiment on both machines running Ubuntu and on the Dell 570 running under Windows 7 (using the Audacity beta for that OS) and got the same results. Humm... I wonder what's going on? I looked through the FAQs and didn't see anything that relates to this. I'm new to Audacity so I may not have it set up right...

Thanks...

By the way, as you can tell from the .wav file, this mic has very good fidelity. It is specked at 35Hz to 18kHz. In the low end that's about six keys from the bottom of a piano keyboard, but the recording indicates that it works well all the way down. The upper key is around 4kHz so that's easily covered -- with room to spare for higher pitched instruments.

[bgravato]

There's not attachments in your post...

How big was the wav? You can only upload up to 1MB I think. Files bigger than that you'll have to upload to somewhere else (megaupload, zshare, mediafire, etc. pick your favourite).

BTW, advertised specs usually don't mean a thing... you can't usually measure the quality of equipment solely by its advertised specs... Even if the claim those freqs it doesn't mean you'll get an accurate response through all the freq range.

[SaberCat]

Hi again,
I removed the start of the file. It now runs from middle C up and is now under 687 KB. I will attach it. The playback audio speeds up as it approaches the end. I'm sure I didn't play it that fast (my fingers can't move that fast!).

[bgravato]

Do you have Sound Activated Recording on? (Transport menu)

[SaberCat]

Yes, I have sound activated recording on. I think I will try the ALSA arecord and aplay programs this afternoon to see if I see the same thing. Maybe its the mic -- although that's hard to believe. It shouldn't care about what it is sampling. Anyway, it may be a way to isolate the problem...

[SaberCat]

OK, I tried the same experiment with and without Sound activated Recording (SAR). Without SAR the playback rate was fine; with it, I got the indicated playback rate increase. To avoid the attachment size constraint I will attach the .wav file for the one without SAR (pianoScanWO_SAR.wav) to this post. Then I will send another post with the one with SAR (pianoScanW_SAR.wav). So it looks like the problem has something to do with SAR...

[SaberCat]

I have attached the .wav file for the recording with SAR to this post. Hope this is helpful...

[steve]

Sound Activated Recording should be off (not selected).

When SAR is enabled it will automatically pause whenever the audio level drops below the threshold value. This is what is causing some of your notes to be cut short, and so speeding up the playback.

[bgravato]

Sorry for the late reply. Tour problem is that you have sound activated recording. There's no speeding up. Like steve said, what happens is that with SAR the recording pauses when it goes below a certain threshold... High pitched notes have a faster drop in volume, so it stops sooner.

Why do you have SAR on? It should be off.

[SaberCat]

Hi again,
Thanks for the reply. I had SAR on so I would have time to get from my computer to the piano. But that's not a big deal. I can always remove the startup interval. Thanks for clarifying all this! I will run from now on with SAR off. It works fine for me that way. This is a fine tool. I appreciate you making it available...