Recordings play back at slower speed

[Peter S]

I recorded music from cassette tape using Audacity. On playback the music was clearly running slow compared with the original.

Could this be because my computer was starting to run out of memory when making a continuous recording of one side of a C90 tape (45 min)? My PC has a relatively modest ¾ gig of RAM.

But other similar recordings I made have played back fine.

As a matter of interest I managed to retrieve the situation by using Effects->Change Speed. I satisfied myself that playback pitch was around one semitone low. I figured that a semitone was 1/12 of an octave, which works out to around 8.3%. I boosted the speed by this amount, which restored the recording to the speed and pitch of the original – to my ears, at least. But I shouldn’t have needed to fiddle around like this.

Comments, anyone?

[steve]

Were you monitoring the music while it was recording? If Audacity "usually" works OK, it is most likely that your cassette was slipping (hence playing slow) while you were recording. Cassette mechanisms require cleaning from time to time - there are commercial products for doing this, or you can use a drop of pure alcohol (IPA works well) on a cotton bud. Be careful not to spill alcohol on the tape machine as it may melt the plastic, and be sure to allow it to dry thoroughly before you play a tape. The important parts to clean are the tape head(s), the capstan (spindle) and the capstan wheel. Also, the drive belts gradually deteriorate and eventually require replacement.

[Peter S]

Thanks stevethefiddle.

Yes, I did monitor while recording and the source seemed fine. I'm well schooled in cleaning tape heads and capstans and have a kit made by Bib which includes a liquid head cleaner and various cleaning pads. I used this before these recordings.

If the cassette deck had been at fault, I would have expected wow and flutter rather than a steady slow speed.

It's a mystery!

[steve]

A mystery indeed, although my suspicions would still be with the tape deck as it's rare for them to have good speed regulation. Some even use AC motors that rely on the mains frequency for controlling motor speed and that can vary by more than 10%. Some cheaper cassette players have no speed regulation at all and just rely on voltage regulation to the motor. Unless you have "perfect pitch" a constant semi-tone is easy to miss.

Anyway, the good thing is that you can correct it with Audacity

[n0ctis]

I'm not sure if this is related, but I had a slow playback issue with a track recorded in Kristal once. I recorded the track in Kristal and when I attempted to open the wave file in Audacity, despite the sample rates matching up, the playback was as if I had slowed it down. This last for about a week and some manner of tweaking I didn't keep track of nixed the issue.

[11l]

please help me i made a beat and i put it as track one... as i played along in my headphones, i played along with the beat, and when i played it back it wasnt on time! im pretty sure this wasnt happening before and i really need help because this is my passion! i tried to clap along with the beat and within a couple of seconds, the beat was off... please post a reply or send a email 11l@live.ca please help

[steve]

please help me i made a beat and i put it as track one... as i played along in my headphones, i played along with the beat, and when i played it back it wasnt on time! im pretty sure this wasnt happening before and i really need help because this is my passion! i tried to clap along with the beat and within a couple of seconds, the beat was off... please post a reply or send a email 11l@live.ca please help

Try Audacity 1.3.4 beta and set up latency correction as described in this topic: viewtopic.php?p=5535#p5535

[Peter S]

My second posting on this subject. Audacity is still running slow on my PC - but not consistently so!

I timed a piece of music on the source input (cassette deck) and compared it with Audacity's timeline on playback. I found the recording to be running about 9.8% slow compared to the original. I fixed it as before using Change Speed. The other side of the same C90 tape recorded perfectly, with no audible drop in pitch! I am satisfied there's nothing wrong with the cassette deck.

I am at a loss to understand how digital recording software can run slow. I would expect it either to work properly or not at all. Or at least be uniformly slow!

Does it need hefty RAM or disk space to work properly? I've got around 3/4 Gig memory and (according to Audacity) enough disk space for 20 hours more recording. (Must delete some of those surplus files!)

[n0ctis]

I think this is an obscure bug somewhere deep in Audacity's code.
Having 768MiB ( 3/4GiB) of RAM should be sufficient, but you could look at CPU as well... perhaps?

I can imagine low system resources stuttering and "skipping" a recording, but slowing down is another story.

[alatham]

Peter S

I found the recording to be running about 9.8% slow compared to the original.

Is that accurate, or is it an educated guess? If you play a 48000 Hz file at 44100 it will run 9% slower than it should.

I am at a loss to understand how digital recording software can run slow. I would expect it either to work properly or not at all. Or at least be uniformly slow!

The most important thing that a sound card should have is properly written drivers. Having bad drivers is the source of a good chunk of the problems we see here.

If your sound card runs at 48000 Hz natively (many, if not most, do), and you ask it to record at 44100 Hz, sometimes the drivers don't seem to do their job properly. So you may have recorded a file at 48000, but Audacity thought it was running at 44100.

I'd bet that if you set Audacity's default sample rate (edit -> preferences -> quality) to 48000 Hz, the problem will disappear. If you do decide to do that, r8brain is freeware for Windows that will quickly convert bunches of wav files to a different sample rate. I don't recommend using Audacity to resample audio (even though it can) due to a bug that adds unneeded silence to the end of resampled files.