Cassette-to-PC audio troubles

March 9, 2008 6:57 PM

Recording from cassette tapes to PC...why does it sound all weird?

So I went and bought a used cassette player today to try to transfer some old cassettes over to the PC. I've everything hooked up correctly, and the levels are fine. Indeed, when I press play on the tape player, I can hear the tape through the headphones connected to the PC. But when I try recording (in Tracktion, AVS Audio Utilities, and Sound Recorder), the volume seems to shift up and down cyclically. Here's a sample. What am I doing wrong?

posted by greatgefilte to technology (9 comments total)

"Page not found"
posted by Flunkie at 7:02 PM on March 9

Since I can't hear your recording (your link is a 404), I can't tell for sure, but the symptoms you're describing could be the result of "automatic gain control".

It means that something is diddling the amplification dynamically in order to make the loudness constant, which means the quiet parts get louder, and the loud parts get quiet. But there's always lag in the detection, so when the original suddenly gets louder, the recorded output goes up fast and then rapidly fades back down. When the original suddenly gets quieter, the recorded output goes way down and then fades up again, which usually brings up the background hiss.

It's hard to say which of your components is doing it. You'd have to search and see if you can find a checkbox or a physical switch or something to disable it. Or if it turns out one of your components is doing AGC and it can't be disabled, you'd need to replace that piece.

A different possibility is that your tape is Dolby-encoded. Dolby is a "compander" and tries to even out the amplitude of the audio. Or your tape may not be Dolby-encoded but you've got a Dolby-decoder running anyway. In that case the amplitude changes will go utterly wild.

(If I could hear your audio, it would be much easier to diagnose your problem.)
posted by Class Goat at 8:15 PM on March 9

Sorry about the link, should be ok now!
posted by greatgefilte at 8:23 PM on March 9

Class Goat, that sounds like exactly the problem. But why am I only hearing it when I play back the recorded file and not when I'm listening live?
posted by greatgefilte at 8:38 PM on March 9

Having listened to it, it doesn't sound like an AGC issue. AGC would be reacting to silence and sound, and so would Dolby strangeness. Your wowing doesn't seem to correlate to that. Also, AGC usually cranks the volume a lot faster than that.

As to why live and Memorex are different, that would suggest that this is an encoding issue, during creation of the MP3. But aside from that, I'm afraid I don't have any idea what's happening. Sorry.
posted by Class Goat at 10:23 PM on March 9

Let's have a think about what's happening here. There is a different chain of processes happening when you are listening to the recording live:

Tape -> Soundcard line input -> soundcard headphone output

And when you're recording it on your computer:

Tape -> Soundcard line input -> A/D Converter -> Software -> D/A Converter -> soundcard headphone output

Making me think something is happening in those digital stages. The first thing I thought of was some kind of stereo phasing effect - somehow, the left channel from the tape is getting overlaid with the right channel from the tape at some point, and if you were recording in mono, this might result in a cancellation effect that might vary due to misaligned tape heads / stretched or wonky tape.

However, it would have to undergo a conversion to mono for this to happen, and listening to the reverb in your recording, it's pretty clear it's true stereo (sorry, don't have Audacity installed on this PC so I can't take a closer look).

Perhaps there's a problem with your soundcard's A/D converter, where left- and right-channels are overlapping slightly, and canceling each other out? I have had soundcards that have leaked audio from one channel to other, introduced DC-offsets etc. so it's not unheard of.

Solution? Maybe you'll just need to compress the audio a bit to even out those volume fluctuations.
posted by Jimbob at 10:37 PM on March 9

the rhythm of the level changes tells me this is tape wear/oxide loss/de-mag consistent with a cross section of the cassette 'reel.' if you sped up the cassette, the level changes would follow.

you could probably solve this with a compressor after the fact. gotta say tho, it sounds pretty good for an old tape.
posted by tremspeed at 10:39 PM on March 9

If it is tape wear, why does it sound ok on the headphones in real time?
posted by bystander at 2:39 AM on March 10

Thanks, Jimbob. I tried compressing it and it sounds decent, but still a bit wonky because the tape hiss still seems a bit variable in volume. Maybe I'll try getting myself a cheap USB sound card and see if that works any better.
posted by greatgefilte at 5:11 AM on March 10