posted 22 April 2001 03:20            

I have been restoring some old tape recordings from about 15-20 years ago, and pretty consistently, they all show a subaudible carrier throughout the audio portion at around 15765 Hz with a very narrow bandwidth, on the order of 10 Hz.

The carrier lives down around -65 dBFS. In some cases there are sidebands at +/- 60 Hz from a carrier at 15705 Hz, so it could be some kind of AM signal imposed on the recording.

What is this? Is it some kind of ID coding? Or was it used as a carrier for recovering from preemphasis EQ? What kind of equipment produces this signal?

[...Oh! You know? I just had a hunch, and so I looked at some of the recordings of my own music from back then...

Back before there were DAT's I used a VCR to FM record my stuff on the audio tracks of video tape... Sure enough the carrier is in my own recordings too! I'll bet it is bleed through from the horizontal sync signal for standard TV... hmmm... ]

Cheers,

- DM

posted 22 April 2001 03:56            

I have a digital mixing console/recorder that runs at 48 KHz. When I dump the recordings into the computer for editing, all of my recordings show another subaudible carrier (quite narrow again) at around 22150 Hz. This carrier also lives at around -65 dBFS.

Why does this happen? My PC audio card does not produce this, nor does the console when fed live signals. Only the 48 KHz/16 bit recordings show it. Is this the manufacturer's idea of "dithering"?

- DM

posted 22 April 2001 17:12            

Thinking back I did first record these songs on a DBX encoded 4-track cassette deck. Only later did I transfer these to the VCR.

I had forgotten all about tape bias, and that was supposed to be up around 15-16 KHz.

Thanks a bunch!