Extracting Subsonic and Ultrasonic Signals from Analog Tape Recordings

[Craig Maier]

Sometimes, it may be necessary to look for subsonic (lower than 20 Hz) or ultrasonic (greater than 20 KHz) signals on an analog Forensics audio tape. I will not get into the details of these circumstances, but will explain a method which will improve the probability of extracting those signals from the tape.

1. Always set up your Diamond Cut recorder in these circumstances to transfer at an 88.2 KHz or greater sampling rate, 24 bit resolution.

2. To improve the transfer of subsonic signals (such as "thuds or sub-harmonics of the power line), transfer the tape at a higher speed than it was originally recorded at by a factor of 2 X or 4 X. This increased speed can be corrected later to the proper speed by applying the Speed Change Filter, but by using a high speed transfer, the low frequency response of the playback machine will be improved.

3. To improve the transfer of ultrasonic signals (such as bias components, etc), transfer the recording at a factor of 0.5 X or 0.25 X compared to what it was originally recorded at. The decreased speed can also be corrected later to the proper speed by applying the Speed Change Filter after the transfer. By using a high frequency transfer, the high frequency response of the playback machine will be improved.

[Dan McDonald]

This would also work for lps and 45s, etc. right, Craig? If you want to listen to John Lennon's experiments or Varese's "electroniques" or whatever, you could transfer an lp at 16rpm or 45 rpm, depending on what you were looking for. If the stylus is appropriate, the speed wouldn't hurt the record, would it?

Dan

[Craig Maier]

Yes, you are right as far as I know when playing a 33.33 RPM record at 16 RPM. However, I am not sure if damage is done to a 33.33 RPM Vinyl LP when played at 45 RPM, although I know that some people do exactly that. The reason that some folks do that with Diamond Cut is to reduce the transfer time of the record. They then use the Speed Change filter to correct it later.

I just have to wonder about the increased friction and a resultant elevated stylus tip temperature when this is done at high speed; does this do any damage to the record? Maybe there is no practical negative effect, but I really do not know. Maybe someone else knows for sure.