Frequency sweep on audio tape

[Tom41]

Recently I recorded a frequency sweep (low to high) on an analogue audio tape, trying to see what is the highest frequency it could properly reproduce. To my surprise, when I played it back I heard other tones going all over the place - downward sweeps as well as upward!

I've attached a picture of the spectrogram. The top shows what I recorded to the tape (generated in Audacity), and the bottom is what I got on playback.

Anyone know why this might have happened? Something to do with harmonics or 'reflecting' signals? I wonder if tapes inherently have some sort of 'reflecting' effect on certain frequencies...

[Th3_uN1Qu3]

Re: Frequency sweep on audio tape

Your input signal was clipping at the recording device.

[Tom41]

Re: Frequency sweep on audio tape

I thought 'clipping' was only to do with the strength (amplitude) of a signal, not its frequency.

I generated the sound with Audacity, burned it to an CD as full uncompressed CD-DA audio, then put the CD in an expensive hi-fi system. I played the CD there and used the hi-fi to record to cassette.
To rule out any distortion caused by the CD decoding, I also recorded the CD output from the hi-fi and the trace looked almost exactly like the signal I generated in the first place. Only when playing back the cassette did these extra tones appear.

[Uniballer]

Re: Frequency sweep on audio tape

Might be partly due to print-through effects. Especially if the signal is strong.

[Th3_uN1Qu3]

Re: Frequency sweep on audio tape

Like i said, it's not the CD causing the issue, or the CD playback device. I stated explicitly that the recording device was clipping. Either the tape preamp, the tape head or the tape itself was overloaded, causing these audible artifacts.

A better method is to use pink noise and a RTA (can be done with free software) to check the frequency response.

[Towncivilian]

Re: Frequency sweep on audio tape

http://www.tapeheads.net/showpost.ph...76&postcount=9

[stj]

Re: Frequency sweep on audio tape

the cd player will be filtering the beat frequency out so you dont hear the laser too.
you should link the soundcard to the tapedeck.

also, i wouldnt be surprised if your input is picking up noise from inside the pc.

[Tom41]

Re: Frequency sweep on audio tape

I'd say the theory of the tape being saturated/clipping seems right. I recorded over the sweep with some speech, and could still faintly hear the sweeping tone upon playback - almost as if the tape was too far saturated to be fully erased.

I also recorded the same sweep (from the same CD) twice; once on a cheap boombox and again on the expensive hi-fi. Looking at the spectrograms, the pattern of tones going up and down was quite different on the two recordings! Must be some sort of resonant frequency - perhaps these could be used to 'benchmark' cassette recorders lol

[SteveNielsen]

Re: Frequency sweep on audio tape

Over-saturation will do that to a tape. Use a bulk tape eraser to eliminate it.

The sweeps are showing you the differences in the frequency responses between the two machines.

[momaka]

Re: Frequency sweep on audio tape

Are you using a metal tape (CrO2, FeCr, Metal) by any chance? If yes, that can cause problems too if you don't have a hi-fi deck with metal-tape capability. Most late 80's stuff and newer don't.

Also, try erasing the tape several times and turn off any Dolby or MPX filters that your deck might have (if that is possible at all - 90's decks and newer lack all of these option altogether and usually employ Dolby B/C that cannot be turned off). Lastly, make sure you're not saturating the tape. Does your deck have power level meters? If yes, make sure you don't go over 0 dB point while recording.

By the way, what did you use to create the sweeping signal and do you mind posting it here? I'm curious to try this on my Marantz SD4000 tape deck.