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Topic: Waveform abnormalities during cassette transfer (Read 6581 times) previous topic - next topic
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Waveform abnormalities during cassette transfer

I've noticed a strange phenomenon when making cassette transfers with my Tascam 122MKIII cassette deck. When I put a tape to digital, the peaks on the bottom of the waveform are always flatter than the peaks at the top of the waveform.

Here is a picture to demonstrate. Also, notice how at the very beginning the peaks on the very bottom are much more attenuated (I've made a diagonal line to show this).



This issue happens with any tape I use (I have dozens of them, all different kinds) - and it only happens with the Tascam deck. I have a lower-end TEAC deck, and the peaks are always more even on the top and bottom.

I purchased the Tascam deck from an authorized dealer/service center, and it was fully calibrated when I bought it. However, I never had much time to use the unit until recently and now I'm noticing the problem is more serious than I had previously thought.

Like I said, I've tried every combination of variables to pinpoint the problem, but it seems clear to me something within the machine is out of wack - something that's beyond my expertise to fix.

The service center is not local, and it's real pain to haul/ship this machine (it's very heavy and bulky). So if anyone here could give me a clue as to what's wrong before I go on a wild goose chase shipping the deck around and trying to get it fixed, I'd really appreciate it.

Thanks in advance.

Waveform abnormalities during cassette transfer

Reply #1 –
It looks to me as though it's just down to bad circuit design. The waveform is being squashed in the negative half-cycles and not in the positive half-cycles probably due to an internal DC offset that doesn't make its way as far as the output sockets.

I doubt if there's anything you can do about it personally. It's just designed badly in the first place by the looks of it.

Cheers, Slipstreem. 

Waveform abnormalities during cassette transfer

Reply #2 –
Quote
I purchased the Tascam deck from an authorized dealer/service center, and it was fully calibrated when I bought it.
  Something like this might be missed during standard "calibration". 

It's impossible to tell if this a design flaw or fault in your particular unit.  The problem is in both channels, right?  (I think that's what I see...)    That would lead me to believe it's a design flaw, which the service center wouldn't be able to repair.   

Quote
The service center is not local, and it's real pain to haul/ship this machine (it's very heavy and bulky). So if anyone here could give me a clue as to what's wrong before I go on a wild goose chase shipping the deck around and trying to get it fixed, I'd really appreciate it.
  And, it might be something that the service center can't diagnose/repair.  Hopefully, you can contact the service center and make sure they understand the problem before you attempt that (maybe you can email them a picture).  Otherwise, they might just do a simple test and say "it's recording and playing back fine".

For now, it's probably best to use your other deck (unless the sound quality is worse from the cheaper deck).  But, if you wanted to "fix" the file, a high-pass (subsonic) filter might help.  I had a file with a similar asymmetric  problem recently.  First, I tried "offset removal", (with GoldWave). but that actually created an obvious offset that you could clearly see at the fade-in and fade-out...  I assume the new average offset was zero, but now I could see an offset during fade-in and fade-out.    The subsonic filter was a better solution.    (The filter didn't have any affect on the sound...  But, the waveform looked much better!    )

Waveform abnormalities during cassette transfer

Reply #3 –
It may be worth phoning the service centre and asking them if they can confirm or deny whether it uses the same basic playback electronics as the TEAC V-1050 which, as far as I'm aware, was the domestic version of an earlier version of the Tascam 122.

I've had my oscilloscope out this evening and checked out my V-1050 and, surprise surprise, it seems to exhibit exactly the same problem. I've never noticed it to listen to so I won't let it bother me, but I thought it best to pass the news along.

Cheers, Slipstreem. 

Waveform abnormalities during cassette transfer

Reply #4 –
Thanks for the info, everyone.


Quote from: Slipstreem on
It may be worth phoning the service centre and asking them if they can confirm or deny whether it uses the same basic playback electronics as the TEAC V-1050 which, as far as I'm aware, was the domestic version of an earlier version of the Tascam 122.

I've had my oscilloscope out this evening and checked out my V-1050 and, surprise surprise, it seems to exhibit exactly the same problem. I've never noticed it to listen to so I won't let it bother me, but I thought it best to pass the news along.


I'll try to find out if the V-1050 uses the same electronics as my 122MKIII, but after what you told me there it sounds like it's a design flaw. 

And yes, this does happen with both channels. It looks more accentuated on the left channel in my pic since the left channel is louder on that particular tape.

Maybe it's really not a super severe problem, but it still bugs me. I've got dozens of tapes I want to preserve (so that I never have to use a cassette deck again), and I really thought this was the one to do it with. After all, it's *supposed* to be a professional grade deck. 

And well, I really don't feel like spending all kinds of time fussing with things like azimuth and whatnot just to  get some ugly-looking waveforms. Looks like it's time to get a new deck (sigh).

My cheaper TEAC deck (R-H500) has good quality sound, but it doesn't have a very good wow-and-flutter spec, and as far as I know there's no way to calibrate the azimuth without tearing it apart, so I don't think it's a very good for archival work.

Hmmm...

Waveform abnormalities during cassette transfer

Reply #5 –
If your deck mechanically resembles the TEAC V-1050 then it should be possible to remove the cassette loading door by pushing it upwards when open to remove it. This allows you to access the head adjustment screws for azimuth adjustment if necessary. I must say that I'm a little disappointed with the TEAC, especially after reading the rave reviews I've read about it. As I say though, I've never noticed the problem from a listening point of view.

Cheers, Slipstreem. 

Waveform abnormalities during cassette transfer

Reply #6 –
Magnetised heads could be causing an offset.
Cheers,
Alan

Waveform abnormalities during cassette transfer

Reply #7 –
That's a valid point, although I've used a tape head demagnetiser on the machine and my results haven't varied noticeably as a result. The problem seems to lie in the biasing in the first transistor stage in the tape head preamp in my particular deck. I suspect that a high value resistor (1MegOhm maybe) strapped between the base and collector may fix the issue, but I can't be bothered to be honest. I'll just add it to the long list of mistakes I've made by equating a supposedly high-quality brand name with actual real-world quality.

Cheers, Slipstreem. 

Waveform abnormalities during cassette transfer

Reply #8 –
Sorry, but do you have the schematics of the deck?
How do you know that it's not having a FET in the first stage? Of couse, unless you haven' opened it
If age or weaknes doe prohibyte bloudletting you must use boxing

Waveform abnormalities during cassette transfer

Reply #9 –
I'm not sure about faults in the deck itself, but my advice would be to
1. Demagnetize
2. Record your stuff at about 3 to 6 dB lower level (you're too close to clipping, neither the top nor bottom should be "flat".
3. Use any old audio software to "remove DC offset" after recording.

Waveform abnormalities during cassette transfer

Reply #10 –
Quote from: eevan on
Sorry, but do you have the schematics of the deck?
How do you know that it's not having a FET in the first stage? Of couse, unless you haven' opened it

I had the deck apart on day one to calibrate it according to a professional calibration tape. I don't have a schematic for it but the biasing arrangement around the first transistor stage looked like a typical bipolar biasing arrangement rather than one that you'd expect to see around a small-signal FET. Regardless of whether the device at the front end is a bipolar transistor or a FET, the symptoms are still typical of incorrect DC biasing in one or more stages of the playback amplifier.

I remember having to increase the playback gain on both channels significantly from their factory settings to get the playback gain high enough to match the 0dB reference tone to 0dB on the meters and the appropriate RMS voltage level at the output sockets. Maybe TEAC use a non-standard lower 0dB reference level that wouldn't lead to this asymmetric clipping on a factory-calibrated deck? If this is the case, then having one professionally calibrated could actually make it worse rather than better as far as this particular problem is concerned.

One possible cure might be to open the deck up and adjust the trim-pots for playback gain down a little. You could make a recording of a 1kHz test tone on both channels that corresponds to a meter reading of 0dB on both channels on playback with the current calibration and then back off the trim-pots until it plays back at, say, -3dB on both channels and see if it helps at all. If it does, then it might be worth recalibrating the meter gains so that this new level reads back as 0dB for both channels.

Having said all of this, the fact that my particular TEAC V-1050 is 'wrong' despite being calibrated correctly doesn't alter the fact that it's still one of the best sounding cassette decks I've ever owned, so I'm inclined to leave mine calibrated to the correct 0dB level personally and I'll make sure that any future recordings I make on it peak at 0dB and no higher.

Cheers, Slipstreem. 

Waveform abnormalities during cassette transfer

Reply #11 –
Quote from: uart on
I'm not sure about faults in the deck itself, but my advice would be to
1. Demagnetize
2. Record your stuff at about 3 to 6 dB lower level (you're too close to clipping, neither the top nor bottom should be "flat".
3. Use any old audio software to "remove DC offset" after recording.


Well, the unit was demagnetized when I got it, but I will try doing that again in the near future.

As for being too close to clipping, the waveform in the picture was amplified several decibels to more clearly show the problem.

As for removing the offset with software, there is no measurable offset. It reads as 0.000% on any program!


Ahhh... A lot of these other technical details involving calibrating the machine are beyond me, but to answer Slipstreem's question, yes the door pops off just like you describe and that's how you access the azimuth screw, etc.

For listening purposes I could live with this, but for archiving a bunch of recordings, I really don't want to have bunch of mangled-looking waveforms as my finished product.

I'll continue to look into this, or maybe just get a different deck if I have to...

Thanks again everyone.

Waveform abnormalities during cassette transfer

Reply #12 –
I've had cassettes come out like this, and thought nothing of it. They sounded fine (excellent, in fact). I never thought it could be a playback machine issue, so I've learnt something from this thread - thank you!

You can also find particular microphones (obviously faulty), and maybe even particular instruments, that give this effect - e.g. I've often seen it on solo trumpet passages, though it's always possible the trumpet was overloading the microphone in an inaudible way.

Cheers,
David.
 

Waveform abnormalities during cassette transfer

Reply #13 –
It's surprising how many people just take it for granted that cassette decks are poor performers in one respect or another. I remember designing and building my own cassette tape head playback preamp over 20 years ago and it was surprisingly easy to get an overall flat frequency response with -3dB points at 10Hz and 21kHz using Metal tapes with a budget head. The noise level was well below the noise floor of the best Metal formulation tapes of the day and distortion was below 0.1% THD at 1kHz. It had nearly 20dB of headroom above the standard 0dB reference level too, so I could guarantee that there wouldn't be any signal clipping going on. Total cost of the electronics to achieve this, £15UK at 1985 prices.

The addition of a self-designed perceptually transparent single-ended (ie, playback only) noise reduction system had it outperforming a friend's Nakamichi Dragon by a country-mile in terms of signal-to-noise ratio as well for an additional £10UK.

The machine I grafted it into is still sitting under my bed, so after discovering just how awful the output on my expensive TEAC looks by comparison, I may drag the old beasty out again over the weekend, dust it down and see if I can ABX them.

Cheers, Slipstreem.