Tascam 38 Bass Artifacts

F

fstrat76

Member
I was just adding a bass track to a song on track 3 of my 38.

I used some very light compression and went direct from my M2600 board to the 38. Recording levels were good, extending occasionally into +1 or +2 VU. Using RMGI SM911 tape. While monitoring from the sync head during recording, everything was crystal clear.

Upon playback however, I noticed that while still clear and punchy, there were some strange artifacts on almost every note played, almost like a speaker rumbling or small tear in a speaker. Impossible of course since I went direct and the monitor sound was perfect. Its a small annoyance and can't really be heard when sitting in the mix.

Still, I re-recorded the track on track 4 and again on track 6. Same thing. What is odd is that all the other tracks like guitar and acoustic guitar sound great, just this bass track has the artifacts.

I don't recall anything being off on any low frequency tests when I calibrated the machine a couple months ago, so I'm not sure it's a calibration issue.

It's not a major issue but was wondering if anyone has run across this or has other suggestions of things to try differently?
 
jpmorris said:
As a first guess, I'd say 'Bias'. Do you have an example clip?
Click to expand...

I can try to get a digital clip this weekend. It almost sounds like a small paper rattle.

Are you talking about erase bias or tape/record bias? I did the procedures for this SM911 tape/record bias. All frequencies showed within +/- 3db as per the manual. I can run it again though.
 
Without hearing it, it's hard to say, but insufficient record bias does produce weird sounds at certain frequency ranges. IIRC bass it makes something of a 'gravelly sound', like rocks sliding around. With high frequencies I got hiss.

Have you tried recording test tones at 100, 1000 and 10000 Hz on a blank section and listening for anything weird?
 
Attached is an mp3 version of the artifacts. Hopefully the artifacts can be heard in this 256k mp3 file.
 

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  • bass.mp3
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jpmorris said:
Have you tried recording test tones at 100, 1000 and 10000 Hz on a blank section and listening for anything weird?
Click to expand...

Just recorded the tones. The low frequencies 60hz, 100Hz, sound noisy as well. I readjusted bias, rec frequency, everything is on. Still a slight distortion on those very low freqs.

Higher frequencies sound great.

Could it be erase bias?
 
I'm honestly not sure what to make of that. For the most part erase just seems to be a current adjustment - what I did for my A807 was simply to record a sinewave for 10 minutes or so, then erase it, listening to the playback with the volume cranked up and adjusting the trimmer until the residuals went away. Kind of ham-fisted, I will admit.
 
Hey FSTRAT, did you ever figure out what was happening with those artifacts? I have a tascam 48 and have the exact same situation. Any ideas would be great!!
 
ecs787s said:
Hey FSTRAT, did you ever figure out what was happening with those artifacts? I have a tascam 48 and have the exact same situation. Any ideas would be great!!
Click to expand...

No, I have not yet resolved this. I was going to buy a new tape to try but have not yet had the chance.

What tape are you using? Mine is a RMGI SM911 from 2010.

Please post back if you get this resolved.
 
It's called "bias rocks".If you record a low freq.tone,(40HZ for example), and view the playback on a O-scope,you will actually see a very short dropout of a piece of the waveform,and your brain interprets it as a pop or click instead of the waveform stopping and resuming.Sometimes over biasing the unit can mimimize the noise.
 
Yeah, I'm using SM911 too. I didn't notice it as much when I was running Quantegy 499... but I probably didn't have my machine biased the same. I'm using a Tascam 48. I also have been reading about using the 40-50hz tone and biasing for minimum noise. It works really well. I just listen to the distortion and artifacts when biasing. The noise is still there, but not as bad. I figure, I'll just eq those out when I'm recording bass...

Good luck!
 
That's what I did; the LF biasing procedure by ear to minimize the noise as best as possible, but it is still there and some eq can help. It's also evident on the low strings on acoustic guitar pieces. I don't recall this happening on my previous recordings from my new original 38 back in the 80's early 90's though. I'll re-bias again soon to a new tape once I get one.
 
Hmmm I wonder if it has anything to do with the electrolytic caps on the bias cards, channel cards or even power supply... You'd have to think that after 25 or 30 years these machines aren't going to be up to spec...I got my machine on Ebay and don't really know its history of refurbishments, if there were any. I bet its time to recap the whole machine, but that would be a little bit of a large project.
 
I don't think its the channel cards since it happens on every channel. Does it happen on every channel of your 48?
 
Probably something you have already tried or maybe not possible, but have you considered trying the tape on someone else's machine? Then you might be able to eliminate whether it is a record or playback issue. A recorded tone played back into a scope might tell you something about the type of distortion, if you have a scope. Good info there from wkrbee! Main thing is to record a pure sine wave so you can trigger it on the scope... I run both a 38 and 48 and have never had this problem...:-/
 
Thanks for the idea... I wish I could try on somebody elses machine, but I don't know anybody else who records to tape!:) Maybe a good thing. BUT, I've not run the recorded signal through a scope. I have a computer program scope and my buddy has a real scope. What frequencies should I be looking at, or should I be looking at all? The problem is most evident on bass sounds and on hot transients from something like a guitar just as FSTRAT76 had said...

Also, what should I be looking for on the scope? And once I find it, what will it tell me? would it tell me that a certain type of distortion is from a certain electrical problem??

Now that I have read about "Bias Rocks", I can confirm that I think it is exactly my problem. WKRBEE, thank you! It does seem minimized as I had said before when I bias using the 40hz-50hz tone and adjust for minimum distortion...

Thanks all who have chimed in. I've tried getting a little out of another forum on biasing this Tascam 48, but have had much better luck here! My main issue before what the calibration procedure listed in the manual. It is the exact opposite of what I've used before with the whole "turn counterclockwise to minimum signal, then turn clockwise to peak, then (in the 48 manual) turn counterclockwise to remove 3-5dB. Usually it is keep turning clockwise over the hump and overbias 3-5db. Well my pots are all screwed I can just keep turning and turning so I have no clue what the current position is. BUT, again, I have adjusted for the best sound (by ear) and I think that is most important...

Still can't get any movement from the HiFreq record cal trimmer. Not sure they really work on any tascam... The manual uses them in a totally different scenario than most other recorders...

Ok, enough rambling.
 
My gut is there are other things off in the electronic side of calibration... calling for a full calibration from scratch... and/or possibly extreme head wear. Next to bias issues, poor head azimuth alignment is another cause of funky bass frequencies. Mechanical interference like recording with the bass amp too close/to loud to the deck is another.

If the input/output calibration is off, the bass and mid-bass frequencies could be hitting the tape too hard, but the meters aren't telling you. And it can happen on all channels if someone calibrated each channel with the same bad technique.

I've always thought the bias procedure in the 48 manual that I've seen was a typo. It makes no sense when looking at the bias circuit, which is the same as other Tascam decks. If everything else is in order, for SM911 tape, the 3 dB down from peak method for 15 ips using the prescribed 10k tone should eliminate any rolling rock artifacts. It's done this way on the 38, 58 and every other Tascam deck I'm familiar with, except for the two-head models, of course.
 
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