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-   -   Sometimes getting hum on VHS and VHS-C tapes? (http://www.digitalfaq.com/forum/video-restore/12328-hum-vhs-vhs.html)

waloshin 12-07-2021 10:38 PM

Sometimes getting hum on VHS and VHS-C tapes?
 
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Is this caused by interreference, the cord, the VCR or the tape? I usually can eliminate the sound with a notch filter in Premiere Pro.

JVC Hr 7900U

latreche34 12-08-2021 02:26 AM

JVC VCR's are known to have muffed linear audio track, I've read somewhere that the audio head pre amp may have some bad filter capacitors, this also applies to any VCR used JVC's preamp PCBs. Clean the audio head just to make sure it is not dirt, Some had success re-aligning the head stack but I would not recommend it.

timtape 12-08-2021 02:39 AM

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There seem to be two separate problems, the muffled audio, and the hums and buzzes. The muffled voices are made even harder to hear by the hums and buzzes.

To test what is causing the hums and buzzes, play an unrelated tape in the same VCR at a quiet passage and listen.

Attached screenshot of the spectral display. Many nasty interference tones there. Not easy to remove them all I suspect.

dpalomaki 12-09-2021 08:05 PM

The sample has a significant 60 and an even larger 120 Hz noise component. To me this implies power supply ripple getting into the audio, especially the 120 Hz component. The 60 Hz component could also include leakage of vertical sync signals into the audio. And I suspect it maybe more pronounced with linear track audio due to the high amplification of the signal read from the linear audio head. Weakening filter caps in the B+ circuits may be a cause.

timtape 12-10-2021 04:21 AM

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Yes there is significant 60 Hz and 120 Hz in the playback but there is another series of tones starting at around 1.2 kHz. These are in the the speech intelligibility zone. (I've reposted the spectral scan of the file but this time in mono to make it larger and easier to read the fine detail).

Maybe there are actually two separate sources of interference, and I guess we cant be absolutely certain some of this isnt on the original tape.

In this case I would suggest a separate audio capture on a VCR with excellent low noise playback. In cases like this the lower the playback deck's own noise the better chance of extracting the most from the audio track.
The gap in speech frequencies from about 2 kHz to 3 kHz suggests a possible null due to azimuth misalignment on playback. If I was making the transfer I would check/adjust audio head azimuth for the clearest, brightest sound on that particular shoot. This alone might usefully lift the wanted speech clarity above the noise.

It seems a low level voice recording so even with the very best treatment it will probably never sound pristine but every small improvement helps.

dpalomaki 12-10-2021 08:31 AM

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Here is another look at the frequency/amplitude content of a piece of the sample without any voice present. Note the peaks at 60 and 120 Hz, the horizontal sync at 15,750 Hz, and the peak at about 1080 Hz with some comb-like peaks at around 1200, 1260, 1320, 1380, 1440, 1500, and 1560 Hz. The frequency spacing of the last seven is interesting. I wonder if they were some noise source in the room backgroud such as HVAC?

It can be cleaned up quite a bit. The attached is a quick and dirty tweak in Adobe Audition, combining 16 dB of gain with medium level noise reduction based on a noise sample taken where there is no audible speaking. With a bit more time it could be made better.

(It also contains some room reverb typical of recordings made using the camcorder's built-in mic several feet from the talent.)

RobustReviews 12-10-2021 11:35 AM

Quote:

Originally Posted by dpalomaki (Post 81093)
Here is another look at the frequency/amplitude content of a piece of the sample without any voice present. Note the peaks at 60 and 120 Hz, the horizontal sync at 15,750 Hz, and the peak at about 1080 Hz with some comb-like peaks at around 1200, 1260, 1320, 1380, 1440, 1500, and 1560 Hz. The frequency spacing of the last seven is interesting. I wonder if they were some noise source in the room backgroud such as HVAC?

It can be cleaned up quite a bit. The attached is a quick and dirty tweak in Adobe Audition, combining 16 dB of gain with medium level noise reduction based on a noise sample taken where there is no audible speaking. With a bit more time it could be made better.

(It also contains some room reverb typical of recordings made using the camcorder's built-in mic several feet from the talent.)

Good work, (assuming the 60Hz fundamental) to still be struggling with 20th-plus order harmonics encroaching appreciably on to the wave something went awry here like you said I'd also be inclined to believe there's an HVAC unit using a VFD or something similar, and not a very good quality one or one that's about to need a repair!

hodgey 12-10-2021 12:53 PM

I haven't experienced it myself, but I did come across this (you can skip a bit out as he gets the cause wrong first) video featuring a vcr with the same mech where there was issues caused by a grounding spring thing on the audio head assembly not making proper contact.

Me and some other users here have had issues with linear audio on the later jvcs models with a different mech but I haven't had issues on ones with this mech other than the audio head possibly being a bit misaligned (though that on it's own should just cause muffled audio i.e the "comb-like peaks", but not the excess noise.)

One other quick thing you could try is turning off the rf output, though I doubt it will have much impact. (Turn off, hold stop/eject for 5 seconds untill it shows channel number in the display, and press ch+/ch- to change it to off, then OK.)

timtape 12-10-2021 08:56 PM

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Thanks Hodgey,

That brings to mind another potential source of noise entry into the linear audio preamp: electromagnetic induction of the audio head itself since the head is designed to be very sensitive to it.

Cassette decks even incorporate a small mumetal hum shield in each cassette tape, right behind the pressure pad (pictured)

Many open reel tape machines incorporate a mumetal hum shield flap which actually swings out of the way for loading the tape and then automatically swings back against the head when playing tapes.

Sometimes the flap has to be manually operated such as in some professional Nagra recorders (pictured)

I've experimented with temporary mumetal shields on VCR A/C heads, sometimes to try and diagnose where excessive noise emanates from inside the deck. Often the flap reduces induced noise.

The problem in fitting a such a shield in VHS or Beta is the limited space in the mechanism and finding a way to automatically pull the flap out of the way when the tape is loaded and unloaded. I'm still working on that aspect!

I note too that some VCR SM Power Supplies are built into a metal box to reduce noise transmission. Others have no such shielding box. Obviously the closer the offending noise source is to the audio head, the worse will be the electromagnetic interference.

These measures can be important when dealing very low level recordings such as Waloshin's may be, and the limit to clearer audio is the playback deck's own noise.

Where the original VCR tape is no longer available for a superior transfer, DeHum or Declick tools can make a great improvement in reducing hum and whistle in the digital copy. But these "after the fact" tools are not ideal. They remove not only the hums but also any wanted audio in the same band. What's more, the more bands needing to be removed, as in Waloshin's example, the more likely wanted speech or music will also be removed or compromised. Getting the audio sounding as good as possible at the VCR end is usually the best way.

waloshin 12-12-2021 05:54 PM

Seems to have been dirty heads on the VCR. Thank you everyone.

timtape 12-12-2021 07:17 PM

Quote:

Originally Posted by waloshin (Post 81121)
Seems to have been dirty heads on the VCR. Thank you everyone.

That's good. Cleaning the AC head could have improved muffled audio but the hum should have remained unchanged. Perhaps the head was so dirty that about all the head could read was the hum.

dpalomaki 12-13-2021 10:06 AM

Dirt on the heads could have prevented good tape contact resulting in lower playback levels. In the sample it was perhaps 10-15 dB below expected levels. Because the electronics hum and noise floor remain unchanged, when playback level was increased to hear the talent the hum noise floor was increased as well and became more apparent..

timtape 12-13-2021 12:48 PM

Quote:

Originally Posted by waloshin (Post 81121)
Seems to have been dirty heads on the VCR. Thank you everyone.

It would be good to hear the same speech section after the head cleaning. Any chance of an uploaded file for comparison?

waloshin 12-13-2021 09:24 PM

Quote:

Originally Posted by timtape (Post 81137)
It would be good to hear the same speech section after the head cleaning. Any chance of an uploaded file for comparison?

Will get one uploaded this week.


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