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Dumb question (for various reasons, I can't listen to the file at the moment). 
Were these cassettes all recorded on small mono portable machines? Tapes made 
on these almost always have a crackly or noisy right channel when played back 
on good decks and in stereo. The solution here is simply to play only the left 
channel.

dl

Mike Hirst wrote:
> Thanks Richard,
> 
> your advice is, as ever, well informed sagely and intelligent. I suspect 
> that I'm not going to find a definitives answer here, but for your 
> interest, and for anyone else who may be interested, I've posted a 10sec 
>  (wav) clip of the kind of crackle I'm hearing. this sort of thing will 
> continue through the full length of the recording and can be heard when 
> the tape is played back using multiple recorders, all of which have been 
> tested using other tapes, which exhibit no such problems.
> 
> http://www.mikehirst.netfirms.com/audio/rhclick.wav
> 
> filesize= 1392640 byte(s)
> riffsize= 1764036 byte(s)
> format  = Straight-PCM
> channel = 2
> depth   = 16 bit(s)
> blk.size= 4 byte(s)
> smp.rate= 44100 Hz
> samples = 441000
> playtime= 0:10.010
> 
> you will note from the clip that the crackle can only be heard in the rh 
> channel. this is typical of the phenomenon, but it can sometimes be 
> heard in both channels (with a bias towards the rh ch).
> 
> I have experimented with some of the de-crackle filters I use when 
> working with disc transfers and as Richard suggests, the crackle can be 
> removed, I am however curious as to where the crackle comes from.
> 
> Richard L. Hess wrote:
>> Hello, Mike,
>>
>> Tom Fine has already posted a number of good explanations.
>>
>> There are, however, less-common explanations that you may wish to be 
>> aware of, just in case.
>>
>> (1) If there is a mismatch between the record machine record and erase 
>> head track position, perturbations in the record bias and/or erase MAY 
>> print to a tape like this. DC-(i.e. permanent magnet) erase may also 
>> cause something like this, but it is usually more of a "burbling" or 
>> what is sometimes called "rocks".
>>
>> (2) Static electricity and PLAYBACK machine "glitches" CAN print to a 
>> tape without the recorder being in record mode. It's uncommon 
>> (thankfully),  but it can happen. Static can be generated by fast 
>> winding in a very dry environment, and depends on cassette materials 
>> including the shell and slip sheets. This is more prevalent with reels 
>> than cassettes.
>>
>> This clicks can usually be removed (depending on their source) by a 
>> declick/decrackle plug-in for your favourite DAW. The Magix 
>> restoration tools version of this is the best I've yet owned, but I 
>> haven't owned either DC7 nor the high-end Algorithmix version.
>>
>> At 08:53 AM 2008-09-18, Mike Hirst
>>> Here's a thing that's been confusing me for some time. I have spent 
>>> the past six months working my way through a large number of cassette 
>>> tapes mostly recorded between 1985 and 1995. every now and again I 
>>> notice light, but significant crackling. This is often more 
>>> noticeable in the right channel, but not exclusively so. On some 
>>> recordings this is louder, on most recordings this is not evident at 
>>> all. This is not restricted to any one brand of cassette, nor is it 
>>> associated with any one playback machine and/or soundcard. Can anyone 
>>> explain this for me?
>>
>> Richard L. Hess                   email: [log in to unmask]
>> Aurora, Ontario, Canada       (905) 713 6733     1-877-TAPE-FIX
>> Detailed contact information: http://www.richardhess.com/tape/contact.htm
>> Quality tape transfers -- even from hard-to-play tapes.
>>
>