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Author Topic: Cassette Tape Equalization
MacManKrisK

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Icon 1 posted December 26, 2007 08:56      Profile for MacManKrisK     Send New Private Message       Edit/Delete Post   Reply With Quote 
*calling all audiophiles*

I have recently discovered the magic of the RIAA Equalization Curve for recording old albums and records into digital formats. It's about +20dB on the low end and about -20dB on the high end, which makes some kind of sense as most pops, crackles, and hiss are on the high end; so you'd want to record an album with the inverse of that curve to emphasize the actual music at the high end so it can overcome the expected pops and hisses.

What I'm wondering, though, is if there's any "magic" EQ curve for cassette tapes. They seem to have a quality opposite that of a record -- that is overemphasized bass and underemphasized treble.

Audacity, which has a nice 31-band EQ (and lets you draw your own custom EQ curves) has an RIAA setting built in for albums, but doesn't have anything for cassettes. I've looked on google and wikipedia and didn't find anything except for a controversy about "the 4dB ambiguity at 16KHz."

So, is there a "magic" EQ curve for cassette tapes? Or is one of those "just play with it 'till it sounds right" kind of things?

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GMx

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Icon 1 posted December 26, 2007 09:13      Profile for GMx     Send New Private Message       Edit/Delete Post   Reply With Quote 
Any tapes that have been encoded with Dolby noise reduction has to have that enabled on your tape deck if it has it. I've noticed that with the reduction ON, you do lose some high end. Unles somebody has some better advice, I guess you'll just have to play around with the EQ. I LOVE GarageBand's "Refresh Dull Recordings" EQ setting. I'll have to look and see what it adds or subtracts, maybe that will help. (I'm not at home right now)
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GrumpySteen

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Icon 1 posted December 26, 2007 10:39      Profile for GrumpySteen     Send New Private Message       Edit/Delete Post   Reply With Quote 
http://www.freepatentsonline.com/US4500932.html

You'll have to register to get to the PDF, but this particular patent has a pretty good description of the equalization that's done on various types of cassettes along with a circuit that approximately (but not completely) compensates for it.

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TheMoMan
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Icon 1 posted December 26, 2007 11:21      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
MacManKrisK ____________________ Did you ever open up a big can of worms. Depending on the oxide used to make the tape, there is a different curve. Metal tape one curve, Iron oxide different curve, some tapes near the do not overwrite window may also have a second or third window to set the playback machine to the right curve, not all players take the tape in far enough to see the windows, some players look to see if the tape conducts electricity if so it sets the metal curve. GMx also is right that there are Dolby curves that also can vary. I used to have a very old tube (valve for you Brits) preamp that had six different curves built into its circuits. There are different curves for Phonograph depending on country of origin and company of origin. COL=CBS RIAA= most USA pressings, most of europe and some of Canada you would use COL. Now for the kicker, there is a bias setting at record time that must match the oxide, it is there to act like a AM radio signal that the audio is then modulated onto, during playback the Bias frequency is too high to be heard but is still along for the ride.

So, is there a "magic" EQ curve for cassette tapes? Or is one of those "just play with it 'till it sounds right" kind of things?

Works for me.

Have I confused you enough?

Thought I better add this some of the packaged units (speakers wired directly to the amp output) had a matched output the AMP had the inverse curve of the speaker, so if you upgraged the speakers you got worse sound.

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Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
Colonel Panic
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Icon 1 posted December 27, 2007 11:32      Profile for Colonel Panic         Edit/Delete Post   Reply With Quote 
Kris,

If you wait long enough either MoMan or I will bring out our baskets of 8-track tapes. That was the portable music format folks used before Dolby and Philips got together and made cassette tapes listenable.

Before 8-track there was reel-to-reel tape. High quality masters were made at 15 inches per second. Dubs were played at 7-1/2 ips. Cassette tapes are played at half that speed. The high speed of r-2-r masters allowed a lot of information on a tape and kept tape hiss to a minimum. Cassette tapes are portable, use less tape, but play at 3-3/4 ips. Information overloads those tapes and tape hiss is distracting at low levels.

To address those issues Dolby laboratories developed a compression/equalization system that encoded music onto tape at a level above the tape hiss. And then they developed other formats. Dolby "B" is commonly found on most commercially produced tapes, although Dolby "C" was available, too. Dolby "S" was introduced when CDs and high-end VCR sound was becoming popular.

If you have a Dolby "B" cassette tape you must use a deck with Dolby "B" decoding enabled to get accurate sound. You will notice a perceived brightness in the tape playback if you do not use the decoding, but in reality what you are hearing is the encoded tape without decoding. People with crappy tweeters would often choose not to enable the decoding because they thought they were losing something. But they are wrong.

MoMan is also correct in noting tape bias. At the peak of cassette tape popularity tape was available in three popular bias formats, Ferric Oxide, Chromium Oxide and Metal.

My last tape deck had stationary, one-direction heads, Dolby B and C noise reduction and bias adjustment for Ferrous and Metal Tape. Chrome tape used the Ferrous bias setting, but provided better sound and offered greater durability.

Here is a link that will inform you about the Dolby formats:

http://www.dolby.com/assets/pdf/tech_library/212_Dolby_B,_C_and_S_Noise_Reduction_Systems.pdf

Oh, and one more thing. Audio compression is a whole new world of audio tinkering. Tread lightly in this area. Once you recognize it you will never be able to listen to a cheap-ass Sony home theatre system again without suffering projectile vomiting. (Their amps compress high and low end to match their cheap-ass speakers and it sounds evil -- a lot of teen-oriented sound systems do the same thing).

Colonel Panic

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MacManKrisK

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Icon 1 posted December 27, 2007 12:09      Profile for MacManKrisK     Send New Private Message       Edit/Delete Post   Reply With Quote 
I have a tape from someone who is no longer living in our plane. He recorded it from a couple of old record albums. The tape is rated for Type II Bias (CrO2), but I would doubt it was recorded with the correct bias setting which is just as well as my deck won't do Ferric, Cr02 or Metal bias. I doubt any Dolby encoding was done, either. There's a real noisy hiss up around 3-4KHz, but clipping that also clips the nice clean crisp high-end of the spoken word performance that I'm trying to capture.

I guess I'll just have to play around with it...

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TheMoMan
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Icon 1 posted December 27, 2007 14:25      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Colonel Panic & MacManKrisK ___________________ I used to have a reel to reel Ampex with big hubs so that I could use 14" reels of tape and run the machine at 30"/sec. now that was HiFi 1/4" tape full width head Mono. and a lot of us audio types got ticked off when the decision was made to bidirection tape, heck you are losing half the sound. Just think of a cassette tape 1/8" wide and they put four tracks on there and enough space between them to prevent cross talk. On that old Ampex there were two DC servo drives for the reels a syncroness motor for the capstan and a cooling fan motor, this was suposedly a portable machine, Actually about a 26" cube and weighed about 90Lbs.

Throw in a box of tape and you're over a hundred weight. Plus cords, cables and Mikes, and you have a lot of money tied up as well.

Now think about how much sound gets throw out to make an Mp3? I was tring to capture all of the sound and now it gets throw away.

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Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
GMx

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Icon 1 posted December 27, 2007 15:09      Profile for GMx     Send New Private Message       Edit/Delete Post   Reply With Quote 
Here's a shot of GarageBand's Refresh Dull Recordings setting:


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TheMoMan
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Icon 1 posted December 27, 2007 16:22      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
GMx & MacManKrisK ______________________ From what I can gather from your posts is that in the range of 2.4 through 4 Khz the audio is clipped? I do not know what to do to restore the wave peak if it is truely flat topped. If you had access to a high end Graffic equalizer you may be able to bring up those frequencys but I doubt that you would be able to restore their waveform. Also if the flat topping occured on a transister machine each corner will have a harmonic also. That is why tubes are so much better. I have done some editing with audacity but do not know of a way of being able to redraw the wave.

On a some what different note My big tube amp had a subsonic roll off curve so that if the turntable was putting rumble into the audio it was killed in the final. That said I would have to turn on the subsonic killer on tapes made from a vinal recording because of rumble. Ah yes the power company loved that Amp. it drew about the same power as an electric oven when in use. Turn the power on and the lights blinked.

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Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
The Famous Druid

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Icon 1 posted December 28, 2007 11:40      Profile for The Famous Druid     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by MacManKrisK:
I doubt any Dolby encoding was done, either. There's a real noisy hiss up around 3-4KHz, but clipping that also clips the nice clean crisp high-end of the spoken word performance that I'm trying to capture.

Could be Dolby.

IIRC, the amount of boost Dolby applies depends on the signal strength. When recording, weak signals get lots of boost, strong signals get little or none. With spoken word, this would mean the boost fluctuates wildly, as there's lots of silence in between the words (unless you're my friend Margaret, then there's no silence at all).

On playback, you apply the reverse process, reducing strong signals a little bit, weak signals a lot. As the hiss tends to be fairly weak, it gets reduced a lot, hence the good noise reduction.

If you're applying a constant correction factor, it'll sound really crappy.

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Dick Moore
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Icon 1 posted December 29, 2007 12:27      Profile for Dick Moore   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Some notes on cassette tape EQs -- All tapes have a playbak equalization curve that begins a -6dB/octave roll-off at a time constant of 3180uS, or about 50Hz. Ferric oxide (normal/standard) has a flattening out at 120uS or about 1326Hz, while CrO2 (chrome/ferrichrome) and metal have a flattening at 75uS or about 2122Hz. On the recording side, the bias current levels are different for each tape type and for best performance, a tape deck should be set for the tape that is going to be used. Of course, your playback deck will do the playback EQ for you, so the sound variables have to do with whether the tape's record EQ and bias were correct for the tape used. They generally aren't which was why Nakamichi had high-end decks that let you adjust those elements from the front panel before recording -- pretty nifty.

To get the best sound on your dub to digital, it's a good idea to use a multi-band equalizer -- either analog or digital, and tweak it until it sounds best to you.

Prior to the Dolby IC chips, Dolby B circuits were notoriously poorly caibrated, and even after the ICs came in, levels were often mis-calibrated to the point that a tape would only sound good on the machine it was recorded on -- Dolby expansion on playback is usually done through the same circuits used for compression on recording, so they are exact inverses of each other. No amount of fiddling with an equalizer can fix a Dolby problem (especially Dolby used to record, but not used for playback, which sounds awful), only help a bit.

Prior to the Dolby IC chips, Dolby B circuits were notoriously poorly calibrated, and even after the ICs came in, levels were often mis-calibrated to the point that a tape would only sound good on the machine it was recorded on. No amount of fiddling with an equalizer can fix that problem, only help a bit.

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Saxima
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Icon 1 posted August 14, 2009 23:57      Profile for Saxima     Send New Private Message       Edit/Delete Post   Reply With Quote 
Here is a riddle I'm hoping someone can help me with:

I am in the process of transferring a rare & precious 1979 cassette tape to digital. I'm using pro Tascam studio equipment - Tascam 322 & a CD-RW402. Before starting the transfer I cleaned and demagnetized the tape heads and installed the tape in a new cassette tape case.

As I'm transferring the tape it is going very well. But about half way through the transfer a squeaking screeching sound is coming through the audio. I stop the tape, put it in the other transport and it plays perfectly, noise free. But after a few minutes the screeching starts slowly again until it is so loud the audio on the tape is not audible.

I tried 3 other tapes in the deck and the problem doesn't happen. Only when I play this tape the screeching sound occurs.

Here's the mystery: the screeching sound is not on the tape. It only occurs when I play the tape 20 minutes or so into the tape at different spots on the tape.

If anybody can enlighten me as to what is happening PLEASE tell me. This is driving me nuts! [crazy]

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MacManKrisK

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Icon 2 posted August 15, 2009 10:18      Profile for MacManKrisK     Send New Private Message       Edit/Delete Post   Reply With Quote 
Saxima: Since this is my own thread I'll comment here.

http://en.wikipedia.org/wiki/Netiquette <<--- read that

As an addendum to that article: posting the exact same thing in multiple threads is not something we in this forum consider to be polite nor appreciate. Furthermore, the likelihood of receiving a response to your question goes *down* with each consecutive identical post you make. That is to say: the number of identical posts you make is *inversely* proportional to the likelihood of you receiving an informational response.

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get rich and you still die"


Posts: 2331 | From: Southwest Michigan, USA | Registered: Oct 2001  |  IP: Logged


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