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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 18th Jan 2022, 11:32 pm   #21
jamesperrett
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Default Re: Tapes at wrong speed

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Originally Posted by TIMTAPE View Post
I believe speed change software is better than it used to be but it does things fundamentally differently from analog. Digital is happier with an even numbered speed change like double speed or half speed. Intermediate speed changes work if they're not too large but eventually fidelity is compromised.
This advice sounds 20 years out of date. Modern digital sample rate conversion is almost completely transparent - no matter what the conversion ratio. I believe that you can use the high quality Sox sample rate convertor in Audacity. The only issue is going to be the shifting of the Nyquist frequency downwards if the recordings need to be slowed down. However, you would be lucky for a hifi cassette recording to have much information above about 16kHz so, at 44.1kHz sampling rate, your tape could play up to 30% fast and you would still be able to retrieve the full bandwidth the recorded signal when it is slowed down to the correct speed.

Last edited by jamesperrett; 18th Jan 2022 at 11:52 pm.
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Old 19th Jan 2022, 3:40 am   #22
TIMTAPE
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Default Re: Tapes at wrong speed

Thanks James. I seem to recall a different view on SOS forum but maybe I imagined it. Still with many tapes there will be the need to process each one digitally. In this case it's known in advance that all of the many tapes are off speed and by the same amount. So correcting that with one initial speed adjustment at the tape machine seems like a no brainer.
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Old 19th Jan 2022, 8:29 am   #23
Ted Kendall
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Default Re: Tapes at wrong speed

The essential point is to get the most information off the tape. A crude stab at the correct speed is good enough for this and is easily corrected in software. Azimuth, however, is critical - get this wrong and the information lost is gone for good. Trouble is, it's next to impossible to tweak azimuth whilst running at double speed. Take the trouble to get the azimuth right and digitise at about baseband. Then start tweaking. Keying to mains hum is generally OK, providing you find the right hum to follow - I've had many tapes where there are several hums around supply frequency, all drifting in different directions. Granted, a lot of these tapes were dubs, but even so, beware. Incidentally, if the cassette speed is within 0.5% beginning to end, I'd be surprised.
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Old 20th Jan 2022, 9:37 am   #24
TIMTAPE
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Default Re: Tapes at wrong speed

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The essential point is to get the most information off the tape. A crude stab at the correct speed is good enough for this and is easily corrected in software. Azimuth, however, is critical - get this wrong and the information lost is gone for good.
So true. Plus especially with slow speed, narrow track formats like cassette, the wanted signal can be weak at best. Playback pre amp noise is often barely quieter than the tape's own baseline noise. Azimuth error can further reduce the already poor signal to noise, and even completely cancel out certain bands, allowing preamp noise to predominate. All avoidable by proper capture in the first place.

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Trouble is, it's next to impossible to tweak azimuth whilst running at double speed.
True. Since I can no longer hear anything above 10 kHz I sometimes slow the tape to half speed to aid azimuth align. Another help I also take advantage of is viewing the signal on an FFT display so I can see the parts of the recording my ears may no longer hear.
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Old 21st Jan 2022, 3:30 am   #25
arjoll
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Default Re: Tapes at wrong speed

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Some of us take a slightly more engineering based approach.
I must say I agree with what you're saying. If you can digitise something played back at the correct speed you get to keep all the samples with no software interpolation to a sensible sample rate. Something digitised at 44.1 kHz that's say 10% fast would have an effective rate of what 39.69 kHz? Or 40.09 kHz? to be ok without software having to interpolate what those "missing" samples should be.

This is the reason I sampled at 88.2 kHz when digitising some of my late father in law's 1.875 ips recordings on my Akai which only does 3.75 and 7.5. Just switch the sample rate to 44.1 in Audition and they were good to go.
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Old 21st Jan 2022, 3:07 pm   #26
TIMTAPE
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Actually as I said I defer to Tanuka and James on this subject. I knew the digital respeed process was improved these days but didnt know by how much it had improved.

By a more engineering based approach I was thinking of getting EQ correct when high speed dubbing. If tape playback speed is doubled then the playback EQ corner frequency for example must also be doubled. It goes up an octave just as the voice or music goes up an octave at double speed.

A decent dual cassette deck with high speed dubbing (x2 speed) would have both its playback EQ and its record EQ settings automatically switched to match the high speed. The settings would only apply at that speed. It was the only way to get a tolerably flat response in the copy, consistent with good signal to noise and distortion.

The playback EQ on good multi speed reel to reel machines was designed to meet the relevent standards as per test tapes. The record EQ was designed to achieve a flat playback response over the relevent frequency range at that tape speed. But doubling of tape speed didnt normally double the corner or resonant frequencies of the record and playback EQ's. Again, EQ was matched to each particular tape speed and had no necessary relationship to the other speeds. So playing a 1 7/8 ips tapes at 3 3/4ips was not strictly correct. An EQ error would have crept in. Halving speed digitally would not have corrected the error.

So unlike well designed dual cassette decks with high speed dubbing function, the various speeds on open reel tape machines were not really designed for proper high speed dubbing. That's my understanding anyway.
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Old 21st Jan 2022, 5:31 pm   #27
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The tapes were all Radio Hallam top 40 tracks, so I have a good idea what many of the tracks would should like at the correct speed. Once you get one good well known track at the right speed the setting will be the same. A C90 tape has 13 tracks on instead of the more typical 12. So I am guessing it was running slightly slower, but not much giving an extra say four minutes to a tape.
I checked my tape log book and it said that the deck was a WYE SC501 - a silver fronted machine. I think somebody had fitted a 9 volt motor instead of a 12 Volt one. I remember trying to get the deck to play a pre-recorder cassette at the right speed and it couldn't do it, even with the speed control on top of the motor at full!
I suspect somebody had also adjusted the head screw.
I think I will go with trying to get the speed right in software mode than tape deck. But I am currently bidding on tape decks on a certain auction site to get a decent playback machine. My current Technics model (not quick on rewind) has a motor speed control built into the motor. I don't like adjusting that type as you can't mark the correct place for it. You can with the the ones on PC boards. But you never know which model has which control. So if the new deck has one on the PC board I can adjust that. If not then I will go with software adjustment.
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Old 21st Jan 2022, 6:45 pm   #28
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Digital has no problem doing non integer speed changes, all it takes is a bit of arithmetic. Modern computers take this in their stride. If you sample at, or above, the Nyquist rate (2X the maximum frequency) ALL the information is there, and given the right sums ALL will be at the end of said sums. Noise is your friend here, if there is zero noise it will take an infinite amount of time to do the sums. There is never no noise so it is doable.
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Old 22nd Jan 2022, 3:18 am   #29
TIMTAPE
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I am currently bidding on tape decks on a certain auction site to get a decent playback machine.
It's hard to advise on good cassette decks to purchase because these days they will be old, and parts, and service technicians, are in short supply. Even when the machine is sitting idle, unused rubber parts deteriorate. Capacitors fail. You might be better off buying a "2 motor" deck as there was often less to go wrong and maintain mechanically, but there can still be problems. All the best in your search.

Last edited by TIMTAPE; 22nd Jan 2022 at 3:35 am.
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