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Digitising Cassettes

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Re: Digitising Cassettes

Postby Tomás Mulcahy » Thu Apr 09, 2020 10:45 am

James Perrett wrote:However, undecoded Dolby B sounds too wispy to me so I usually use Uhe's Satin tape emulator which includes encoding and decoding for various noise reductions systems. I play around with the input level on Satin to obtain the best match while listening to things like ride cymbals.
That's a BRILLIANT idea! Thanks.
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Re: Digitising Cassettes

Postby James Perrett » Thu Apr 09, 2020 1:22 pm

ef37a wrote:Re "mechanisms". Some months ago I scored 3 5 packs of TDK SA 90 from the Scope shop in town. Back home I settled down to copy some PC audio to cassette (for the jam jar). Machine is an excellent Sony Dolby S deck.

All seemed well on playback until about 20 minutes into the tape when the most appalling wow and flutter began and got steadily worse, like the music was under water. I examined the tape and it seemed be 'concave' and shiny not flat and matte.

The tape in cassettes often appears concave or convex and shiny so I'm not sure that this is a fault. I suspect that the belts in your machine are on the way out and that the housings have a higher friction than other tapes. As I'm sure you are aware, belts are a very common problem and every cassette machine I own has either had replacement belts or is in need of them.

Good point about de-maggers Mike. In all my years using and fixing cassette machines I have never known one suffer noise due to magged up heads and since the vast majority use a common rec/replay *head it has AC bias running though it from time to time. Even play only machines however, in cars an factory PA racks never seemed to have a problem.

I've only ever had one instance where demagnetising a machine made a dramatic difference. However, it can give you a subtle improvement to the high frequencies so it is worth doing every few weeks or so. Yes, it needs to be done correctly as it relies on the alternating field being slowly reduced so you need to s-l-o-w-l-y (as it says on the label on my demagnetiser) move it away from the head before switching off.

I would like to suggest to any absolute newb that maybe does not want to get too deeply into the matter and wants to keep costs low, buy a Behringer UCA 202 (222?) .

I would just add the proviso that, if your deck doesn't have an output level control, you may need some kind of input attenuator with these as the one that I have is easily overloaded (tried with both a Rotel reciever and an Aiwa cassette deck).
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Re: Digitising Cassettes

Postby ef37a » Thu Apr 09, 2020 1:40 pm

Hi James, I replaced the belts in the Sony only around a year ago (TC-K611S) and in any case it plays all my other cassettes of which I have scores of various brands but many SAs.
The look of the tape maybe is not a clue? Just an observation.

I also have a Denon DRM 550 and that gives exactly the same symptoms. I suspect the tapes might have been exposed to a high temperature, against a radiator perhaps? I can send you a couple to check if you like!

I take your point about the low headroom of the 202's inputs but it can handle just about 1V rms and I have never had a problem with 'domestic' hi fi. Maybe your machines are bit butcher than mine!

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Re: Digitising Cassettes

Postby Tim Gillett » Thu Apr 09, 2020 1:44 pm

Tomás Mulcahy wrote:
James Perrett wrote:However, undecoded Dolby B sounds too wispy to me so I usually use Uhe's Satin tape emulator which includes encoding and decoding for various noise reductions systems. I play around with the input level on Satin to obtain the best match while listening to things like ride cymbals.
That's a BRILLIANT idea! Thanks.

Practitioners have been adjusting pre decoder gain for a long time.

But even back in the heyday of cassettes, Yamaha and NAD included a perhaps more targeted pre decoder front panel adjuster on some of their better model decks named "Play Trim". It allowed some fine adjustment of the highs pre the decoder, on the assumption that since with cassettes the recorded highs are very sensitive to bias levels, and that especially Ferric cassettes tended to slight demagnetisation, which shows itself first in a loss of HF information, some manual trimming of the HF was a good idea. It was.

I've incorporated both ideas for many years: a flat gain trim, plus selective boost and cut of the highs pre decoder. All manually adjustable of course and independently on each of the stereo tracks. Much more range and power of adjustment to minimise Dolby mistracking.

What I've never done and never in all my research on the topic even heard one person mention let alone advocate, until yesterday, is denoising pre the decoder. But who knows? :roll:
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Re: Digitising Cassettes

Postby James Perrett » Thu Apr 09, 2020 1:53 pm

ef37a wrote:Maybe your machines are bit butcher than mine!

Just bog standard Aiwa decks from around 20 years ago which weren't far enough up the range to get an output level control. I get a few tapes where people haven't bothered checking the recording level and end up with the meters hitting the end stops. These are the sort of tapes that the Behringer can't handle. 12" singles also often overload the inputs when fed from a Rotel receiver. I ended up making some attenuating cables for mine - I used 22k ohm resistors in series with the input (which already has a 22k resistor from signal to ground inside the unit) but if I was doing it again I'd go for a slightly higher value.

Edited to add: yes, I'll take a look one or two of those problem tapes if you like Dave.
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Re: Digitising Cassettes

Postby Tim Gillett » Thu Apr 09, 2020 2:26 pm

Hugh Robjohns wrote:Tim, I'm not sure you're actually reading what James is explaining.... but his process sequencing seems entirely valid and appropriate to me. It's how I would do it too, and the necessity for unity-gain with the Dolby decoding is taken care of by adjusting the input level to the decoder, as James described -- something that is typically needed anyway when playing a tape back on a different machine to the one it was recorded on because of notoriously unreliable stock alignments.

The aim is to reduce tape noise further than the capabilities of Dolby on its own, while maintaining an accurate reproduction frequency response from the Dolby decoder.

Agreed but Hugh what do you think specifically of Denoising upstream of Dolby decoding and why?
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Re: Digitising Cassettes

Postby Hugh Robjohns » Thu Apr 09, 2020 2:46 pm

It's all in the text you quoted! :think:
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Re: Digitising Cassettes

Postby ef37a » Fri Apr 10, 2020 10:02 am

James Perrett wrote:
ef37a wrote:Maybe your machines are bit butcher than mine!

Just bog standard Aiwa decks from around 20 years ago which weren't far enough up the range to get an output level control. I get a few tapes where people haven't bothered checking the recording level and end up with the meters hitting the end stops. These are the sort of tapes that the Behringer can't handle. 12" singles also often overload the inputs when fed from a Rotel receiver. I ended up making some attenuating cables for mine - I used 22k ohm resistors in series with the input (which already has a 22k resistor from signal to ground inside the unit) but if I was doing it again I'd go for a slightly higher value.

Edited to add: yes, I'll take a look one or two of those problem tapes if you like Dave.
I

I get you now James. All my tapes are done peaking to Dolby level or just over and with B engaged. This gives me a good level in the old Proton and the Dolby 'lift' helps with any alignment issues and this old deaf'un! Even tapes done decades ago were made on a decent Yamaha deck and again peaking to DL. IIRC the Sony puts out about 500mV at DL.

I shall sort out a couple of tapes over the weekend, thanks.

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Re: Digitising Cassettes

Postby Tomás Mulcahy » Fri Apr 10, 2020 1:21 pm

@ef37a FWIW I've found the TDK SX cassette shells wobbly even when I used them back in the early nineties at double speed in a four track. Sony UX-S were better, with the Maxell XL series being better again, and the XL-IIS seem to be the smoothest (although probably identical to XL-II).

Tim Gillett wrote:Practitioners have been adjusting pre decoder gain for a long time.
This is (another) good example of your selective reading. I did mention the technique in my first post... Perhaps I wasn't clear in the post you're quoting, so I'll clarify. The clever bit I was referring to in James's post is doing the transfer without decoding, and leaving the level adjustment, with decoding, to the Uhe plugin afterwards.
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Re: Digitising Cassettes

Postby James Perrett » Fri Apr 10, 2020 2:53 pm

Tomás Mulcahy wrote:
Tim Gillett wrote:Practitioners have been adjusting pre decoder gain for a long time.
This is (another) good example of your selective reading. I did mention the technique in my first post... Perhaps I wasn't clear in the post you're quoting, so I'll clarify. The clever bit I was referring to in James's post is doing the transfer without decoding, and leaving the level adjustment, with decoding, to the Uhe plugin afterwards.

Doing a flat transfer and then noise reduction afterwards also allows you to go back and try a different type of noise reduction - I now have multiple decoders for Dbx and Dolby A so I can choose the one that works best. I think that there may be an alternative to Satin for Dolby B in the works too.
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Re: Digitising Cassettes

Postby ef37a » Fri Apr 10, 2020 2:59 pm

Tomás Mulcahy wrote:@ef37a FWIW I've found the TDK SX cassette shells wobbly even when I used them back in the early nineties at double speed in a four track. Sony UX-S were better, with the Maxell XL series being better again, and the XL-IIS seem to be the smoothest (although probably identical to XL-II).

Tim Gillett wrote:Practitioners have been adjusting pre decoder gain for a long time.
This is (another) good example of your selective reading. I did mention the technique in my first post... Perhaps I wasn't clear in the post you're quoting, so I'll clarify. The clever bit I was referring to in James's post is doing the transfer without decoding, and leaving the level adjustment, with decoding, to the Uhe plugin afterwards.

FYI these are TDK SA 90 and on the 'spine' of the shells is printed AHKJ217-36.

I never had any problem with TDK mechs back in the day nor Maxell but my machine was biased for SA and AD so that was my main buy. If I had a machine in with a tangled tape the cassette was invariably a cheap one.

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Re: Digitising Cassettes

Postby Tim Gillett » Fri Apr 10, 2020 3:38 pm

Tomás Mulcahy wrote:
Tim Gillett wrote:Practitioners have been adjusting pre decoder gain for a long time.
This is (another) good example of your selective reading.
We can all be selective at times. If you can show me where I've been selective I'd be grateful.

Tomás Mulcahy wrote: I did mention the technique in my first post...
Yes you seemed to refer to play trim. I've just read that post in detail. Glad you are aware of it and its importance in achieving Dolby tracking.

Tomás Mulcahy wrote: Perhaps I wasn't clear in the post you're quoting, so I'll clarify. The clever bit I was referring to in James's post is doing the transfer without decoding, and leaving the level adjustment, with decoding, to the Uhe plugin afterwards.

Sure, but again that's hardly new. I started doing this many years ago with an outboard hardware analog decoder (gain staging needs to be done well) and I'm sure others did it well before then.

The only difference here is using a software decoder rather than a hardware.
Software is easier but it still needs a "home" reference IMO. Whenever I used this technique I always recorded in the file a reference tone for Dolby level, 200 nWbm from the playback machine which itself was well calibrated. Even if the actual recording doesn't strictly line up with Dolby level tone, the transferred 400Hz tone is a baseline of sorts from which to branch out, gain up or gain down.

You're already familiar with the "play trim" principle. The input gain adjustment is important too as Hugh mentioned. The two together are IMO the standard tools for this. Both are often needed although often it's the "play trim" type of tool which is more decisive because as we know many cassettes are vulnerable to loss of highs. Knowing that, (I'm speaking generally here, not to anybody in particular) why persist in just adjusting overall input gain? It makes no sense. If the tracking problem seems mainly due to a loss of highs, try and correct the highs with EQ. Target the actual error. Don't simply adjust overall gain. It will only introduce another error. (Happy to explain that if needed). Glad you're aware of both adjustment methods.

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Re: Digitising Cassettes

Postby Tomás Mulcahy » Fri Apr 10, 2020 5:52 pm

:beamup:
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