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Author Topic: Dolby A Decoders  (Read 25276 times)

Offline astrotoy

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Dolby A Decoders
« on: August 28, 2008, 03:40:30 PM »
In the recent Irvington list, there are several 15ips 2 track for sale that are dolby A encoded. I have a dolby B decoder (Advent 100) but have not seen a dolby A decoder. When I googled it, I got pages of dolby digital and other decoders - but nothing on dolby A. Any suggestions if I want to buy a dloby A decoder?  Thanks, Larry
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Offline ironbut

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Re: Dolby A Decoders
« Reply #1 on: August 28, 2008, 05:37:30 PM »
Hi Larry ( having problems posting those pic BTW but never fear, I'll figure it out) , the only Dolby A decoders that I know of are the Pro audio ones from Dolby Labs. Model 361 with the cat 22 card is a mono unit so you'd need two. You can find them for very cheap (I got a couple for $50 a piece). There is also a model 301 but they're pretty hard to find. The Dolby A is a 4 band unit while the B is a 1 band system. The only drawback with these is that it's a balance unit but I seem to recall that you got one of Doc's Repro amps which has balanced (+4dB) outs and considering the rest of your system, I wouldn't doubt if you have a balanced input on your preamp. If you're interested in getting a pair, I can check with a guy that did have a bunch, does his own calibration and servicing of these things. Email me if you are.
steve koto
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Re: Dolby A Decoders
« Reply #2 on: August 28, 2008, 06:08:41 PM »
I seem to recall other postings here at TP on these Dolby units which indicated that they were intended for cassette use and were ill suited for rtr applications.
As a result I have not considered them for purchase.
What is the scoop fearless leader?

Offline U47

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Re: Dolby A Decoders
« Reply #3 on: August 28, 2008, 06:50:49 PM »
Dolby Labs were the only makers of the A type noise reduction. They came out with it in 1965 and Decca was one of their first customers. The first model produced was the A-301, which is an all discrete 2 channel encode/decode transformer coupled device. It is as large as a medium sized Rowland Stereo amp! Great sound, which the later 360 and 361 could not compete with for sonics. They were 1/10th the size of the 301. I've used the 301 for years and gave it back to Charlie(Stellavox) a few years ago- it is sorely missed. Dolby A was used on all<?> the Lyrita recordings and all the Decca classical LP recordings since 1966. Dolby B units made by Dolby Labs(London) were the 330 and 334. One was for Dolby FM and the other for cassette. Their appearance is almost identical to the 360- slimline 1 rack unit pieces.

Rich Brown
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Offline ironbut

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Re: Dolby A Decoders
« Reply #4 on: August 28, 2008, 08:02:30 PM »
The Dolby "B" type decoders/encoders were a simplified version of the "A". Like I told Larry, the "A" has 4 bands of processing and the "B" has only one (centered around the frequencies that tape hiss resides). Dolby "A" was intended for use in a studio environment in part of the recording chain to reduce noise by compressing and attenuating certain frequencies (encode) , then expanding and boosting those frequencies on playback (decode). All this is horrendously complicated and requires tape to tape calibration and very close component tolerances and matching. The "B" type is much simpler (but still has a very high component count) and the introduction of standardized Dolby B chips went a long way to keeping the home units working. In fact, if there was a strength to Dolby B, it  was pretty universal adherence to the Standard. What this means to us is that whatever the medium, cassette, open reel, or even digital storage devices which Dolby B encoded material may be directly transfered to, when played back, the Dolby B decoder should treat them the same way.
Well that's the theory anyway. The reality of the situation is that of all the units I've tried (I still have about 10) I'm not happy with any of them.
In a way, Charles was right. Dolby B seems to be fine for many cassette fans but I don't believe that cassettes have as much to loose as reel to reel. With their narrow track width and slow tape speed, there just isn't as much real estate that gets imprinted.
Having said that, I wouldn't give up hope on a decent sounding Dolby B decoder and Dolby A is inherently better (but source material is rare ).
steve koto
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Offline astrotoy

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Re: Dolby A Decoders
« Reply #5 on: August 28, 2008, 11:20:43 PM »
Thanks fellows for all the information. I'll PM Steve on the possibilities. Right now I don't have any dolby A tapes. What I think is that I may be able to get 10 or 20 maximum so I have to judge whether it is worth the hassle of getting a pair of 301's or just not bother about those tapes. I do have more stuff to listen to than time before I kick the proverbial bucket.   Larry
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Offline intell

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Re: Dolby A Decoders
« Reply #6 on: August 29, 2008, 04:49:40 PM »
Here is a photo of cat.22 module PCB.


Notice a quantity of yellow drop-tantalium capacitor - not a best solution for good sonics.
Best regards! Anton

Offline ironbut

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Re: Dolby A Decoders
« Reply #7 on: August 29, 2008, 05:22:14 PM »
Hi Anton, that is just one of the unfortunate realities of the Dolby Labs units. The reason the tant's were chosen was for stability (from what I gather). You'll find that many of the components were chosen just for that reason. Supposedly, the components had to be ordered with strict tolerances in materials that were resistant to drifting values. Just think about the complexity of splitting the music into 4 frequency bands, sending it all through that board and then reassembling it into the full spectrum at the other end. It's a wonder they ever got the prototype to work. All that circuitry and that's one channel folks. These cards had to be as close to identical as possible since a studio might send the encoded recording to another facility for production of mastering. And with Dolby B, the model 330 was for high speed duplication of cassettes so it needed to encode the signal so that every home/car/radio station would get the same product for decoding.
Even though the pro audio designers that I've spoken to about Dolby NR seem to agree that it was a bad choice (fidelity wise) to adopt the Dolby standard, I can't help but admire the team that made this work.
steve koto
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Offline swissavox

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Re: Dolby A Decoders
« Reply #8 on: November 19, 2008, 09:51:18 AM »
For what it is worth, just a few personal thoughts from my own limited experience of Dolby A and the 361 series unit in particular.

I think it is fair to say that any form of processing will be a balance of good and bad. Dolby A was the first virtually transparent form of audio noise reduction, and probably single handed gave rise to the (once) modern multitrack recording industry. Sure there were other and rather later devices from people such as DBX and Telex (their telcom system even plugged into a 361 frame) but they were thought at the time to be less transparent or have fairly obvious artefacts.

The problem, was that it was becoming increasingly obvious that the cumulative build up of noise (both in decrease of S/N ratio and in the increase in modulation noise) was making the newly developed multitrack tape formats of the early 1970?s almost unusable. But Ray Dolby?s new system almost by chance made multutrack workable enough to become the new way of manufacturing popular music. No longer need a group of musicians come together to make a recording, but various bits of a ?track? could be stuck together and at various times like the components of a motor car to make complete the end ?product?. Backing track, lead and vocals all recorded at different times and sometimes even on different continents could be glued together (during the reduction as it was known) to make a seamless whole. And all this mostly because of Dolby A, which was probably the most significant innovation in the audio business since the invention of the tape machine itself. Later the different though still analogue Dolby SR system was developed to combat the then up and coming digital formats. By then though virtually all analogue master tapes would have been Dolby A encoded, and every disk cutting suite would have had a pair of 361s in the playback mastering chain.

Now while U47 is of course correct about Decca using the first A-301 systems they did however change over to their own digital mastering system (for classical music) in the late 1970s. Interesting that the 361/Cat 22 is thought to be a step down in sonics to the 301, certainly when I was a trainee recording engineer (tape op) in a minor London studio in the early 70?s, racks of 361s working with Ampex or Studer machines were thought to be the last word in quality back then, and were to become the de-facto standard for years to come. I do however remember that Angus Mackenzie wrote in his review of the 301 that one could by removing certain boards use it to clean up old 78 recordings(!). Which you can?t do that with a Cat 22 module. 

The fully interchangeable Cat 22 modules in their 1U 361 rack units were a much more convenient package than the first Dolby 301s once it became obvious that the multitrack studio was to be Dolby A?s true home. Partly because they were much smaller (though surprisingly heavy) but more importantly that they had remote control of record and replay switching, so that a single signal from say the tape machine?s control panel could switch a whole bank of (8, 16, 24) Dolbys from record to replay mode. The Dolby 301 units contained a stereo pair of record/replay channels (and separate boards for the 4 individually processed frequency bands) while the 361 was an integrated single channel of record and replay processing with one Cat 22 module. Later, Cat 22 modules would be fitted to even more compact multiple rack mounts. I am not sure how many of those Tant beads were used in the signal lines as the circuit was pretty secret but naturally they may have had some detrimental effect. (I found that the much more expensive metal cased tantalums could sound even worse). But of course professional audio equipment was never designed as hi-fi equipment in the first place, it just had to work reliably day in and day out.

My own personal experience of the A-361 system was with a pair bought from the BBC some years ago. Once used in their Mada Vale studios, I used them with both my Stellavox SU8 and an Akai GX77 for mostly recording Jazz FM  broadcasts from a Yamaha CT7000 tuner. Interfacing was a pain as they had balanced (of course) ins and outs, no gain controls (only fine trimmers) and needed a healthy input signal at something like + 4dBm. They were of after all designed to work from the output of a professional mixing desk. Once set up and working though the most obvious effect was a considerable reduction of tape noise with none of the treble and transient  problems of Dolby B (or C), but there were other more subtle effects in that these units reduced crosstalk, modulation noise and made the recordings (even at 3 ? IPS) sound cleaner and ?nicer?. While of course daft, taped Dolby A recordings of Lps seemed to sound rather better than the originals?

I don?t use them now for in this world of exotic cables and connectors having a couple of transformer coupled audio processors in the signal path seems to be a bit of a problem philosophically. And anyway for simple ? track recording with a slight amount of tape noise seems not a problem.

Sorry to go on.
 
Gino Mancini Hi-Fi enthusiast and modest collector. Marantz CD12/DA12, Yamaha CT7000, Audio Research LS7, Quad II (rebuilt), Quad ELS57 + Townsend Ribbons, Nagra SNN, Stellavox SU8, Akai GX77, Ferrograph Logic7, Nagra T-Audio, Decca PCM system.

Offline GabrieleP

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Re: Dolby A Decoders
« Reply #9 on: September 15, 2012, 01:05:56 AM »
Hello everybody!
OK, TPF tells me it's been 120 days since the last post but I thought I might better keep this thread going and not start yet another new one...
My 1st post here at the forum, I am a sound engineer.
I've been learning about the Dolby A301 old trick that recording engineers did: pulling out cards to disengage compression of certan frequency bands when in record mode in order to get "sonic benefits".
I have been offered a Dolby B 320, which I understand is a totally different beast, but since it's so cheap I decided to give it a go.
Question: is the Dolby B 320 working at pro level, +4 dB?
Thank you in advance for your contribution.
Regards, Gabe

Offline ironbut

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Re: Dolby A Decoders
« Reply #10 on: September 15, 2012, 12:08:20 PM »
Hi Gabe,
I think you'll find that all of the Dolby Labs products were balanced +4dB input/output.
If you're worried about your source having enough voltage to drive the unit to calibration levels, most solid state components will but it can be pretty close sometimes. You just need to try.
I'm not personally familiar with the 320 and I've only seen pictures. Let us know how it goes!
steve koto
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Offline c1ferrari

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Re: Dolby A Decoders
« Reply #11 on: September 15, 2012, 10:40:28 PM »
There's also a Dolby Model 363 -- two-channel SR/A processor (Cat 300 modules).
Sam Lucero
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:-)

Offline ironbut

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Re: Dolby A Decoders
« Reply #12 on: September 15, 2012, 11:39:58 PM »
Hey Sam,

There have been a dozen or so different Dolby Labs analog processors. Some are just variations of well known models such as the 334 which is just a broadcast version of the 330 while others are specific to cinema work (talking pre Dolby Digital of course).
All of them that I've worked on are incredibly durable and overbuilt. Out of the 20 odd units I've bought and/or worked on, only one was DOA and it looked like it had been run over by a truck!
Of course, they were, and still are, (I think the 442 is still in production) pretty darn expensive.
steve koto
 Sony scd 777es(R. Kern mods)> Vpi Aires>Dynavector XX-2mkll>Bent mu>CAT ultimate>CJ premeir 140>Magnepan 1.6qr(Jensen xover)Headphone Eddie Current Zana Deux>AT ad2000,HD800 ,Metric Halo ULN-2 (battery powered),
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Offline GabrieleP

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Re: Dolby A Decoders
« Reply #13 on: September 16, 2012, 12:44:30 AM »
Steve,
my concern is about operating level mismatch... since signal level is what Dolby machines look at to perform well their task.
I guess I'll find out in a few days when I'll have the chance to try out the specific piece of gear.
Thanks for now!

Offline c1ferrari

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Re: Dolby A Decoders
« Reply #14 on: September 16, 2012, 12:37:08 PM »
For what it is worth, just a few personal thoughts from my own limited experience of Dolby A and the 361 series unit in particular.

I think it is fair to say that any form of processing will be a balance of good and bad. Dolby A was the first virtually transparent form of audio noise reduction, and probably single handed gave rise to the (once) modern multitrack recording industry. Sure there were other and rather later devices from people such as DBX and Telex (their telcom system even plugged into a 361 frame) but they were thought at the time to be less transparent or have fairly obvious artefacts.

The problem, was that it was becoming increasingly obvious that the cumulative build up of noise (both in decrease of S/N ratio and in the increase in modulation noise) was making the newly developed multitrack tape formats of the early 1970?s almost unusable. But Ray Dolby?s new system almost by chance made multutrack workable enough to become the new way of manufacturing popular music. No longer need a group of musicians come together to make a recording, but various bits of a ?track? could be stuck together and at various times like the components of a motor car to make complete the end ?product?. Backing track, lead and vocals all recorded at different times and sometimes even on different continents could be glued together (during the reduction as it was known) to make a seamless whole. And all this mostly because of Dolby A, which was probably the most significant innovation in the audio business since the invention of the tape machine itself. Later the different though still analogue Dolby SR system was developed to combat the then up and coming digital formats. By then though virtually all analogue master tapes would have been Dolby A encoded, and every disk cutting suite would have had a pair of 361s in the playback mastering chain.

Now while U47 is of course correct about Decca using the first A-301 systems they did however change over to their own digital mastering system (for classical music) in the late 1970s. Interesting that the 361/Cat 22 is thought to be a step down in sonics to the 301, certainly when I was a trainee recording engineer (tape op) in a minor London studio in the early 70?s, racks of 361s working with Ampex or Studer machines were thought to be the last word in quality back then, and were to become the de-facto standard for years to come. I do however remember that Angus Mackenzie wrote in his review of the 301 that one could by removing certain boards use it to clean up old 78 recordings(!). Which you can?t do that with a Cat 22 module. 

The fully interchangeable Cat 22 modules in their 1U 361 rack units were a much more convenient package than the first Dolby 301s once it became obvious that the multitrack studio was to be Dolby A?s true home. Partly because they were much smaller (though surprisingly heavy) but more importantly that they had remote control of record and replay switching, so that a single signal from say the tape machine?s control panel could switch a whole bank of (8, 16, 24) Dolbys from record to replay mode. The Dolby 301 units contained a stereo pair of record/replay channels (and separate boards for the 4 individually processed frequency bands) while the 361 was an integrated single channel of record and replay processing with one Cat 22 module. Later, Cat 22 modules would be fitted to even more compact multiple rack mounts. I am not sure how many of those Tant beads were used in the signal lines as the circuit was pretty secret but naturally they may have had some detrimental effect. (I found that the much more expensive metal cased tantalums could sound even worse). But of course professional audio equipment was never designed as hi-fi equipment in the first place, it just had to work reliably day in and day out.

My own personal experience of the A-361 system was with a pair bought from the BBC some years ago. Once used in their Mada Vale studios, I used them with both my Stellavox SU8 and an Akai GX77 for mostly recording Jazz FM  broadcasts from a Yamaha CT7000 tuner. Interfacing was a pain as they had balanced (of course) ins and outs, no gain controls (only fine trimmers) and needed a healthy input signal at something like + 4dBm. They were of after all designed to work from the output of a professional mixing desk. Once set up and working though the most obvious effect was a considerable reduction of tape noise with none of the treble and transient  problems of Dolby B (or C), but there were other more subtle effects in that these units reduced crosstalk, modulation noise and made the recordings (even at 3 ? IPS) sound cleaner and ?nicer?. While of course daft, taped Dolby A recordings of Lps seemed to sound rather better than the originals?

I don?t use them now for in this world of exotic cables and connectors having a couple of transformer coupled audio processors in the signal path seems to be a bit of a problem philosophically. And anyway for simple ? track recording with a slight amount of tape noise seems not a problem.

Sorry to go on.
 

WHAT?  Are you kidding ;-)
That was wonderful and I loved reading about it!

I have a Dolby 363 with Cat 300 modules to repro/decode Dolby A reels (I don't envision recording with A or SR, at this time).  I believe there exist, at least, a few mods for this unit.  Would you or anyone have a recommendation for the "best-sounding" Dolby A decoding unit?

Unfortunately, it would appear few tapes have the proper "Dolby tone" for A decoding (I believe it's referred to as "noise" for SR decoding) :-(
Anyone have a work-around for correct Dolby A decoding absent Dolby tone?
Thanks :-)
Sam Lucero
SPQCV
:-)