Can a Dolby emphasis curve be constructed passively?

[xcortes]

I know that the Dolby NR system curve varies according to the signal level and to get it 100% correct you would need a Dolby processor or chip but what are the thoughts of emulating the curve passively (or actively modifing a tube preamp)? Could it be done? It would be a nice way of listening to Dolby tapes without old SS Dolby units.

Xavier Cortes

[stellavox]

I don't recommend it.

Let's review the "single band" Dolby B process, which I believe you are referring to.  The encode process continuously monitors the high frequency (HF) music content above 1 Khz or so and boosts this level accordingly. No HF music content - up to 10dB of boost: heavy HF music content - no boost.

The decode process performs a complementary level-dependent cut.

Per your suggestion, if you incorporated a "full time" passive HF rolloff in your preamp it would roll off all the HF all the time.  Music with low level HF content might sound OK but music with a high HF content would sound "dull?"

All Dolby B encoders/decoders had to use the same licensed circuitry.  At first discreet components were used but then National Semiconductor and others developed an IC that could do most of the work and simplified the parts count significantly.   

As you know, many manufacturers offered add-on Dolby "boxes" with the Advent units probably the most prevalent.  I've worked with a number of these units and found that from a sonic perspective, they are typically compromised by "less than adequate" coupling capacitors and unregulated power supplies.

My own DIY solution was to "rob" a Dolby B board scavenged from a Tandberg cassette deck and mount it in a little minibox with a regulated power supply fed from a "wall wart".  Tandberg used quality components throughout and this unit seems to offer minimum sonic degredation.

Charles

PS:

Of course if you are a TRUE master dub "casualty", it won't be too long before you'll also need to add a Dolby A decoder to your arsenal, as many labels used this method of noise reduction in the '70s.  Dolby A used a 4 frequency band encoding/decoding scheme and on their Model 301 it is possible to switch out all but band 4 creating a "more versatile pseudo-B" unit.  Haven't figured out how to do this with the subsequent generation Cat 22 cards yet..

 

[xcortes]

You got my idea. I know that using a "full time" HF roll off could cause some tapes to sound dull. I've read, however, that some people consider that you can listen to Dolby-B tapes without a decoder. I was hoping there could be a middle point. Maybe a variable circuit where you can select the level of attenuation and adjust it depending on the tape you're listening to?

I follow your DIY point. Typically what would be the size of a Dolby B board? What voltage does yours require?

Thanks,

xavier

[stellavox]

Xavier,

Board measures about 3" X 5".  Wall wart puts out 12 VDC so regulated input to board must be around 9V.  Another reason I chose the Tandberg is that the modules all plug into a mother board so removing the Dolby portion took 2 seconds.  Most other cassette recorder manufacturers configured all the electronic circuitry on one or two large boards.  If you're up to it the Dolby portion can be "excised" with an Exacto knife.  Picture attached...

 

[xcortes]

I took the jump and bought a "for parts" Tandberg deck. I also ordered the manual in pdf to be sure.

As Charles said the dolby is an independent board. It is discrete.

Here's the schematic and the board. Also if anyone is interested I can send the pdf.

I bet PJ could design a tube one if there was a market. Maybe a FP3 modification?

[ironbut]

Nice work guys! And welcome back Charles.
At one point I tried to isolate the Dolby circuit in an old Yamaha deck I had. Like Charles said, it had a single board for the audio and another one for the power supply. I gave up after a week or so because the more I looked, the more intertwined the different circuits were.
That's great that you've found that schematic Xavier. It actually looks pretty simple.

[Ben]

Umm I want tubes ... :)
Just thinking here, more crap in your audio path. :(

[ironbut]

I'm with you on that one Ben! You can't imagine how complex some of these things are (or maybe you can). I've heard it from more than one audio designer that the industries decision to go with Dolby was a bad one (I guess there were other less complex choices out there at the time).Now that audio has gotten to the point where it's sounding pretty good, (again!) anyone who wishes to unlock the tapes that are being held hostage by Dolby B have got a tough row ta hoe (I've got a couple of hundred). They sound pretty darn good without decoding but I just have to find out if a decoder can be made to sound decent.

[Ben]

I can't see how dolby-decoding works in the the first place with out delaying the audio while you figure out the gain for that signal main level. I guess nobody planned to play latin stuff with a lot of complex sounds.
 

[ironbut]

Right you are Ben. There's a 27μ sec delay from end to end (at least on the pro version). The delay is global which prevents any smearing of the signal (in theory) but it does add an extra element to the mix. Doing some mods ( eliminating circuitry) could result in phase issues making the sound worse (don't ask me how I know this).
 I made a little progress on the unit I'm fooling with and need to get some parts now. The next thing to work on is converting it from a balanced (+4dB) in/out to a single ended (-10 dB). That will eliminate 2 transformers (and some circuitry that support the trannies) per channel. Anybody reading this who've done this sort of thing, please PM or email me 'cause I haven't!

[xcortes]

In the meantime I purchased a Nakamichi NR-200 unit. Nak decks from the 70s are very well regarded. They have a cult like almost religious group of followers. I've never heard one but they must do something good. The NR-200 was produced in the early 80s. Considering that the celebrated series 700 and 1,000 had Dolby units incorporated in them one would expect the NR-200 to be at least a very decent unit.

[ironbut]

Xavier, let us know how that one sounds. I tried to get one a year or two ago but it went for way too much (around $300 if I recall correctly). If it sounds great, I'd pay that much in a heartbeat though.

[intell]

stellavox, first thing you can do to imrove sonics of a tandberg dolby board - is a remove bias and MPX filter elements (l701,702 and related capacitors).
second thing- bypass all electrolytic capacitors with a 0.1mkf good polypropylene caps.
thrid thing - replace all ceramic caps (if board has it) by polypropylene.

all this things in complex can help to improve sonics very much.


PS sorry for my poor english.
Best regards.
Anton.

[xcortes]

Thanks Anton!

If that's OK I may get in touch with you when I get into modifying the module. In the meantime I have another idea.

[xcortes]

Back to the original question. I have another idea. Let's look at the Dolby B encoding characteristics chart:



What's interesting about this chart is that the dolby encoding effect is very similar to a baffle step effect that varies with level.

Let's focus on the lowest curve. We have a "baffle step" of 10 db centered around 2k. Paul Joppa has published a passive circuit to compensate for baffle diffraction step that does exactly what we want to achieve here (remember, we?re concentrated on the lowest curve now):

http://www.t-linespeakers.org/tech/bafflestep/passive-BSC.html

But as Charles pointed out before, a fixed compensation doesn't work because the level of the compensation is volume dependent. So what if we use a voltage controlled resistor in PJs circuit which resistance is set by the signal level? Maybe a FET could be used in this application?:

http://www.t-linespeakers.org/tech/bafflestep/passive-BSC.html

Still PJ's filter is only good for 6dB so we would have a 2 db deviation in the higher and upper frequencies at lower levels. Or maybe stack two circuits?

Add your favorite line stage (aka Foreplay 3) afterwards to get back your gain and you're set with a simple circuit that "decodes" Dolby B passing the signal only through a couple of resistors and caps.

Yes, very incipient and at this stage very theoretical but I think something can be achieved here that would be a good compromise between Ironbut's do-nothing technique and the complicated and color accretive Doby B decoders.