Limiters & VU meters

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
ricardo
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Re: Limiters & VU meters

Post by ricardo »

http://www.thatcorp.com/datashts/dn03.pdf

The stuff here is EXACTLY what you want. Get your EE friend to study it carefully and configure it for your application including single supply. All the info is there.

The circuit you got from THAT also needs to be configured but there are no equations to help you do that.

You need to work out the GAIN STRUCTURE for your application so the limiter can work properly. This means deciding the operating levels and gains at the output of your 1512 so that it doesn't overload before the Limiter gets a chance to do its stuff.

Also if you want release Time Constant to be different from attack TC. These choices need to be made with a built up device and listening tests.

JR, when you've done Limiters, do you have different TCs for attack & release? Compressor/Limiter (& EQ) design was considered a Black Art at Calrec. Clem, my mike mentor, was considered da man for this.
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Carl, Vref in your circuit from THAT is our 1/2 Vcc
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JR.
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Re: Limiters & VU meters

Post by JR. »

ricardo wrote:
JR, when you've done Limiters, do you have different TCs for attack & release? Compressor/Limiter (& EQ) design was considered a Black Art at Calrec. Clem, my mike mentor, was considered da man for this.
I don't wan't to create too much of a veer but yes... Pure limiters are generally fast attack and relatively fast release, but the release is somewhat slower than the attack, compression is generally slower and adjustable both attack and release times. Not only slow release but even adjustable "hold" times when nothing happens and even rate dependent attack/release based on how far the current VCA gain is from the current signal level. Some of the best dynamic processing work I did with smart side-chains was for tape noise reductions back in the late '70s. When I made 2:1 compandors that tracked well and were transparent. But I could not compete with DBX NRs made in Japan for less than the cost of my unassembled kits. :oops:

One issue I was already running into with comp/limiters is too many adjustable controls for typical operators. Things like the one-knob DBX were very successful but far from an optimal solution.

Some day I may finish a computer/smart phone interface so I can make a comp/limiter with all 100 adjustments available, but only if the customer wants to tweak. :lol:

JR
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carlmart
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Re: Limiters & VU meters

Post by carlmart »

Isn't a circuit with four setup trimmers per channel, just on the limiter, a bit impractical for a commercial product or even for customer personalization?
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JR.
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Re: Limiters & VU meters

Post by JR. »

carlmart wrote:
Isn't a circuit with four setup trimmers per channel, just on the limiter, a bit impractical for a commercial product or even for customer personalization?
It's an application note, not a production schematic. Application notes show you what can be done.... I wouldn't complain too loudly about the free design advice.

=====
I found that non-linear capacitor (figure 6) strangely familiar... :lol: I did the same thing in one of my old tape NR compander kits back in the early '80s, but I did a few more tricks in addition.

Image

More recently I posted a variant on how to make the capacitor variable.

Image

I never bench tested the later scribble so it is not a certified proved design, but looks like it should work perhaps with some value tweaking. Sorry that it uses an adjustment pot, but how else can you vary the variable capacitor?

JR
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carlmart
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Re: Limiters & VU meters

Post by carlmart »

Far from me to complain about free design advice. It would be quite hypocritic and suicidal to do so. :D

One trimmer it's always necessary. I complained about having four! Per channel!
ricardo
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Re: Limiters & VU meters

Post by ricardo »

carlmart wrote:
Isn't a circuit with four setup trimmers per channel, just on the limiter, a bit impractical for a commercial product or even for customer personalization?
The Design Note is adjustable for different amps/power level/desired THD etc.

You or your EE friend need to study DN03 and work out for YOUR GAIN STRUCTURE and levels, what needs to be done using the eqns. THAT so thoughtfully provide.

But for what you want (ENG), the final product should have NO twiddle pots .. unless you make a mess of working out YOUR GAIN STRUCTURE & levels.
______________

JR, NE572 brings back happy (?!) memories. I tried to make one emulate a DBX 122 (?) but ended up building a couple of Dolby B's instead. There was a John Vanderkooy article in Wireless World with a much more complicated version.

I still have some Dolby B master tapes but sadly no working Dolby B decoders.

Anyone know if there is a software Dolby B decoder?
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JR.
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Re: Limiters & VU meters

Post by JR. »

ricardo wrote:
I still have some Dolby B master tapes but sadly no working Dolby B decoders.

Anyone know if there is a software Dolby B decoder?
When I was working at Peavey we were a Dolby licensee to manage the NR in the 4 track cassette deck I had to engineer.

The Dolby decoders were typically built into chipsets.

You could probably roll your own but I don't recall all the curves... it was a below threshold 1-2 expander mainly operating on HF and limited to only X dB of NR.

FWIW I was kind of impressed by the rigor that Dolby exercised over their licensees to get it right. As I recall one critical part about dolby tracking was getting the playback levels off tape accurate. We had to use certified reference tapes to set up the decks for production.

JR
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ricardo
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Re: Limiters & VU meters

Post by ricardo »

JR. wrote:
ricardo wrote:Anyone know if there is a software Dolby B decoder?
The Dolby decoders were typically built into chipsets.

You could probably roll your own but I don't recall all the curves... it was a below threshold 1-2 expander mainly operating on HF and limited to only X dB of NR.

FWIW I was kind of impressed by the rigor that Dolby exercised over their licensees to get it right. As I recall one critical part about dolby tracking was getting the playback levels off tape accurate. We had to use certified reference tapes to set up the decks for production.
We were Dolby licencees too.

R&D had a number of calibration tapes and we used to log their use .. especially the 10kHz ones. As we did for the vinyl Test Records.

I think the Hitachi chipset was the best.

Alas, only a few expensive cassette recorders allowed user adjustments for tape sensitivity .. let alone HF response. I used to do 'factory' adjustments for friend's cassette decks and their specific favourite cassette tapes. Dem wer de days :roll:
______________________

Carl, to re-iterate, you might need trimpots on your prototype but once you have tested it and selected the values that sound right, your production item should NOT need any trims (unless you need 1ppzillion THD).

The THAT solutions are far more repeatable than stuff using Vactrols etc.
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Re: Limiters & VU meters

Post by emrr »

I'd use the non-linear capacitor option for this purpose, and I think I'd avoid using the feedback path in favor of feed-forward side chain as seen in most THAT app notes. Set it high ratio, give yourself a threshold knob, maybe a make-up gain knob and you should be in good shape.
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JR.
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Re: Limiters & VU meters

Post by JR. »

ricardo wrote:
JR. wrote:
ricardo wrote:Anyone know if there is a software Dolby B decoder?
The Dolby decoders were typically built into chipsets.

You could probably roll your own but I don't recall all the curves... it was a below threshold 1-2 expander mainly operating on HF and limited to only X dB of NR.

FWIW I was kind of impressed by the rigor that Dolby exercised over their licensees to get it right. As I recall one critical part about dolby tracking was getting the playback levels off tape accurate. We had to use certified reference tapes to set up the decks for production.
We were Dolby licencees too.

R&D had a number of calibration tapes and we used to log their use .. especially the 10kHz ones. As we did for the vinyl Test Records.

I think the Hitachi chipset was the best.

Alas, only a few expensive cassette recorders allowed user adjustments for tape sensitivity .. let alone HF response. I used to do 'factory' adjustments for friend's cassette decks and their specific favourite cassette tapes. Dem wer de days :roll:
______________________

Carl, to re-iterate, you might need trimpots on your prototype but once you have tested it and selected the values that sound right, your production item should NOT need any trims (unless you need 1ppzillion THD).

The THAT solutions are far more repeatable than stuff using Vactrols etc.
Funny speaking of adjustments.... while working at Peavey we went to great lengths to avoid factory set trimpots. The power amp division even tightly specified the power transistor Vbe and used a tightly specified diode pair to set the class A bias for the class A/B amp stage without a trim. That 4 track cassette deck that I pretty much inherited as an almost finished ready to build SKU, and ended up redesigning almost every single internal PCB, was a basket case/poster boy for too many factory set adjustment. After I got the machine working well enough to sell, my next battle was building them fast enough. We ended up doubling the final test station with a complete second set of expensive test gear and test technician. It was still hard to get more than a few out a day from each tech. I have forgotten all the many trims or adjustments but remember that each of these was 4x. Besides the obvious need for accurate record/playback levels for Dolby NR tracking, another quirk of the Dolby NR and facility of the 4track to do over-dubs or print to one track at a time while listening to the other tracks, the play back path required hand trimmed bias traps to suppress bias noise that could cause the NR to mis-track.

Lots of adjustments and not trivial stuff, Had I done the original design I might have been able to engineer out some of the trims but the engineer before me and I use the term engineer loosely, just cobbled together a bunch of typical cassette deck circuits, then added to them to make them work (mostly). The packaging was a nightmare too, I recall one connector rep commenting that I had engineered out several tens of dollars of connectors from the design when I repackaged it. It was still a money loser due to all the technician time to build them... But it was an important product for the fledgling AMR recording division so Peavey shipped some money with every unit for it's lifetime.

JR

PS: IIRC we used a Hitachi Dolby chip... as i recall the entire recorder audio path was integrated into the IC.
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