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Tact Dynamic Room Correction Question

post #1 of 32
Thread Starter 
Hey everyone,

I guess I am going to show my ignorance here, but I have a question about the Tact Dynamic Room Correction functionality and I would assume this is the forum where people are most likely to be able to answer my question.

If I understand correctly, the Dynamic Room Correction ability allows the room correction adjustment to take account of the volume of the music being played in real time, and therefor account for the fact that humans perceive frequencies differently at different levels. The Tact website is very clear about their ability to apply the appropriate level of correction for each 0.1 dB volume change.

If I understand these units correctly, they perform analysis of the in-room response during initial set up and then apply the correction in the digital domain.

Now, if the unit has a DAC installed and is acting as a preamp with a volume control then I can understand how the Tact unit is aware of the volume of music being played and can make the level-dependent correction.

What I do not understand is if the Tact unit is being used with an outboard DAC (as mentioned in the user's manual) and therefore Volume control is downstream in the analog domain, how does the Tact unit know what the volume of the music is and therefore what level-dependent correction to apply? After initial setup, there is no mic attached. However, the manual does not imply there is any sacrificing of functionality if an outboard DAC is used.

If someone could help me see blindingly obvious, I would appreciate it. And please - in the kind of terms that a non-rocket scientist can understand.

Kind regards,

Ash
post #2 of 32
Well, if that's a dumb question, then I'm dumb too. I'd definitely like to know how they accomplish it.
post #3 of 32
Just got back from Dallas where I heard my audiophile buddies room and TacT equalized. He uses an ARC Ref 3 into the Tact (bypassing the preamp portion of the Tact) and then to his amps
post #4 of 32
Quote:
Originally Posted by skirmash View Post

...how does the Tact unit know what the volume of the music is and therefore what level-dependent correction to apply?

During initial calibration, each channel is set to reference level (like calibrating any pre-pro or receiver), zeroing out the volume knob in the process. After that, whenever you change the volume level, the Tact unit can see how far you're offset from reference (how many dB away from the zero setting) and apply the appropriate amount of correction.

Sanjay
post #5 of 32
Quote:
Originally Posted by skirmash View Post

What I do not understand is if the Tact unit is being used with an outboard DAC (as mentioned in the user's manual) and therefore Volume control is downstream in the analog domain, how does the Tact unit know what the volume of the music is and therefore what level-dependent correction to apply? After initial setup, there is no mic attached. However, the manual does not imply there is any sacrificing of functionality if an outboard DAC is used.

A mic?

I think this would be a great question to e-mail TACT. Then come back and tell us the answer.
post #6 of 32
Thread Starter 
Quote:
Originally Posted by sdurani View Post

During initial calibration, each channel is set to reference level (like calibrating any pre-pro or receiver), zeroing out the volume knob in the process. After that, whenever you change the volume level, the Tact unit can see how far you're offset from reference (how many dB away from the zero setting) and apply the appropriate amount of correction.

Sanjay

Hey Sanjay,

I understand this to be true when the Tact unit is itself acting as the preamp. The Tact unit would know the offset from reference and be able to apply the appropriate level-dependent correction.

However, my question relates to when an outboard DAC and downstream volume control is being used. I did not understand how the Tact unit would be aware of the offset from reference.

I said "did" in the previous paragraph because I think Oneobgyn may have provided the answer. It is my belief (I really don't know for sure until I email Tact) that when they say outboard DAC, they mean an amplifier that can take a digital input. This would include some of the Tact amps and I believe Goldmund has some amps that take a digital feed too.

Such an implementation would still put the Tact in control of the volume (presumably in the digital domain) and therefore able to apply the appropriate level-dependent correction. I will attempt to ascertain the validity of this idea directly with Tact.

Presumably, with enough bits, there is no reason why a digital volume control should not sacrifice quality?

Thanks for the responses. I don't feel so dumb now.

Kind regards,

Ash

PS. Queue .. I was being lazy and typed Mic instead of microphone.
post #7 of 32
I don't believe that Tact is doing volume-dependent correction at all. They used ordinary FIR filters, which by themselves would not allow this because they can only process signals linearly. The psychoacoustic component which Tact refers to is I think simply the use of coarser frequency resolution at higher frequencies. This makes correction more logarithmic in frequency, which is the way we perceive sound. Also, their FIR filter coefficient calculation algorithm perhaps uses psychoacoustic-based averaging, like Audyssey does.

As far as I know, Tact uses 3 frequency ranges, which have different sample rates: 3 kHz for low frequencies, 12 kHz for medium, and 96 kHz for high. This sort of multi-rate FIR filtering is pretty common these days, as it has the very practical consequence of limiting the filters to reasonable lengths.

Regards,
Terry
post #8 of 32
Terry,
What they do is employ a different FIR filter at different loudness levels.
post #9 of 32
Quote:
Originally Posted by skirmash View Post

Such an implementation would still put the Tact in control of the volume (presumably in the digital domain) and therefore able to apply the appropriate level-dependent correction.

That would be the only way they could do volume dependent correction. It's also how the better pre-pros handle Loudness Compensation and how Audyssey handles their (upcoming) dynamic EQ.

Sanjay
post #10 of 32
Quote:
Originally Posted by Michael Grant View Post

Terry,
What they do is employ a different FIR filter at different loudness levels.

Okay, so then the nasty bit would be how to interpolate/transition between two filters without audible artifacts.

Regards,
Terry
post #11 of 32
I doubt it's that big of a deal, since the filter is altered only when the volume is changed.
post #12 of 32
Here is their marketroid speak on it:

http://www.tactlab.com/Resources/Dow...Correction.pdf
post #13 of 32
Okay, I get it now, Michael. Loudness is what is set on the volume control, not a changing property of the sound. Thanks!

- Terry
post #14 of 32
Quote:
Originally Posted by skirmash View Post

What I do not understand is if the Tact unit is being used with an outboard DAC (as mentioned in the user's manual) and therefore Volume control is downstream in the analog domain, how does the Tact unit know what the volume of the music is and therefore what level-dependent correction to apply?

Ash,

If you want to use the Dynamic Room Correction feature - then you HAVE to use the
Tact's volume control as the volume control for the system. The unit keys off the
position of its own volume control.

Therefore, volume controls that precede or follow the Tact can't be used with DRC.
post #15 of 32
Quote:
Originally Posted by Terry Montlick View Post

Okay, so then the nasty bit would be how to interpolate/transition between two filters without audible artifacts.

Terry,

That is EXACTLY what the Tact does - it interpolates between the filters.

http://www.tactlab.com/Resources/Dow...Correction.pdf
post #16 of 32
Yes, but when Terry said that he was assuming that was being done dynamically as the music rose and fell in loudness---even with the volume level set at constant. Thankfully the TacT is not doing anything that complicated; the filter is determined solely by the master volume control position of the TacT.
post #17 of 32
Quote:
Originally Posted by Michael Grant View Post

Here is their marketroid speak on it:

http://www.tactlab.com/Resources/Dow...Correction.pdf

That reads like the "dynamic" part is just loudness compensation, which has been done on pre-pros and receivers for years. Is that the dynamic correction that Tact is doing or are they actually changing other parts of their room correction based on volume?

Sanjay
post #18 of 32
Quote:
Originally Posted by sdurani View Post

That reads like the "dynamic" part is just loudness compensation, which has been done on pre-pros and receivers for years. Is that the dynamic correction that Tact is doing or are they actually changing other parts of their room correction based on volume?

Sanjay

It is basically just a fancy "loudness button" Sanjay, but one which is continuous and slaved to the volume control of the Tact. The word "dynamic" is nice marketing lingo, but had me confused as well.

Regards,
Terry
post #19 of 32
Agreed. What they call "dynamic" is just that they compute the new filter and insert it into the signal chain without having to bring the system down into calibration mode.
post #20 of 32
I was just at Tact the other day, and Boz gave me a comprehensive and fascinating demo of the system, taking me through all of the software.

As noted above regarding loudness compensation, it does interpolate between different loudness curves.

The software will show five loudness correction curves across the frequency spectrum, each of these for a given volume level. Any volume level set that falls between two curves, will yield some combination of these two curves depending on where the level is relative to each. 'Dynamic' can be a bit of a misnomer as -- there are no changes on the fly w/the music which would essentially be taking the recording mixer out of the equation. It is dynamic in that the profile of the loudness compensation is specific to the volume set and adjusts in very small increments.

I'm a big fan of the concept and execution. I've missed the old loudness controls as I'm almost always wanting more bass presence at lower volumes.
post #21 of 32
Quote:
Originally Posted by Michael Grant View Post

Yes, but when Terry said that he was assuming that was being done dynamically as the music rose and fell in loudness---even with the volume level set at constant. Thankfully the TacT is not doing anything that complicated; the filter is determined solely by the master volume control position of the TacT.

Michael,

Yes - I didn't realize that he was thinking about the loudness of the signal rather than
the position of the volume control when I wrote that post.

Sometimes I should read the entire thread through before I respond, rather than responding
as I go. You will also note that I cited the same TacT URL that you did. If I had read on,
I need not have reposted something you already posted.
post #22 of 32
Quote:
Originally Posted by Terry Montlick View Post

It is basically just a fancy "loudness button" Sanjay, but one which is continuous and slaved to the volume control of the Tact. The word "dynamic" is nice marketing lingo, but had me confused as well.

Terry,

The dynamic room compensation is more than just a "loudness button".

As you know, a loudness button just adds a fixed emphasis curve to the response of
the system. The TacT DRC is one level removed from that.

The thing that is being interpolated is NOT the filter response itself. The thing that is
being interpolated is the "target curve". The TacT interpolates within a set of 5 curves
to compute a correction to the reference target curve. These 5 curves are tweekable
by the user.

Once the correction curve is interpolated, it is then added to the reference target curve
chosen by the user at calibration time. The sum is the desired effective target curve.

The TacT 2.2 XP then uses the measured room response which was measured at
calibration time to compute a new filter that will give the desired target curve response,
but tailored to the particular room in which it is employed. [ That is something no
"loudness button" can do - the engineer that designs the loudness circuit has no idea
what the room response is ] The new filter is then automatically loaded into the DSP
chips.

I use a TacT 2.2 XP in my system. I used a separate frequency response program
running on the laptop to tweek the target curves and measure the effect with the
frequency response program as the system plays various test tracks.

The TacT 2.2 XP system does work as advertised - and I enjoy the results very much.
post #23 of 32
Quote:
Originally Posted by Terry Montlick View Post

It is basically just a fancy "loudness button" Sanjay, but one which is continuous and slaved to the volume control of the Tact. The word "dynamic" is nice marketing lingo, but had me confused as well.

Thanx Terry. IF that is what Tact is doing, then I don't see the big deal. My old Lex processors have been doing this for over a decade, switching to different equiloudness curves depending on volume level. On the MC-12v4EQ, this variable loudness compensation is added to the room correction. Greg, is this along the lines of what you're talking about, or is Tact doing something different?

Sanjay
post #24 of 32
Quote:
Originally Posted by sdurani View Post

Greg, is this along the lines of what you're talking about, or is Tact doing something different?

Sanjay,

NO - the TacT is doing something different than either what Terry told you, or
what the Lexicons are doing.

As I stated above, the TacT is NOT just adding a pre-determined emphasis which is what
a loudness control is doing.

No - the volume dependent interpolation is done to alter the "target curve". The TacT
does that calculation BEFORE it does the room correction calculation.

Let me put it this way. Suppose we had TWO Tact 2.2 XPs in two sonically different
rooms. Suppose we were to feed both TacTs the same set of 5 DRC curves and
reference target curve, and set them to the same volume control level.

The first part of the calcuation would be the same - both TacT units would calculate
the same target curves. However, because they are in different rooms with different
measured responses - the two TacTs will calculate DIFFERENT filters to apply to
the signal.

If they were just "loudness buttons" like Terry said, the amount of emphasis would
be the same for the two units. But that's NOT what the TacT is doing. The calculated
response filter would be DIFFERENT because the room response is different - even
though the target curves would be the same.

The TacT is computing a "loudness emphasis" that is TAILORED FOR the room it is in.
post #25 of 32
OK Greg, let's see if I'm understanding this. Changing the loudness curve based on volume level is to compensate for our human hearing. In addition to that, is the Tact changing the room correction based on volume level to compensate for measured room response?

There are two corrections going on here: one based on equi-loudness curves for our hearing and the other to address the room's unwanted contributions. I'm trying to figure out whether either one or both are being dynamically corrected. Or are they tied together and not separate corrections?

Sanjay
post #26 of 32
Thread Starter 
I will attempt a shot at this.

I believe what Greg is implying is that there are effectively 2 sets of calculations.

The first takes the target curve and in fact determines how this should change according to the volume level being played.

The second essentially applies the appropriate correction given the room response to attain the "volume-modified" curve identified in the first step.

Presumably, once room response has been measured, new target curves can be entered at any time. The modifications to this curve to account for changes in perception at different volume levels can actually be calculated upfront and therefore off the critical path.

It is the second step that must be performed "on the fly" so to speak.

I have some more questions but I believe its worth making sure we are on the same page before I progress.

Thanks to everyone in responding and Greg in particular for helping me.

Kind regards,

Ash
post #27 of 32
I've been trying to convince other companies to do this for years. Nice to see it showing up finally. Seemed like a no brainer given the power of DSP these days. I guess Audyssey is doing something similar now, but not sure of the specifics.

Edit - more info - http://www.audyssey.com/technology/dynamicEQ.html

Funny thing, they show the Eq'd measured FR as being the same as the non EQ'd response. But that is the opposite of how it would work. I guess they were trying to show the *perceived* FR.
post #28 of 32
Quote:
Originally Posted by skirmash View Post

I will attempt a shot at this.

I believe what Greg is implying is that there are effectively 2 sets of calculations.

The first takes the target curve and in fact determines how this should change according to the volume level being played.

The second essentially applies the appropriate correction given the room response to attain the "volume-modified" curve identified in the first step.


Ash,

You got it EXACTLY CORRECT on all counts above.

Every time the volume control is changed, the TacT re-interpolates the target curve, and then recomputes
the room dependent filters, and loads them into the DSP chips, "on the fly".

Once the filter is loaded into the DSP chips; the filter is applied to the signal as per usual operation of the
TacT.
post #29 of 32
Thread Starter 
Again, thanks for the assistance.

So, I have a couple of follow up questions because I need to make some decisions regarding the system in my front room. More on that in a later post. First, Greg's explanation of the 2 step process leaves me with another question.

If logic serves me correctly, then there must be a reference point volume level associated with the Target Curve specified by the user. The user presumably "auditioned" various curves and chose the target one they prefer. All level dependent adjustments must be made to the chosen curve for volume levels other than the one at which the user preferred the target curve.

I guess alternatively ... I could be talking hogwash. The curves may have already been adjusted for volume at the point they are "auditioned". However, if the user wants to specify a curve that is not one that comes with the unit ... then I think the above question remains.

Am I making any sense ?

Kind regards,

Ash
post #30 of 32
Hi All,
I think that this is a very intriguing thread. Room correction is , for me, the next step in my system. I have been researching as best as possible the best systems out there. Unfortunately, some of them need very educated input. "We do the work" , such as Tact and Lyngdorf, are very enticing, although with the Lyngdorf, there is no provision for user adjustabiltiy. Tact seems to allow this, but are they state of the art? DEQX and the Dolby Lake processor all need an experienced hand at the helm. Of course, there are no dealers in the area. However, a local sound guy, Mark Seaton, does have experience with the Dolby. Some systems, such as DEQX, seem more aimed at speaker crossover and integration rather then room correction.
Another issue, more for home theater than 2-channel audio, is that the new listening codecs all seem to require an HDMI interface. I don't believe that any of these have HDMI, so they are not applicable to the next generation. ..Yet.
I should add that I'm interested in correction purely in the digital domain, since I had poor experience with the Rives Parc, which is analog bass attenuation. In my system, it tended to depress the dynamics. As always, YMMV.

David
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